Topic 7a Chemical substances, chemical properties, chemical changes, colloids
Updated 2008-07-16
Please send comments to: J.Elfick@uq.edu.au
See also: Interesting websites
7.5.0 Particles
7.6.0 Suspensions and precipitates
7.7.0 Solutions, solubility, molar solution, solubility equilibrium, solubility product, solubility rules
3.9.0 Solubility
7.8.0 Colloids, sols, emulsions, gels, aerosols, foams, types of colloids
7.9.0 Chemistry terminology
7.5.0 Particles
3.55 Brownian movement
3.56 Particles of matter and dilution
3.57 Size of a molecule
3.58 Clay soil suspension
7.6.0 Suspensions and precipitates
7.6.1 Shake clay soils in water
7.6.2 Add salts to clay suspension
7.6.2.1 Add limewater to clay suspension
7.6.2.2 Add potassium alum or aluminium sulfate to a clay suspension
7.6.3 Test clay suspensions with a centrifuge
7.6.4 Prepare aluminium hydroxide precipitate, remove colour with aluminium hydroxide
7.7.0 Solutions, solubility, molar solution, solubility equilibrium, solubility product, solubility rules
7.7.0.1 Solution
7.7.0.2 Solubility
7.7.0.2a Solubility of different salts
7.7.0.2b Solubility of gases and temperature, carbon dioxide
7.7.0.3 Molar solution
7.7.0.4 Solubility equilibrium, solubility product
7.7.0.5 Solubility rules
7.7.1 Separate soluble from insoluble substances, sand and sodium chloride, ammonium chloride and sulfur
7.7.5 Solubility of different salts and temperature
7.7.5.1 Measure the solubility of sodium chloride in water at room temperature
7.7.6 Solubility and temperature, plot solubility curves
7.7.6.1 Measure effect of temperature on solubility of potassium dichromate
7.7.8 Measure solubility and particle size, copper (II) sulfate
7.7.9 Measure solubility of sucrose (cane sugar)
7.7.10 Solutions contain more than one solute
7.7.10.1 Solubility of sodium chloride and potassium dichromate in sugar solution
7.7.12 Rates of solution
7.7.13 Weight of solids dissolved in tap water
7.7.13.1 Volume of gas dissolved in tap water
7.7.14 Fractional crystallization of sea water
3.9.0 Solubility
3.9 Solubility in water
3.10 Solubility and temperature, solubility of salts in water
3.11 Solubility of a substance in water at a given temperature
3.12 Solubility and particle size
3.13 Solubility and solvents
3.14 Solubility and agitation
3.15 Volume of solutions
3.16 Miscible liquids
3.17 Heat of solution
3.17.1 "Magnetic" sugar cube dissolves
3.71.1 Solubility table and solubility rules
3.71.2 Add sodium chloride solution to copper (II) sulfate solution
3.71.3 Test if a precipitate forms when solutions of salts are mixed
3.71.4 Test if precipitate forms when solutions are added to lead (II) nitrate
3.71.5 Solubility of blackboard chalk in water
3.72 Displacement of copper from solution of copper ions
3.2.5 Secret writing inks (invisible ink) with cobalt (II) chloride, sucrose, starch, ammonium iron (II) sulfate, sodium chloride
4.19 Invisible inks (Primary)
7.8.0 Colloids, sols, emulsions, gels, aerosols, foams, types of colloids
7.8.0.1 Colloids and crystalloids
7.8.0.2 Sols
7.8.0.3 Emulsions
7.8.0.4 Gels
7.8.0.5 Aerosols (fogs)
7.8.0.6 Foams
7.8.0.7 Types of colloids
7.8.1.0 Examine common colloids, mayonnaise, cod liver oil
7.8.1.1 Ferric hydroxide colloid
7.8.1.2 Sulfur in methylated spirits colloid
7.8.1.3 Sodium thiosulfate (hypo) with dilute hydrochloric acid colloid
7.8.1.4 Size of colloidal particles
7.8.2.0 Emulsions
7.8.2.1 Examine emulsions with a microscope
7.8.2.2 Prepare face cream emulsion
7.8.2.3 Prepare bean curd
7.8.2.4 Temporary emulsions and permanent emulsions, kerosene, detergent
7.8.3.0 Gels
7.8.3.1 Prepare silica gel
7.8.3.2 Prepare gelatine (gelatine) gel
7.8.3.2.1 Gels in the home kitchen
7.8.3.2.2 Metallic salts gel, calcium carbonate gel, calcium acetate gel
7.8.4.0 Sol, starch solution, smoke
7.8.4.1 Smoke and aerosol colloids
7.8.4.2 Foam colloid, bubble bath solution
7.8.5.0 The Tyndall effect
7.8.6.0 Use soap as an emulsifying agent
7.8.7.0 Chemical changes in photography, make photographic print, silver nitrate
7.6.0 Suspensions and precipitates
1. Suspensions are heterogeneous systems of particles containing many molecules in a liquid, e.g. clay particles in water. Examine the suspension particles. Suspension particles eventually settle on standing. The smaller the size of the suspension particles the longer the time to settle. Under a microscope the suspension particles show Brownian movement. A suspension has large solid solute particles that can be seen, settle out on standing and do not pass through filters or permeable membranes, e.g. milk of magnesia, Mg(OH)2, calamine lotion and muddy water. The two or more components of a suspension are easily visible.
2. Precipitates are solids that form in a solution. Precipitates form when the particles of dissolved substances join, and fall down, to leave the solution. A precipitate may appear as a cloudy suspension in solution or as coagulated lumps. It may be white or coloured.
3. When water is added to clear solutions of some alcoholic drinks, e.g. ouzo, pastis, sambuca, the drinks become milky white because the terpenes used for flavouring are soluble in alcohol but not in water. The added water creates a suspension of  terpenes that have left the alcohol solution.
7.6.1 Shake clay soils in water
Fill to 3/4 a measuring cylinder with mixture of clay soil and water. Shake the mixture then leave it to settle. Note which particles settle and in what order. For particles that do not float the smaller the particles the longer they take to settle. Filter the milky coloured liquid. The filter paper stops some particles, but the filtrate is still cloudy because clay particles can pass through the filter paper. Some particles remain in suspension for a long time because they are small and water molecules hit them on all sides because of Brownian movement.
7.6.2 Add salts to a clay suspension
In many hot countries, salt crystallizes from pans built on clay beds near the mouths of rivers. Divide the clay filtrate into two test-tubes. Keep one as a control. To the other add drops of sodium chloride solution. The filtrate becomes clear. The effect occurs when a clay suspension in a river meets the salts in sea water.
7.6.2.1 Add limewater to a clay suspension
Repeat the experiment by adding drops of limewater.
7.6.2.2 Add potassium alum or aluminium sulfate to a clay suspension
Half fill two measuring cylinders with a clay suspension or muddy water. Add aluminium potassium sulfate [potassium alum, Al2(SO4)3.K2(SO4).24H2O] or add aluminium sulfate [Al2(SO4)3.18H2O,water filter powder] and sodium carbonate to one measuring cylinder. Shake both measuring cylinders and leave to stand. Compare the clarity of the water in the two measuring cylinders.
7.6.3 Clay suspensions in a centrifuge
1. Make a low-cost centrifuge from a meat grinder. Put a test-tube of clay suspension in a sling and whirl it around the head BE CAREFUL! Another method is to attach the test-tube to the spoke of an upturned bicycle then turn the pedals with your hand. During the whirling, the heavier particles move to the closed end of the tube and separate partly from the liquid.
2. Shake an emulsion in water then centrifuge it. The liquids separate faster.
3. Centrifuge a solution of copper (II) sulfate. No separation occurs.
7.6.4 Prepare aluminium hydroxide precipitate, remove colour with aluminium hydroxide
1. Add 1 cm depth of potassium alum solution [Al2(SO4)3.K2(SO4).24H2O] to equal volume of dilute ammonia solution or dilute washing soda solution, Na2CO3.10H2O. A white jelly-like precipitate of aluminium hydroxide forms. Add dilute citric acid or sulfuric acid and the precipitate disappears.
2. Put in a test-tube 1 cm depth of water and one drop of a common dye or ink, e.g. cochineal (pink cake colouring) Congo red, black ink, red ink. Add an equal amount of dilute ammonia solution. Fill the rest of the test-tube with potassium alum solution and leave to stand. Aluminium hydroxide precipitate forms as a coloured jelly leaving the liquid above it colourless.
7.7.0.1 Solution
A solution is a homogeneous system (no boundaries) that consists of a solute dissolved in a solvent. The particles are usually molecules or ions. An alloy, e.g. steel, is an example of solutions of solids dissolved in solids. A saturated solution is a solution in which no more solute can dissolve at that temperature. In a solution, the solid particles are very small, cannot be seen, and do not settle. A solution is uniform in appearance, clear or coloured, the solute may be regained from the solvent by evaporation and can pass through fine filters and permeable membranes.
7.7.0.2 Solubility
The solubility of a solute in a solvent is the number of moles or grams of the solute that can dissolve in a volume, usually 100 g of the solvent (1 dm3 of water) at room temperature, usually 20oC. The solubility of a substance is the weight of solute that can dissolve in a solvent at a particular temperature. For example the solubility of sodium chloride is 36 g /100 g of water at 20oC. The solubility of gases decreases as the temperature rises. When a more concentrated solution is diluted with a solvent, C1V1 = C2V2, where C1= original concentration, V1 = original volume, C2= final concentration, V2 = final volume.
7.7.0.2a Solubility of different salts
Half fill three identical beakers with water. Add teaspoons measures of a different substance to each beaker until no more can dissolve, e.g. sugar, table salt, sodium bicarbonate. Note how many teaspoons of the substance can be added to the water until the solution becomes saturated and no more of the substance can dissolve. You can probably dissolve 20 spoonfuls of sugar in a cup of coffeee before the coffee solution becomes saturated and undrinkable.
7.7.0.2b Solubility of gases and temperature, carbon dioxide
Use two identical plastic bottles or aluminium drink cans of aerated water, fizzy drink, cola. Leave the two drinks on the table for some time until you are sure that the contents are at the same room temperature. Put one drink in the refrigerator, not in the freezer. The next day take the drink out of the refrigerator and quickly open both drinks without any shaking. Note which drink releases the most carbon dioxide gas as fizzy bubbles. The warmer drink releases the most gas because more carbon dioxide remains dissolved in the cooler solution.
7.7.0.3 Molar solution
See also 7.9: Amount of substance, the mole
A molar solution, 1 M, contains one mole of the substance per litre of the solution. The energy change when 1 mole of solute dissolves in the solvent is called the heat of solution. Exothermic dissolving processes of a solid in a solvent are associated with high solubility. Endothermic dissolving processes of a solid in a solvent are associated with low solubility. A substance is "soluble" in a solvent if it dissolves to give a concentration > 0.1 M. The dissolving process of a gas in a liquid is exothermic. Solubility of gases in liquids decreases with temperature. When an ionic solid dissolves in water the cations and anions separate. Usually, ionic solids, e.g. sodium chloride NaCl, are soluble in water and non-ionic substances are insoluble. However, some ionic solids, e.g. silver iodide AgI, is insoluble in water. A precipitate is a solid produced in solution. Differences in solubility can be used to separate mixtures of ions.
7.7.0.4 Solubility equilibrium, solubility product
When the forward process continues as the same rate as the reverse process in a reversible chemical reaction, the system is at equilibrium and its properties will not change, e.g. colour. A dissolving solid and its solution reach equilibrium when the rate of crystallization equals the rate of dissolving. Silver chloride has a low solubility in water. AgCl(s) <---> Ag+(aq) + Cl-(aq) Let [Ag+] = concentration of silver ions and [Cl-] = concentration of chloride ions. The solubility at fixed temperature is constant. Therefore, constant = [Ag+] X [Cl-] called the solubility product, Ksp for a saturated solution of silver chloride at room temperature. It is the equilibrium constant for that solid-solution system.
Some solubility products
Compound Ksp
1. AgCl2 2 X 10-10
2. CaSO4 3 X 10-4
3. BaSO4 2 X 10-9
If the solubility product is greater than Ksp the a precipitate will form until the product of the ion concentrations equals Ksp is large, then at equilibrium, the concentration of products is much greater than the concentration of the reactants.
7.7.0.5 Solubility rules
All sodium, potassium and ammonium salts are soluble. All nitrates are soluble. All acetates are soluble. All chlorides are soluble except silver chloride and lead chloride. Lead chloride is slightly soluble in cold water and is more soluble in hot water. All carbonates are insoluble except lead sulfate and barium sulfate. Calcium sulfate is only slightly soluble. All carbonates are insoluble sodium, potassium and ammonium carbonate.
1. Shake powdered blackboard chalk with water in a test-tube. Filter the mixture and collect the filtrate. Evaporate the water by heating the basin over a beaker of boiling water. Examine the inside of the evaporating basin. If you see any residue, part of the solid did dissolve.
2. Put 20 mL of tap water and deionized water or demineralized water in clean evaporating basins and evaporate each to dryness. Examine the inside of each evaporating basin for any residue. A residue indicates that the water contains dissolved solids.
3. Shake a small quantity (on a little finger nail) of each of the following salts with 10 mL deionized water or demineralized water: Ammonium chloride, sodium acetate, sodium sulfate, sodium carbonate, barium chloride, barium nitrate, barium sulfate, copper nitrate, copper (II) sulfate, copper carbonate, lead chloride, lead nitrate, lead sulfate, lead carbonate, calcium nitrate, calcium sulfate. If the salt dissolves, note any change in the temperature of the mixture. Classify each salt as soluble or slightly soluble or insoluble.
4. Mix the following pairs of substances in small quantities and observe whether a solution forms: Sodium chloride and kerosene, olive oil and water, methylated spirit and water, petrol and olive oil, petrol and kerosene, methyl alcohol and copper (II) sulfate crystals, ethyl alcohol and copper (II) sulfate crystals, kerosene and petroleum jelly.
7.7.1 Separate soluble from insoluble substances, sand and sodium chloride, ammonium chloride and sulfur
Shake a mixture of sand and sodium chloride in a test-tube containing water. Filter the mixture into an evaporating basin. Heat the filtrate to form sodium chloride crystals. Sodium chloride, the solute, dissolves in water, the solvent, to form a solution. Sand is insoluble in water.
Repeat the experiment with a mixture of ammonium chloride and sulfur. The sulfur is insoluble in water. The ammonium chloride is soluble in water and can be recovered by filtration and evaporation of the filtrate.
7.7.5 Solubility of different salts and temperature
Most ionic substances increase in solubility with increases of temperature. After completing the experiment, keep the recrystallized salts in special jars and record each solubility on the label of the jar. Record as solubility in 100 g of water, to nearest gram and oC.
Chemical 20oC 30oC 40oC 50oC
Sodium nitrate 88 g 95 g 102 g 109 g
Potassium nitrate 31 g 46 g 62 g 82 g
Potassium chloride 34 g 36 g 39 g 42 g
Sodium chloride 35 g 36 g 37 g 38 g
7.7.5.1 Solubility of sodium chloride in water at room temperature
Dissolve sodium chloride in 25 mL of water at room temperature. Stir until no more dissolves. The solution is saturated. Filter the solution to remove undissolved salt. Record the temperature of the saturated solution. Weigh an evaporating dish (W1). Pour the saturated solution into the dish and heat the dish slowly to evaporate the solution to dryness. Cool the dish and weigh again (W2). Mass of the salt dissolved = W2 - W1. Mass of water evaporated = mass of 25 mL. The solubility of the salt = (W2 - W1) / 25 X 100 g per 100 mL water at room temperature.
7.7.6 Solubility and temperature, plot solubility curves
See diagram 7.7.6
clear="all"> Measure the solubility of different salts at different temperatures, e.g. sodium nitrate, potassium nitrate and potassium chloride. Use water at different temperatures. Construct a table to compare the amount of each salt dissolved at different temperatures. Draw a graph showing the results and the experimental results above. Plot the solubility expressed as the number of grams of solute dissolved in 100 mL water along the vertical axis. Plot the temperatures expressed in oC along the horizontal axis.
7.7.6.1 Effect of temperature on solubility of potassium dichromate
Dissolve potassium dichromate in 50 mL of water at 60oC, until no more dissolves. The solution is saturated. Pour the clear solution into a second beaker. Let the temperature drop slowly to 40oC. Crystals form in the second beaker. Pour the clear solution from this beaker into a third beaker. Leave it cooling to room temperature to form more crystals. The experiment shows that a saturated solution contains fewer dissolved solids at a low temperature than at a higher temperature.
7.7.8 Effect of particle size on the rate of dissolving of copper (II) sulfate
See also 3.12: Solubility and particle size
Separate large crystals from small crystals of copper (II) sulfate. Add 5 g of each to a test-tube containing the same amount of water. Shake both test-tubes equally and simultaneously. Note the amount of undissolved salt left in each tube after the same number of shakes. Small particles dissolve faster than large particles.
7.7.9 Solubility of sucrose (cane sugar)
Add sucrose to a test-tube of water at room temperature until no more dissolves after stirring. Record the weight of the sucrose dissolved. Sucrose, cane sugar, is very soluble in water.
7.7.9.1 Temperature affects solubility of sucrose
Repeat the experiment at 10oC intervals until 70oC and record the weight of sucrose dissolved. Heat the solution to 100oC. Pour it into an evaporating basin and leave to cool. Observe the cooling solution.
7.7.9.2 Use solubility of sucrose to make toffee candy
Prepare toffee or candy by heating the sucrose solution with milk or butter until it boils. Heat to 125oC. Pour it into paper cups and leave to cool. Constant stirring and temperature control is essential.
7.7.9.3 Prepare crystal rope
Prepare a saturated solution of sucrose (cane sugar). Tie a weight to a cotton thread and suspend it so that the weight and most of the thread are in the solution. As the solution cools, crystals form on the thread and weight. Take out the thread and weight. Hold the weight and set fire to the cotton. A rope of sugar crystals remains.
7.7.10 Solutions containing more than one solute
The presence of one dissolved substance does not prevent other substances dissolving in the solution. As general rule, unless the concentrations are high, one solute does not affect the solubility of others.
7.7.10.1 Dissolve sodium chloride and potassium dichromate in sugar solution
Dissolve some sugar in a small quantity of water. Add sodium chloride crystals to the solution. Note whether it also dissolves. Drop pieces of potassium dichromate into the solution and shake it. The colour change of the solution shows that potassium dichromate is dissolving.
7.7.11.1 Solubility of sodium chloride and iodine in different solvents
Half fill two test-tubes with water and methylated spirit. Add equal weight of sodium chloride. The sodium chloride dissolves more in water than in the methylated spirit. Repeat the experiment with iodine crystals. The iodine crystals are more soluble in the methylated spirit than in the water.
7.7.11.2 Find a solvent for Sudan III
Put drops of Sudan III dye in a test-tube. Add water and shake the test-tube. The dye does not dissolve. Add a little oil or melted butter and shake the test-tube. The dye dissolves in fats and in oils.
7.7.12 Rates of solution
The rate at which some solid dissolves in water can be increased in three ways:
1. Grind the solid into smaller pieces: Take two equal quantities of large crystals of copper (II) sulfate-5-water. Grind one quantity into a fine powder. Put both samples into equal quantities of water in separate test-tubes and shake. Compare the rates at which the different samples dissolve.
2. Shake the solution while the solid is dissolving: Put equal quantities of sugar into separate equal quantities of water in two test-tubes. Shake one tube and leave the other to stand. Compare the rates at which the samples dissolve.
3. Heat the solution: Put equal quantities of potassium nitrate in equal quantities of water in two test-tubes. Shake both test-tubes, holding one of them over a flame. Compare the rates at which the samples dissolve.
7.7.13 Weight of solids dissolved in tap water
Evaporate to dryness equal volumes of tap water and distilled water. Examine the inside surface of each evaporating basin. The residues in the basin show that tap water contains dissolved solids. Weigh the residues.
7.7.13.1 Volume of gas dissolved in tap water
See diagram 7.7.6 | See also 3.25: Gases dissolved in a water sample
Hold a round bottom flask under water. Insert a stopper with a delivery tube so that water completely fills the whole apparatus. Note the original volume of water. Put the end of the delivery tube into a test-tube full of water. Heat the flask to boiling to collect dissolved gases in the test-tube. Leave the apparatus to cool to room temperature. Measure the volume of gases expelled from the water.
7.7.14 Fractional crystallization of sea water
When sea water in a rock pool evaporates, white crystals form along the edge of the pool. If all the sea water evaporates, the final solid will contain about 0.5% carbonates, 3% gypsum, and 96.5% sodium chloride. However, when the sea water starts to evaporate and crystals start to form, calcium is the only cation near saturation. As water evaporates from the sea water, the order of precipitation is mainly calcium carbonate, calcite, and also calcium magnesium carbonate, dolomite, then calcium sulfate dihydrate, gypsum, then sodium chloride when the volume of the sea water is about 10% of the original, then potassium and magnesium salts. If you dissolve more carbon dioxide in the sea water, you would not precipitate more calcium carbonate. The pH of sea water is 7.8 and carbon dioxide is present mainly as bicarbonate (HC03-, about 90%). Dissolving more carbon dioxide lowers the pH and the equilibrium shifts to converting more carbonate to bicarbonate.
7.8.0.1 Colloids and crystalloids
Colloids are glue-like amorphous substances, e.g. gelatine or starch, with particles bigger than most molecules, i.e. 10-9 to 10-6m. The particles are too large to pass through a membrane but too small to be observed under a microscope. Colloids have a dispersed phase of particles scattered through a continuous phase, the medium. Colloids differ from crystalloids, e.g. inorganic salts that can pass through a membrane. Colloids are a type of mixture with properties between that of true solutions and suspensions. Colloids do not separate on standing but can pass through filters. Colloidal particles are uniformly distributed in solution, cannot be seen under the microscope but are still too large to pass through membranes. Colloids can scatter light and the common white colour of colloids is because of the reflection by the particles of all colours of the spectrum.
7.8.0.2 Sols
Sols are dispersions of small groups of molecules in a medium. Sols remain dispersed because Brownian movement prevents the groups of molecules precipitating under the influence of gravity. Lyophilic ("water loving") sols, e.g. starch in water have large dispersed particles with an affinity with the medium. The particles of some protein solutions have a chemical shell of water molecules around them that prevents them flocculating unless a strong salt solution is added to precipitate the colloid. Lyophobic sols, e.g. silver chloride in water, have a dispersed phase with no affinity with the medium. The particles keep apart because of their electrical charge, but eventually they precipitate. Association colloids, e.g. soap in water have a dispersed phase part lyophobic and part lyophilic. Colloidal dust, including chalk dust, may be shown using projector beams.
7.8.0.3 Emulsions
Emulsions are colloids with dispersed and continuous phases both liquids, e.g. oil in water. When you shake two immiscible liquids, droplets of one liquid are dispersed in the other. In temporary emulsions, e.g. kerosene in water or oil in water, there is no attraction between the two liquids. The two phases will disperse in each other if shaken together but will separate on standing. By adding an emulsifying agent, e.g. soap, the two liquids, kerosene and water, will remain dispersed within each other. Mayonnaise is a mixture of olive oil dispersed in vinegar and stabilized by egg yolk or mustard. Margarine is an emulsion of water, flavours, colours, and vitamins in a semi-solid fat. Lotions, cosmetic creams and ointments are mostly emulsions of oils dispersed in water.
7.8.0.4 Gels
Gels are colloids in which liquids are dispersed in solids to form a jelly. They are semi-rigid systems that consist of random networks of colloidal fibres or crystals, with liquid in the spaces. Gels are colloidal systems that have set. Some gels can lose one component by heating to leave a solid, e.g. silica gel. Hydrogels have insoluble chains of polymers in a network that can contain very high concentrations of water. They can be used for breast implants, water-holding granules for dry soils, burn dressings, baby nappies( diapers), sanitary napkins, contact lenses and water sensors. Environmentally sensitive hydrogels can be used to detect specific concentrations of substances or temperature or pH then release the substances they are carrying . Hydrogels include acrylate polymers,  polyvinyl alcohol, sodium polyacrylate and natural hydrogels, e.g. methylcellulose and may have thixotrophy, i.e. become fluid when disturbed but solidify at rest, e.g. hair gels.
7.8.0.5 Aerosols
Aerosols (fogs) are dispersions of liquids or solids in a gas, e.g. fly spray, mist, smoke.
7.8.0.6 Foams
Foams are dispersions of gases in liquids, e.g. fire extinguisher foam or gases in solids, e.g. foam rubber mattress.
7.8.0.7 Types of colloids

Dispersed Medium In solid dispersion medium, disperse phase In liquid dispersion medium, disperse phase In gas dispersion medium, disperse phase
Solid continuous phase Alloys, coloured ruby glass, gemstones, paper, minerals gold in glass Paints, Fe2O3, clay, chocolate drink, gelatin Iodine vapour, cement dust, ammonium chloride, smoke
Liquid continuous phase Gels: celluloid, jelly, glue, jam, gelatine, desserts, mud, pearl Emulsions: milk, mayonnaise, blood, ice cream, kerosene in water, suntan lotion, photography emulsions, butter (water in oil) cream (oil in water) Aerosols, Fogs: agricultural sprays, clouds, visible steam
Gas continuous phase Foams: rubber, pumice, Styrofoam, bread, cake, marshmallow, plaster Foams: lather, froth, soap suds, whipped cream (No colloid gases)
7.8.1.0 Common colloids, mayonnaise, cod liver oil
Note the properties of common colloids, e.g. mayonnaise, glue, fog, smoke, aerosols, soups, human tissue, ice cream, fondants, marshmallows, beaten egg white, face cream, milk, salad dressing (olive oil and vinegar) gravy, soap solution, salad cream, furniture cream, hand lotion, hair cream, ointment, oil in water garden spray (white oil) creamy milk (unstable emulsion) cod liver oil, polyvinyl acetate paint.
7.8.1.1 Ferric hydroxide colloid
Dilute a ferric chloride solution until it appears pale yellow in colour. Pour some of this solution into a test-tube then place the test-tube in a beaker of warm water until the solution turns brown. Do not heat the solution to boiling. The colloid formed is hydrated iron oxide, Fe2O3. Keep the colloid for later use. Fe3+ (aq) + 2H2O (l) --> Fe(OH)3 (colloidal) + 3H+ (aq)
7.8.1.2 Sulfur in methylated spirit colloid
Mix sulfur powder (flowers of sulfur) with methylated spirit. Shake the mixture then filter. Continuously turn the mixture and pour the solution into a beaker of water. A weakly opalescent colloidal sulfur solution forms. Keep the solution.
7.8.1.3 Sodium thiosulfate (hypo) with dilute hydrochloric acid colloid
Dissolve 5 g sodium thiosulfate crystals in 500 mL water. Add 5 mL hydrochloric acid to 50 mL the sodium thiosulfate solution (S2O3)2- (aq) + 2 H+ (aq) --> S (colloidal) + H2O + SO2
7.8.1.4 Size of colloidal particles
1. Use three 50 mL, beakers. Pour 20 mL copper (II) sulfate solution into Beaker 1. Add 20 mL skim milk solution to Beaker 2 and Beaker 3. Add 5 mL 3 M acetic acid to Beaker 2. to form the precipitate casein. Filter the contents of each beaker into three large test-tubes 1, 2 and 3. Use two 20 cm lengths of dialysis tubing, soak for ten minutes in distilled water to soften and tie up one end of each length. Pour the copper (II) sulfate solution from Beaker 1 into one set of dialysis tubing. Pour the skim milk from Beaker 2 into the other set of dialysis tubing. Tie the ends of both sets of tubing, rinse with deionized water and put into two separate beakers of water. Observe what happens after some time.
2. Repeat the last experiment using a mixture of starch and glucose solution inside the dialysis tubing. After 30 minutes test the water outside the dialysis tubing for water for glucose and starch. The next day, repeat the test for glucose and starch. Filter the starch and glucose solutions and observe whether the filter papers contain any residues. State your conclusions the sizes of colloids compared with the true solutions in these experiments.
7.8.2.0 Emulsions
See also 6.9.18.8: Emulsifying agents (for pesticides)
Emulsions are colloidal systems with both the dispersed phase and the continuous phase are liquids, e.g. oil in water. The droplets of a liquid remain suspended in another liquid. Emulsions may be cloudy or opaque. Emulsions are like suspensions because they settle on standing. Emulsifying agents are used to keep the phases dispersed so that the droplets remain suspended and the emulsion remains stabilized. Milk is a poor emulsion. The forces of cohesion between the emulsified cream droplets are greater than the forces of adhesion between the milk and the cream, so the cream floats above the milk. In homogenized milk, the droplets have been broken into smaller particles and dispersed to form one phase.
7.8.2.01 Bile salts are emulsifying agents that let fat particles remain suspended so that enzymes can digest them before absorption into the blood stream. They sell bile salts commercially as sodium tauroglycocholate.
7.8.2.1 Emulsions with a microscope
Put a blob of an emulsion on the end of a microscope slide. Put a drop of paraffin oil on one side of the blob and put a drop of water on the other side. Stir a little of the emulsion into each liquid. Smooth mixing occurs only when the liquid forms the continuous phase.
7.8.2.2 Prepare face cream emulsion
See diagram 3.8.2.2: Emulsifiers used in cosmetics
Heat disodium tetraborate (III) in water until dissolved. Heat a mixture of medicinal paraffin (propan-2-yl tetradecanoate) beeswax and petroleum jelly on a hot plate. Pour the hot disodium tetraborate (III) solution into the mixture of hot oils and stir. When cool, add perfume and colour.
7.8.2.3 Prepare bean curd
Weigh 50 g of moth free and mildew free soybeans and put them in a 500 mL beaker. Add 300 mL water to soak for 24 hours to make the soybeans fully swell. Replace the water if the atmospheric temperature is high. Then pour out the water. Grind the soaked soybeans in 200 mL water by using a household grinder. To make soybean milk, transfer the soybean slurry to a filter fitted with two pieces of filter cloth, and filter by suction. Wash the filter cake many times with 100 mL water to extract the soybean milk fully from the bean dregs. The filtrate obtained is concentrated soybean milk. Pour the concentrated soybean milk (or alternatively use commercial concentrated soybean milk in bags) into a clean 500 mL beaker and heat to about 80oC. Add saturated gypsum aqueous solution to the hot soybean milk. Stir constantly until white wads appear. Stop heating and let the soybean milk stand for five minutes. Solid lumps start to separate out of the bean milk. After standing for about 20 minutes, filter the solidified lumps from the bean milk. Gather the lumps and shape them into a cube folded up in the filter cloth. Place the cube on a clean plate and press it by putting a small beaker containing cold water on it. About 30 minutes later, a cake of bean curd forms. The bean curd will be whiter and more tender if commercial concentrated soybean milk is used. To preserve the freshly made bean curd from deterioration for a few days, soak it in 2-5% table salt aqueous solution and keep it in a cool, shady place.
7.8.2.4 Temporary emulsions and permanent emulsions, kerosene, detergent
1. Add 5 drops of kerosene to 5 mL of water and shake the test-tube. Leave to stand and observe the separation into two layers. Add liquid detergent, shake, and leave to stand. Observe the permanent emulsion.
2. Add 5 drops of oil to 5 mL of water and shake the test-tube. Leave to stand and observe the separation into two layers. Add liquid detergent, shake, and leave to stand. Observe the permanent emulsion.
3. Mix 5 mL of oil and 5 mL vinegar. Leave to stand and observe the separation into two layers. Add mustard or egg yolk, shake, and leave to stand. Observe the salad oil permanent emulsion.
7.8.3.0 Gels
Gels are colloids with a three dimensional structure of a network of linked molecules, e.g. gelatine, jelly, silica gel. Gels easily change their shape under pressure but may flow under high pressure. Gels form when colloidal solutions are left standing to allow a 3 dimensional network to form. Collagen is denatured by heat to form separate molecules called gelatine that form an amorphous network when cooled. When gelatine is dissolved in water, it forms chemical bonds with the water and acts as a semi-solid or gel, jelly. The gel can "dissolve" when heated and then form again when cooled again. When the proteins in egg white are denatured by heating, they form a permanent gel that does not "dissolve" when heated.
7.8.3.1 Prepare silica gel
Some gels can lose water by heating to leave a rigid gel. Silica gel is an amorphous form of hydrated silica, very hygroscopic, and is used to protect delicate machinery from rusting.
Add sodium silicate to water. Add drops of phenolphthalein. Add 3 M hydrochloric acid until the red colour disappears. The solution sets as a gel. Heat the gel to remove moisture. The gel can absorb water again.
7.8.3.2 Prepare gelatine (gelatin) gel
Water-soluble protein from boiling collagen, e.g. horse hooves. Found in photographic emulsions, food jellies, adhesives.
1. Weigh a teaspoon of gelatine and dissolve it in 100 mL of hot water. Cool it to form a jelly.
2. Repeat the experiment to find the smallest concentration of gelatine needed to make a firm jelly.
7.8.3.2.1 Gels in the home kitchen
1. In the home kitchen, dissolve a sachet of household gelatine in 50 mL hot water. Pour into a dish and leave to cool until a firm jelly forms.
2. In the kitchen, repeat using agar or jelly crystals instead of gelatine.
3. Dissolve a packet of household jelly crystals in one cup of hot water. Cool and when the jelly is partially set, beat until frothy. Whip the mixture with one cup of chilled evaporated milk until the mixture is stiff. Add 2 tablespoons of lemon juice. Whip the mixture again. Fold this into the jelly gradually. Pour into pie crust and refrigerate for 3 hours. You can eat this foam colloid!
7.8.3.2.2 Metallic salts gel, calcium carbonate gel, calcium acetate gel
Dissolve 19 g calcium chloride in 25 mL water. Dissolve 28 g potassium carbonate in 25 mL water, then pour into the calcium chloride solution and stir vigorously. A gel forms of CaCO3.nH2O. Prepare a gel by adding a solution of magnesium sulfate (26 g in 100 mL water) to potassium hydroxide solution (107 g in 100 mL water) or sodium hydroxide solution (42 g in 100 mL water). Add 30 mL calcium acetate solution (35 g /L). The gel formed, called "Sterno", is flammable and is used for outdoor stoves.
7.8.4.0 Sol, starch solution, smoke
Sols are dispersions of very small solid particles in a liquid. Lyophobic, or hydrophobic, sols (solvent hating, water hating) are unstable and can form precipitates, e.g. silver chloride precipitates in photography. Lyophilic, or hydrophilic, sols (solvent loving, water loving) are more like solutions and are stable, e.g. starch in water. Aerosols are solutions of solid or liquid particles in a gas, e.g. smoke, fog. Foams are solutions of gases in solids or liquids. Foams, e.g. foam rubber, expanded polystyrene, have been stabilized with surfactants (surface acting agents), e.g. detergents.
7.8.4.1 Smoke and aerosol colloids
1. Pour concentrated hydrochloric acid and concentrated ammonia solution into two watch glasses. BE CAREFUL! Move the watch glasses close to each other and observe the smoke of ammonium chloride that forms.
2. Put a burning splint of wood on a tile and shine a beam at the smoke. Observe what happens.
3. In a darkened laboratory, shine a projector beam at the mist as it emerges from the nozzle of an aerosol spray can.
7.8.4.2 Foam colloid
1. Put bubble bath solution into a container of water and beat strongly until some foam forms. Here a gas is dispersed in a liquid.
2. Beat or whisk some cream (emulsion) until it becomes whipped cream foam.
7.8.5.0 The Tyndall effect
When a beam of light passes through some suspensions and colloids containing particles with diameters < 1/20 the wavelength of light, the scattered light appears mainly blue, e.g. tobacco smoke suspension. Direct a beam of light through the solution. If you can see the solute particles, the solution is colloidal because of the scattering of light.
Fill 250 mL beaker with yellow potassium chromate solution (K2CrO4). This is a true solution so it will not scatter light. Prepare the following test solutions in 250 mL beakers:
1. copper (II) sulfate solution,
2. starch solution,
3. 2 mL olive oil,
4. 10 mL water, 4 drops detergent. Shake and put in a large test-tube
5. weak black tea,
6. instant coffee solution,
7. detergent in a beaker of water,
8.toothpaste shaken in a beaker of water,
9. any other colloid solution.
Put each beaker of test solution next the beaker of potassium chromate solution and pass light from a projector through both solutions. Observe light passing through both solutions to detect which test solutions are colloids.
7.8.6.0 Use soap as an emulsifying agent
Soap in water have molecules that have both lyophilic and lyophobic parts so is called an association colloid. Soap molecules (sodium stearate) can cause droplets of fat to become negatively charged. These droplets remain suspended in the water and can be become washed away during cleaning. Soap is not soluble in salt water. Add drops of vegetable oil or paraffin oil to: 1. water, 2. soap solution, 3. gelatine solution. Leave the solutions to stand and note the separation. The oil separates first in 1, then in 2, and then in 3. If a stable emulsion forms in 3. the liquids may not separate.
7.8.7.0 Chemical changes in photography, make a photographic print, silver nitrate
Mix in a test-tube 4 cc of 2% silver nitrate solution with 4 cc of 5% sodium chloride solution. A white precipitate of silver chloride forms. Wrap a photographic negative around the tube and expose to sunlight for an hour. Remove the negative. You can see the image from the negative on the walls of the test-tube because the light causes reduction of the silver in the silver chloride to black silver.
7.9.0 Chemistry terminology
aerobic: A bacterial processes that occurs only in the presence of oxygen, opposite is anaerobic.
aliquot: A portion is a known fraction of the whole sample.
alkali: A water-soluble base yielding a caustic solution, pH > 7.
alloy: A compound, solution or mixture of two or more metals.
amphoteric: Can act as an acid or a base.
azeotrope: Mixture of liquids that boils at constant temperature, constant boiling mixture, e.g. 4% water and 09 % ethanol.
Bessemer process: Converts pig iron from a blast furnace into steel by blowing air or pure oxygen into the molten impure metal to convert impurities into a separating slag.
borosilicate glass: Addition of borate allows the formation of a glass that melts at a lower temperate than silica, and expands less on heating than soda glass, as well as more plastic over a wider temperature range, e.g. Pyrex and glass wool.
bromine: a halogen, intermediate between chlorine gas and iodine solid, a dense, fuming liquid at room temperature.
buffer: A mixture of substances that tend to hinder large changes in acid or basic properties of a solution.
buffer: A mixture of substances that tend to hinder large changes in acid or basic properties of a solution Used in a more general sense outside chemistry.
bumping: Sudden formation of a large amount of vapour from the bottom of a heated vessel of liquid, rather than the usual controlled boiling.
catalyst: Agent that speeds up a chemical reaction without itself being used up in the process, the transition metals Co, Ni, Pt. Enzymes are catalysts for biological reactions.
caustic: Very alkaline, can dissolve skin and fat to form soap.
continuous phase/outer phase: The continuous "outside" liquid that surrounds a second liquid, its droplets being discontinuous, in an emulsion.
detergent: Synthetic surfactants, not including soaps the sodium salts of natural fats.
efflorescence: Crystals containing water of crystallization lose some of that water by evaporation and begin to powder. Also, when dissolved salts rise to the surface and crystallize out of the ground or on a wall.
electrolysis: Electricity is carried through a solution by ions. At the electrodes, these or other ions are neutralized and discharged as neutral atoms or molecules.
emulsion: The suspension of one liquid as fine droplets in another with which it does not mix.
enzyme: A biological molecule that can promote or catalyse a particular reaction.
equilibrium: Reactions in which the forward and reverse rates are matched so that the composition of the mixture appears unchanging in time.
ferrite: Pure iron without carbon.
flammable: Easily set on fire. Also: non-flammable, not "inflammable". Flammability, explosion, limits: outer limits for the ratio of fuel to air within which the mixture will burn.
flash point: Temperature at which a chemical produces enough vapour to catch fire in the presence flame.
flocculate: Coagulate in fluffy lumps. Also: flocculant.
fluorescence: The rapid emission of light at longer wavelengths than that which is absorbed, e.g. adsorption of ultraviolet light can yield blue fluorescence.
flux: A substance added to lower the melting temperature in metallurgy and soldering.
froth flotation: Adsorption of chemicals on solid particles along with a foam to preferentially float off certain minerals and leave others behind.
fuel cell: Device with a cathode and anode, which converts a fuel directly into electricity without burning. The simplest case is hydrogen bubbled over a porous sintered nickel anode in alkali solution, while oxygen is bubbled over a similar cathode separated by a porous membrane. An electric current is produced in an external circuit. Like a battery, except that fuels, e.g. methanol, rather than metals are consumed, and the reaction is not reversible.
galvanize: Cover metal by electrodeposition of zinc.
group formula: laces atoms together in groups that correspond to the grouping in the actual molecule, e.g. aspirin, CH3CO.O.C6H4COOH.
hardness, calcium: A measure of the amount of dissolved calcium, magnesium, and iron compounds in water.
heavy metals: Higher atomic mass metals tend to form more poisonous compounds, e.g. Hg, Cd, Pb.
hydrophilic: Water loving, polar materials that mix with water. hydrophobic: Water hating, non-polar, often oily, materials that do not mix with water.
hygroscopic: Materials which absorb water from the air.
inhibit: Slow down a chemical reaction by blocking a part of the mechanism.
labile: Unstable, liable to change to another form or to move away
martensite: Solid solution of carbon in iron formed on rapid cooling, responsible for the hardness of quenched steel.
mole: Contains 6.023 x 1023 single units, e.g. atoms, molecules, electrons.
Molecular mass, formerly molecular weight, is the mass of one mole of that material.
napalm: Petrol gelled with the aluminium salts of naphthalenic and palmitic acids, used for warfare in flame throwers and napalm bombs.
non-miscible: Property of two liquids which do not mix, e.g. oil and water.
non-volatile: Not very volatile. The material has a very low, but not negligible volatility, in contrast to involatile materials such as granite, which have no vapour.
petroleum fraction: A fraction of oil selected in a refinery on the basis of boiling point.
photolysis: A chemical reaction brought about by light including ultra-violet light. Radiolysis is the equivalent when radioactive emissions are involved.
pK: A measure of the degree to which an acid or base will dissociate in water. The negative logarithm of the acid dissociation constant K, When the pH of solution is at the value of pKa for a dissolved acid, that acid will be 50% dissociated.
radical: A group of atoms that behaves like a single atom in a chemical reaction, e.g. the ammonium radical, NH4+. A free radical has an unpaired single electron, such as the methyl radical, CH3-.
sequester: To take out of circulation, to tie up metal ions so that they do not interfere, e.g. by precipitating soaps.
sequestering agent: A chemical that ties up metallic ions in solution.
solute: Dissolved material.
solvent: Dissolving material, usually liquid.
spontaneous: A process that has the potential to occur on its own without further input. However, it may occur so slowly it is unmeasurable.
substrate: A basis on which something else is placed, a starting material.
surfactant: A molecule attracted to the surface of water and capable of changing the properties of the surface, generally by lowering the surface tension.
synergism: Where two or more substances together produce an effect that is greater than the sum of the individual separate effects.
tempering: Time temperature treatment for modifying the mechanical properties of complex materials such as steel and chocolate.
varnish: A solution of a natural or synthetic resin in a solvent, sometimes with the addition of a drying oil.
waterglass: Colloidal solution of sodium silicate in water, used in chemical gardens, egg preserving, paper sizing, and much else.