1:03 understand how the results of experiments involving the dilution of coloured solutions and diffusion of gases can be explained
Diffusion is the spreading out of particles in a gas or liquid. There is a net movement of particles from areas of high concentration to areas of low concentration until a uniform concentration is achieved.
i) dilution of coloured solutions
Dissolving potassium manganate(VII) in water demonstrates that the diffusion in liquids is very slow because there are only small gaps between the liquid particles into which other particles diffuse.
The random motion of particles cause the purple colour to eventually be evenly spread out throughout the water.
Adding more water to the solution causes the potassium manganate(VII) particles to spread out further apart therefore the solutions becomes less purple. This is called dilution.
ii) diffusion experiments
When ammonia gas and hydrogen chloride gas mix, they react together to form a white solid called ammonium chloride.
ammonia + hydrogen chloride –> ammonium chloride
NH3(g) + HCl(g) –> NH4Cl(s)
A cotton wool pad was soaked in ammonia solution and another was soaked in hydrogen chloride solution. The two pads were then put into opposite ends of a dry glass tube at the same time.
The white ring of ammonium chloride forms closer to the hydrochloric acid end because ammonia particles are lighter than hydrogen chloride particles and therefore travel faster.
Even though these particles travel at several hundred metres per second, it takes about 5 min for the ring to form. This is because the particles move in random directions and will collide with air particles in the tube.
1:10 describe these experimental techniques for the separation of mixtures: simple distillation, fractional distillation, filtration, crystallisation, paper chromatography
This method is used to separate a liquid from a solution. For example: separating water from salt water.
The salt water is boiled. The water vapour condenses back into a liquid when passed through the condenser. The salt is left behind in the flask.
Note: cold water is passed into the bottom of the condenser and out through the top so that the condenser completely fills up with water.
This method is used to separate a mixture of different liquids that have different boiling points. For example, separating alcohol from a mixture of alcohol and water.
Water boils at 100oC and alcohol boils at 78oC. By using the thermometer to carefully control of temperature of the column, keeping it at 78oC, only the alcohol remains as vapour all the way up to the top of the column and passes into the condenser.
The alcohol vapours then condense back into a liquid.
This method is used to separate an insoluble solid from a liquid. For example: separating sand from a mixture of sand and water.
The mixture is poured into the filter paper. The sand does not pass through and is left behind (residue) but the water passes through the filter paper and is collected in the conical flask (filtrate).
This method is used to obtain a salt which contains water of crystallisation from a salt solution. For example: hydrated copper sulfate crystals (CuSO4.5H2O(s)) from copper sulfate solution (CuSO4(aq)).
Gently heat the copper sulfate solution in an evaporating basin until a hot saturated solution forms. Leave in a warm place to allow the hydrated copper sulfate crystals to form. Remove the crystals by filtration and wash with distilled water. Dry by leaving in a warm place.
If instead the solution is heated until all the water evaporates, you would produce a powder of anhydrous copper sulfate (CuSO4(s)).
This method can be used to separate the parts of a mixture into their components. For example, the different dyes in ink can all be separated and identified.
The coloured mixture to be separated (e.g. a food dye) is dissolved in a solvent like water or ethanol and carefully spotted onto the chromatography paper on the baseline, which is drawn in pencil so it doesn’t ‘run or smudge’.
The paper is carefully dipped into the solvent and suspended so the baseline is above the liquid solvent, otherwise all the spots would dissolve in the solvent. The solvent is absorbed into the paper and rises up it as it soaks into the paper. The choice of solvent depends on the solubility of the dye. If the dye does not dissolve in water then normally an organic solvent (e.g. ethanol) is used.
As the solvent rises up the paper it will carry the dyes with it. Each different dye will move up the paper at different rates depending on how strongly they stick to the paper and how soluble they are in the solvent.
Air is a mixture of different gases.
The abundance of gases in the air is as follows:
2:14 Practical: determine the approximate percentage by volume of oxygen in air using a metal or a non-metal
The following 3 experiments can be used to determine that oxygen (O2) makes up approximately 20% by volume of air.
The copper is in excess and uses up the oxygen to form copper oxide (CuO).
All the oxygen in the air is therefore used up, and so the volume of the air decreases by about 20% (the percentage of oxygen in air).
The iron reacts with the oxygen in the air (rusting).
As long as the iron, oxygen and water are all in excess, the total volume of air enclosed by the apparatus decreases by about a fifth (20%) over several days.
The phosphorus is lit with a hot wire.
It reacts with the oxygen in the air and causes the water level in the bell jar to rise by about 20%.
2:42 practical: prepare a sample of pure, dry hydrated copper(II) sulfate crystals starting from copper(II) oxide
Excess Solid Method:
Preparing pure dry crystals of copper sulfate (CuSO4) from copper oxide (CuO) and sulfuric acid (H2SO4)