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.
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)