A classroom (with a sink) is easily converted into a temporary laboratory. Although we offer several experiments, the most popular ones are those designed to increase measurement, investigatory and cooperation skills and an increased awareness of chemical safety. These are described below.
Many secondary-school teachers may recognise the experiments, but the purpose of running them in primary schools is not to get the pupils to produce written records. They may do this later with their teachers, but the main point is to show the children how exciting and how much fun science can be.
Note: All chemical use should have a risk assessment made by the people conducting the experiment and must take into account local conditions. Primary teachers who do not have a background in chemistry may feel more comfortable obtaining the following solutions from colleagues in their local secondary school.
Investigating the properties of slime and polymorph
Slime is made by mixing borax and poly(vinyl alcohol) (PVA) solutions, then adding a few drops of food colouring. Changing the proportion of borax changes the physical properties of the slime. This (rather yucky) experiment is very popular with children.
Polymorph is a thermoplastic that can be moulded at the comparatively low temperature of 62°C.
For two classes of 35 pupils working in pairs, you will need 3 l of PVA solution and 2 l of borax solution.
To make 1 l of PVA solution:
- Take 40 g of poly(vinyl alcohol) and add water until the total volume is 1 l.
- Stir while heating from 40° to 90°C in water (allow 1 h).To speed up the process, cover the beaker in tin foil to keep the heat in.
To make 1 l of 4% borax solution:
- Take 40 g of borax and add water until the total volume is 1 l.
Note: Borax is also called sodium borate or sodium tetraborate, and is widely used in detergents, water softeners, soaps and disinfectants.
Polymorph can be bought from Middlesex University Teaching Resources, UK for about €30/kg. Allow 4-5 g per pupil (in individual tubes).
- Pour PVA solution to a depth of about 1 cm into a disposable plastic beaker (there is usually a convenient marking on the beakers we use: the PVA solution is too viscous to make sensible use of a measuring cylinder).
- Optional: add 3-5 drops of food colour and stir.
- Add a carefully measured volume of borax and stir it in.
Initially, tell each pair of pupils to use a fixed volume of borax – between 4 and 10 ml.
- Wearing gloves, scoop out the slime and investigate its tactile properties by stretching it.
Introduce the pupils to the thermoplastic polymorph. This comes as colourless granules which, when put into hot water (over 62°C), can be moulded into interesting shapes such as fish, birds or – for the less imaginative – balls. The balls made from this material bounce differently depending on whether the polymer is still warm or has reformed its bonds in the coloured form.
The iodine clock experiment
With this experiment, the children investigate dilution and learn how to measure volume accurately to make the chemical reaction change colour after exactly 30 seconds. We run this activity as a competition, which adds to the fun.
The experiment uses two solutions referred to simply as solution A and solution B (see below). We do not confuse the issue with the names of the chemicals used. Students have three small beakers half-filled with solution A, solution B and water (W) and appropriately marked measuring cylinders to avoid contamination.
For two classes of 35 pupils, you will need 4 l of each solution.
To make 4 l of solution A, mix the following chemicals and add water until the total volume is 4 l:
- 0.2 g soluble starch
- 30 ml ethanoic acid (glacial acetic acid)
- 4.1 g sodium ethanoate (acetate)
- 50 g potassium iodide
- 9.4 g sodium thiosulphate
To make 4 l of solution B, take 200 ml (30%, also described as 100 Vol) hydrogen peroxide and add water until the total volume is 4 l.
Make sure that each pair of students has the correct equipment and can operate a stopwatch, then demonstrate the reaction without dilution. Mix 15 ml of solution A and solution B together. The mixture will turn black after several seconds.
Discuss the effect of adding water to the mixture. Use terms such as ‘less concentrated’ and ‘more dilute’ in the explanation, to give the pupils a mental model of what is happening. Give all instructions verbally to maximise the time spent doing the practical.
Set the children the challenge of making the mixture turn black at a set time – set a target that is between 30 and 60 seconds. Give each pair of children a different time to achieve.
Although the experiment is not strictly a fair test, as both the volume and concentration of the mixture changes, it does excite the pupils as well as giving them practice in measuring, investigating and how to work in a team. If necessary, you could offer a reward to the best pair.
It can be made more complex for secondary-school students, for example by insisting that solution B is diluted such that the same volume of solution B is used each time – which is of course more scientifically appropriate.
After the experiment, carefully pour the iodine solution that has been produced into a sink and wash it away. Wipe and reuse the beakers. If you do not have a sink, use a waste bucket (containing sodium thiosulphate crystals to react with the iodine) and dispose of the solution in a toilet at intervals.
The pupils enjoy investigating the effect of diluting an acid on its reaction with magnesium, especially because they get to collect the hydrogen and set it alight (with a loud pop) at the end.
- Magnesium ribbon cut into 2 cm strips. Each student will need 5 pieces.
- Approximately 2 l of each of four concentrations of hydrochloric acid (HCl), see below.
Table 1: Hydrochloric acid concentrations
|Approximate concentration (molar, M)
||Volume of 11 M hydrochloric acid (ml)
||Volume of water (l)
- Measure 10 ml of one of the acid solutions and put it into a boiling tube.
- Add a piece of magnesium, start a stopwatch and stop it when the fizzing stops.
- Tabulate their results and convert the times into seconds.
- Towards the end of the session, use the fifth pieces of magnesium and trap the liberated hydrogen gas in a second boiling tube, then use a lit wooden spill to ignite the gas safely.
- Compare the results of their experiment with the explosion of the hydrogen balloons in the school hall.
When we do the experiment, the postgraduate scientist shows the pupils how to ignite the gas and supervises them carefully while they do it themselves.