Experiment: Action of heat on salts

Heating a salt can break it down into simpler substances, and the products depend on the salt. This experiment investigates the action of heat on carbonates and nitrates, testing the gases given off and observing the residue left behind. It is a clear, well-defined Form 4 practical that also connects to the reactivity series, and it is a natural Paper 3 context. This guide sets out the method, the observations to expect, and the science process skills the practical papers reward.

Aim

To investigate the effect of heat on salts such as metal carbonates and metal nitrates, by testing the gases given off and observing the change in the solid.

Apparatus and materials

• Test tube (a hard-glass ignition tube for strong heating)
• Bunsen burner
• Delivery tube and a test tube of limewater
• Wooden splint
• Test tube holder
• Spatula
• Samples of salts, for example copper(II) carbonate, and a nitrate such as lead(II) nitrate

Procedure

1. Put a spatula of the salt into a clean, dry hard-glass test tube.
2. Clamp or hold the tube with the test tube holder and heat the salt strongly with the Bunsen burner.
3. As the salt is heated, test any gas given off: pass it through limewater, and separately test it with a glowing splint held at the mouth of the tube.
4. Observe and record any colour change in the solid as it is heated and after it cools.
5. Note the colour and smell of any gas seen at the mouth of the tube, keeping your face to one side.
6. Repeat with a different salt, using a fresh tube each time, and record the observations side by side for comparison.

Expected observations

For a carbonate such as copper(II) carbonate, the green solid turns black on heating, and the gas given off turns limewater milky, showing carbon dioxide. For a nitrate such as lead(II) nitrate, the solid crackles as it is heated, a brown gas with a sharp smell is given off, a gas that relights a glowing splint is also produced, and the residue is a solid that may change colour on heating and cooling. Some very stable salts, such as sodium carbonate, show little or no change on heating. Recording the gas test result and the colour change together is what tells you how the salt has decomposed.

Inference and conclusion

The action of heat depends on the salt and on how stable it is. A metal carbonate decomposes on heating to give a metal oxide and carbon dioxide, which is why the gas turns limewater milky and the solid changes colour. A metal nitrate decomposes to give a metal oxide, a brown gas (nitrogen dioxide) and oxygen, which is why a glowing splint relights. Salts of very reactive metals are more stable to heat and decompose less easily or differently. The conclusion links each observation, the gas identified and the residue seen, to the decomposition that produced it.

Science process skills (Paper 3 style)

Identifying gases operationally. Define the tests operationally: carbon dioxide is the gas that turns limewater milky; oxygen is the gas that relights a glowing splint; nitrogen dioxide is the brown gas with a sharp smell.

Controlling variables. Use the same amount of salt and heat each sample in the same way, so that differences in observation are due to the salt and not to the heating.

Classifying. Classify the salts by what they give off on heating, carbonates giving carbon dioxide, nitrates giving oxygen and a brown gas, and relate this to the type of salt.

Inferring and communicating. From the gas test and the colour change, infer how the salt decomposed, and write a conclusion that names the products and quotes the observations that identify them.

Safety precautions

• Wear safety goggles, because heated salts can spit and the gases are harmful.
• Point the mouth of the test tube away from yourself and others while heating, so that no hot solid or gas is directed at anyone.
• Do not breathe in the brown gas from a nitrate; keep your face to one side and work in a well-ventilated area or fume cupboard, because nitrogen dioxide is toxic.
• Let the tube and residue cool before handling, or use a holder, to avoid burns.

Common errors

• Heating gently instead of strongly. Some salts need strong heating to decompose; a weak flame gives no clear change.
• Testing the gas too late. Test the gas as it comes off, because it is given off only while the salt is decomposing.
• Using a soft-glass tube. Ordinary glass can soften or crack under strong heating; use a hard-glass ignition tube.
• Confusing the gases. Use both tests, limewater and a glowing splint, and note the colour and smell, so carbon dioxide, oxygen and nitrogen dioxide are told apart.
• Vague recording. State the actual colour change of the solid and the result of each gas test, not just “it decomposed”.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we use the action of heat on salts to join two ideas, identifying gases and reading the residue, into a single, well-recorded observation. Because these gas tests and the pattern of carbonate and nitrate decomposition recur across SPM Chemistry, and because heating salts and testing the products is a natural Paper 3 practical (Paper 3 is a practical test assessing science process skills), the habit of testing carefully and writing exactly what is seen is where the marks are secured.