Grade: 10
Subject: Physical Sciences
Term: 2
CAPS Type: Informal Experiment
Topic: Chemical Change, Conservation of Matter, Conservation of Mass, Reactants and Products
Aim
To observe and prove the law of conservation of matter during chemical reactions.
Learning Outcome
- Explain the law of conservation of matter.
- Measure mass before and after a reaction.
- Explain why matter is conserved during physical and chemical changes.
- Understand the importance of a closed system.
- Observe evidence of chemical reactions.
- Compare the total mass of reactants and products.
Time Allocation
Approximately 45 minutes per experiment option.
How Does It Work?
The law of conservation of matter states that matter cannot be created or destroyed during a physical change or chemical reaction.
When water changes into steam, the water particles move further apart and the volume increases, but the water molecules still exist. The total mass remains the same.
During a chemical reaction, atoms are rearranged from the reactants to form new products. The products may have different properties from the reactants, but the atoms are not destroyed or created.
Mass of reactants = Mass of products
If a gas forms during a reaction and escapes into the air, it may seem as if mass has been lost. This is why a closed system is important.
A closed system prevents gas, liquid or solid material from escaping.
Open System
Gas or matter can escape, causing the measured mass to appear different.
Closed System
No matter can escape, so the total mass remains the same.
The French chemist Antoine Lavoisier, known as the father of modern chemistry, helped establish the law of conservation of mass by showing that reactions in closed containers do not lose or gain mass.
Scientific Background
Chemical reactions change substances into new substances. The starting substances are called reactants, and the substances formed are called products.
Although the properties of the products may differ from the reactants, the total number of atoms remains the same.
Chemical equations are balanced because matter is conserved. The same number of each type of atom must appear on both sides of the equation.
A reaction may appear to gain or lose mass if a gas is involved.
For example, rusting increases the mass of a metal object because oxygen from the air combines with the metal. The oxygen is invisible, but it still has mass.
This experiment helps learners understand that gases have mass, even when they cannot be seen.
Hypothesis
If a chemical reaction takes place in a closed system, then the total mass before and after the reaction will remain the same.
Variables
Independent Variable
The stage of the reaction, before and after mixing the reactants.
Dependent Variable
Total mass of the reaction system.
Control Variables
- Same container.
- Same balance.
- Same quantities of reactants.
- Same closed system.
- No gas, liquid or solid allowed to escape.
Experiment 1: Cal-C-Vita and Water
Aim
To observe the law of conservation of matter in the reaction between a Cal-C-Vita tablet and water.
Apparatus
- Cal-C-Vita tablet
- Water
- 2 × clear plastic bags
- Elastic band
- 250 ml measuring cylinder
- Electronic balance
Safety Precautions
- Do not taste any substances used in the experiment.
- Keep the work area dry.
- Make sure the plastic bags are properly sealed.
- Clean up spills immediately.
Method
- Fold one plastic bag so that it can fit inside the second bag and stay upright.
- Place the Cal-C-Vita tablet inside the inner bag.
- Use a measuring cylinder to measure 200 ml of water.
- Pour the water into the outer bag.
- Place the inner bag containing the tablet inside the outer bag, but do not allow the tablet to touch the water yet.
- Push excess air out of the bags.
- Seal the outer bag tightly with an elastic band.
- Measure and record the combined mass of the bags and their contents.
- Turn the bags so that the tablet comes into contact with the water.
- Observe the reaction.
- When the reaction is complete, measure the combined mass again.
- Record the final mass.
Results Table
| Stage | Mass (g) | Observations |
|---|---|---|
| Before reaction | ||
| After reaction |
What Learners Should Observe
- Bubbles forming.
- Gas production.
- Swelling of the bag.
- Little or no change in total mass if the bag is properly sealed.
Expected Results
The total mass before and after the reaction should remain approximately the same because the system is closed and no matter escapes.
Conclusion
Matter is conserved during the reaction. Although gas is produced, the total mass remains approximately the same because the system is sealed.
Experiment 2: Potassium Iodide and Lead(II) Nitrate
Aim
To observe the law of conservation of matter in the reaction between potassium iodide solution and lead(II) nitrate solution.
Apparatus
- Electronic balance
- Distilled water
- 2 × 1000 ml volumetric flasks
- Spatula
- 250 ml Erlenmeyer flask with rubber stopper
- Small test tube
- Measuring cylinder
Chemicals
- Potassium iodide, KI(s)
- Lead(II) nitrate, Pb(NO₃)₂(s)
- Distilled water
Safety Precautions
- Wear safety goggles and protective clothing.
- Lead(II) nitrate is toxic and must be handled carefully.
- Avoid skin contact with lead-containing solutions.
- Do not inhale or ingest any chemicals.
- Collect lead-containing waste separately according to laboratory safety procedures.
- Do not dispose of lead solutions down a normal drain unless approved by school laboratory policy.
- Wash hands after the experiment.
Method
- Weigh 3.3 g of potassium iodide and transfer it into a 1000 ml volumetric flask.
- Dissolve the potassium iodide in distilled water and fill to the 1000 ml mark.
- Weigh 3.3 g of lead(II) nitrate and transfer it into a second 1000 ml volumetric flask.
- Dissolve the lead(II) nitrate in distilled water and fill to the 1000 ml mark.
- Measure 15 ml of potassium iodide solution and pour it into the Erlenmeyer flask.
- Measure 5 ml of lead(II) nitrate solution and pour it into a small test tube.
- Place the test tube carefully inside the Erlenmeyer flask without mixing the solutions.
- Seal the flask tightly with a rubber stopper.
- Measure and record the mass of the sealed flask and its contents.
- Turn the flask upside down to allow the two solutions to mix.
- Observe the reaction.
- Measure the mass of the sealed flask again after the reaction.
- Record the final mass.
Chemical Equation
2KI(aq) + Pb(NO₃)₂(aq) → PbI₂(s) + 2KNO₃(aq)
Results Table
| Stage | Mass (g) | Observations |
|---|---|---|
| Before reaction | ||
| After reaction |
What Learners Should Observe
- A yellow precipitate forming.
- Evidence of a chemical reaction.
- Little or no change in total mass if the flask remains sealed.
Expected Results
The total mass before and after the reaction should remain approximately the same because no matter escapes from the sealed flask.
Conclusion
The formation of a yellow precipitate shows that a chemical reaction occurred. The total mass remains approximately the same because matter is conserved in the closed system.
Experiment 3: Hydrochloric Acid and Sodium Hydroxide
Aim
To observe the law of conservation of matter in the reaction between hydrochloric acid solution and sodium hydroxide solution.
Apparatus
- 2 × 50 ml beakers
- Measuring cylinder
- Electronic balance
Chemicals
- 0.1 mol.dm⁻³ sodium hydroxide solution, NaOH(aq)
- 0.1 mol.dm⁻³ hydrochloric acid solution, HCl(aq)
- Bromothymol blue indicator
Safety Precautions
- Wear safety goggles and protective clothing.
- Sodium hydroxide is corrosive and must be handled carefully.
- Hydrochloric acid must be handled carefully.
- Avoid skin and eye contact.
- Clean up spills immediately.
- Wash hands after the experiment.
- Do not taste or inhale chemicals.
Method
- Measure and record the mass of two clean 50 ml beakers.
- Measure 15 ml of hydrochloric acid solution using a measuring cylinder.
- Pour the hydrochloric acid into the first beaker.
- Measure the mass of the beaker and hydrochloric acid.
- Calculate the mass of the hydrochloric acid solution.
- Measure 15 ml of sodium hydroxide solution.
- Pour the sodium hydroxide solution into the second beaker.
- Add a few drops of bromothymol blue indicator.
- Measure the mass of the beaker and sodium hydroxide solution.
- Calculate the mass of the sodium hydroxide solution.
- Add the hydrochloric acid solution to the sodium hydroxide solution.
- Observe the colour change.
- Measure the mass of the beaker and products.
- Calculate and compare the total mass before and after the reaction.
Chemical Equation
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Results Table
| Measurement | Mass (g) |
|---|---|
| Mass of empty beaker 1 | |
| Mass of beaker 1 + HCl | |
| Mass of HCl solution | |
| Mass of empty beaker 2 | |
| Mass of beaker 2 + NaOH | |
| Mass of NaOH solution | |
| Total mass before reaction | |
| Mass of beaker + product | |
| Mass of product |
What Learners Should Observe
- A colour change due to the indicator.
- Neutralisation between acid and base.
- The calculated mass of products should be close to the total mass of reactants.
Expected Results
The total mass before and after the reaction should remain approximately the same if no liquid is lost during mixing.
Conclusion
The acid-base reaction supports the law of conservation of matter. The total mass before and after the reaction should remain approximately the same when measurements are taken carefully.
Questions for Learners
- What is the law of conservation of matter?
- Why must the system be closed in some reactions?
- What is the difference between reactants and products?
- Why does the total mass remain the same during a chemical reaction?
- Why do gases still have mass even though they are invisible?
- What evidence showed that a reaction occurred in each experiment?
- Why are chemical equations balanced?
- Why might the measured mass be slightly different after the reaction?
- Which experiment gave the clearest visual evidence of a chemical reaction?
- Which experiment best demonstrated a closed system?
Common Mistakes
- Allowing gas to escape before weighing.
- Not sealing the bag or flask properly.
- Spilling liquid while mixing reactants.
- Measuring only part of the system instead of the entire system.
- Reading the balance before it stabilises.
- Forgetting to subtract the mass of empty containers.
- Using incorrect chemical formulae, for example writing HC instead of HCl.
Teacher Notes
- The most reliable proof of conservation of matter uses a closed system.
- The Cal-C-Vita experiment is safer and easier for younger learners.
- The potassium iodide and lead(II) nitrate experiment gives a strong visual result because of the yellow precipitate.
- The hydrochloric acid and sodium hydroxide experiment is useful for calculations, but learners must avoid spills.
- Reinforce that gases have mass, even when they cannot be seen.
- Link conservation of matter to balanced chemical equations.
Teacher Tip
Use the Cal-C-Vita experiment first to show gas formation in a closed system. Then use the potassium iodide and lead(II) nitrate reaction to show that visible product formation also follows the law of conservation of matter.
Extension Activity
Ask learners to compare an open system with a closed system.
They can perform the same gas-producing reaction twice:
- once in an open beaker,
- and once in a sealed bag or flask.
Learners should explain why the open system appears to lose mass while the closed system does not.
Real-World Application
The law of conservation of matter is important in:
- chemical manufacturing,
- laboratory analysis,
- environmental science,
- food chemistry,
- combustion reactions,
- rusting and corrosion,
- and industrial process control.
For example, when a metal rusts, its mass can increase because oxygen from the air combines with the metal. The oxygen is invisible, but it still has mass.