Conservation of Mass KS3 Worksheet - Science Resource - Twinkl - Free Printable
Educational worksheet: Conservation of Mass KS3 Worksheet - Science Resource - Twinkl. Download and print for classroom or home learning activities.
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Step-by-step solution for: Conservation of Mass KS3 Worksheet - Science Resource - Twinkl
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Show Answer Key & Explanations
Step-by-step solution for: Conservation of Mass KS3 Worksheet - Science Resource - Twinkl
The task is to complete a table that explains why it doesn’t always *appear* that the mass of reactants equals the mass of products — even though the Law of Conservation of Mass says they should be equal.
We’ll go row by row, thinking about what happens in each experiment and whether any mass seems to “disappear” or “appear” — and why that’s actually an illusion (because mass is conserved, but sometimes gases escape or enter).
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Row 1: 10g of magnesium ribbon is burnt in oxygen
- What happens? Magnesium burns brightly, turns into white ash (magnesium oxide). It looks like the ash weighs less than the original ribbon.
- Why? Because when you burn magnesium, it reacts with oxygen from the air. The product (magnesium oxide) should weigh MORE than the original magnesium — because you’re adding oxygen atoms! But if you only weigh the solid before and after, you might think mass decreased — because you didn’t account for the oxygen gas that combined with it.
- Explanation: The mass of the products includes the oxygen from the air. If you don’t trap the gas, it seems like mass changed — but it didn’t. Total mass is conserved.
→ So:
What happens to the mass? → Seems to increase (if you include oxygen), or seems unchanged if you measure carefully. But often students think it decreases — which is wrong. Actually, the solid product weighs more than the original magnesium.
Explanation: Oxygen from the air combines with magnesium to form magnesium oxide. The total mass of magnesium + oxygen = mass of magnesium oxide.
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Row 2: 25g of hydrochloric acid has 3g of powdered marble added to it, a gas escapes
- Marble is calcium carbonate. When it reacts with HCl, it makes carbon dioxide gas, water, and calcium chloride.
- The gas escapes into the air — so if you weigh the container before and after, the mass will seem to decrease.
- But the gas still has mass! It just left the system.
- Explanation: The missing mass is the CO₂ gas that escaped. If you trapped the gas, the total mass would stay the same.
→ So:
What happens to the mass? → Seems to decrease (because gas escapes).
Explanation: The gas produced (carbon dioxide) leaves the container, so the remaining stuff weighs less — but the total mass including the gas is unchanged.
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Row 3: 25g of water has 5g of salt added to it and is stirred until all the salt dissolves
- Salt dissolves — no chemical reaction, just mixing.
- No gas escapes, nothing leaves.
- Total mass = 25g + 5g = 30g.
- Mass doesn’t change at all.
→ So:
What happens to the mass? → Stays the same.
Explanation: Dissolving is physical, not chemical. All matter stays in the container. Mass is conserved.
---
Row 4: A can of lemonade is opened and left for 4 hours
- Lemonade has dissolved carbon dioxide (fizz). When you open it, gas escapes slowly over time.
- After 4 hours, some gas has left — so the can + liquid weighs less.
- Again, the gas had mass — it just floated away.
→ So:
What happens to the mass? → Decreases (gas escapes).
Explanation: Carbon dioxide gas escapes from the drink into the air. The lost mass is the gas — total mass is still conserved if you count the air around it.
---
Now let’s write this clearly in table format as requested.
Final Answer:
| Experiment | What happens to the mass? | Explanation |
|-------------------------------------------------|-----------------------------------------------|-----------------------------------------------------------------------------|
| 10g of magnesium ribbon is burnt in oxygen | Mass seems to increase (or stay same if measured right) | Oxygen from air combines with magnesium to make magnesium oxide — total mass conserved. |
| 25g hydrochloric acid + 3g marble, gas escapes | Mass decreases | Gas (CO₂) escapes — its mass is lost from the container, but total mass is still conserved. |
| 25g water + 5g salt, stirred until dissolved | Mass stays the same | Salt just dissolves — no gas escapes, no new substances formed. Mass conserved. |
| Can of lemonade opened, left 4 hours | Mass decreases | CO₂ gas escapes from the drink — mass is lost from the can, but total mass is conserved. |
We’ll go row by row, thinking about what happens in each experiment and whether any mass seems to “disappear” or “appear” — and why that’s actually an illusion (because mass is conserved, but sometimes gases escape or enter).
---
Row 1: 10g of magnesium ribbon is burnt in oxygen
- What happens? Magnesium burns brightly, turns into white ash (magnesium oxide). It looks like the ash weighs less than the original ribbon.
- Why? Because when you burn magnesium, it reacts with oxygen from the air. The product (magnesium oxide) should weigh MORE than the original magnesium — because you’re adding oxygen atoms! But if you only weigh the solid before and after, you might think mass decreased — because you didn’t account for the oxygen gas that combined with it.
- Explanation: The mass of the products includes the oxygen from the air. If you don’t trap the gas, it seems like mass changed — but it didn’t. Total mass is conserved.
→ So:
What happens to the mass? → Seems to increase (if you include oxygen), or seems unchanged if you measure carefully. But often students think it decreases — which is wrong. Actually, the solid product weighs more than the original magnesium.
Explanation: Oxygen from the air combines with magnesium to form magnesium oxide. The total mass of magnesium + oxygen = mass of magnesium oxide.
---
Row 2: 25g of hydrochloric acid has 3g of powdered marble added to it, a gas escapes
- Marble is calcium carbonate. When it reacts with HCl, it makes carbon dioxide gas, water, and calcium chloride.
- The gas escapes into the air — so if you weigh the container before and after, the mass will seem to decrease.
- But the gas still has mass! It just left the system.
- Explanation: The missing mass is the CO₂ gas that escaped. If you trapped the gas, the total mass would stay the same.
→ So:
What happens to the mass? → Seems to decrease (because gas escapes).
Explanation: The gas produced (carbon dioxide) leaves the container, so the remaining stuff weighs less — but the total mass including the gas is unchanged.
---
Row 3: 25g of water has 5g of salt added to it and is stirred until all the salt dissolves
- Salt dissolves — no chemical reaction, just mixing.
- No gas escapes, nothing leaves.
- Total mass = 25g + 5g = 30g.
- Mass doesn’t change at all.
→ So:
What happens to the mass? → Stays the same.
Explanation: Dissolving is physical, not chemical. All matter stays in the container. Mass is conserved.
---
Row 4: A can of lemonade is opened and left for 4 hours
- Lemonade has dissolved carbon dioxide (fizz). When you open it, gas escapes slowly over time.
- After 4 hours, some gas has left — so the can + liquid weighs less.
- Again, the gas had mass — it just floated away.
→ So:
What happens to the mass? → Decreases (gas escapes).
Explanation: Carbon dioxide gas escapes from the drink into the air. The lost mass is the gas — total mass is still conserved if you count the air around it.
---
Now let’s write this clearly in table format as requested.
Final Answer:
| Experiment | What happens to the mass? | Explanation |
|-------------------------------------------------|-----------------------------------------------|-----------------------------------------------------------------------------|
| 10g of magnesium ribbon is burnt in oxygen | Mass seems to increase (or stay same if measured right) | Oxygen from air combines with magnesium to make magnesium oxide — total mass conserved. |
| 25g hydrochloric acid + 3g marble, gas escapes | Mass decreases | Gas (CO₂) escapes — its mass is lost from the container, but total mass is still conserved. |
| 25g water + 5g salt, stirred until dissolved | Mass stays the same | Salt just dissolves — no gas escapes, no new substances formed. Mass conserved. |
| Can of lemonade opened, left 4 hours | Mass decreases | CO₂ gas escapes from the drink — mass is lost from the can, but total mass is conserved. |
Parent Tip: Review the logic above to help your child master the concept of law of conservation of mass worksheet.
