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KS3 Science · Year 7Chapter 10 · Conservation of mass
Year 7 · Chapter 10

Conservation of mass

By the end of this chapter you will be able to

  • State the law of conservation of mass in a chemical reaction
  • Explain why the mass of a sealed system stays the same, and why an open system can seem to lose or gain mass
  • Explain why magnesium gains mass when it burns in air
  • Plan a fair test of mass in a sealed and an open system
  • Use conservation of mass to find a missing mass

You should already know

  • Chemical reactions and word equations (Chapter 9 of this book)
  • Atoms and mass (Chapter 6 of this book)

Where this leads

  • Atoms, elements and compounds in later GCSE chemistry, including balanced equations

National Curriculum for England, Key Stage 3 science, Year 7. Content: C8 mass is conserved in a chemical reaction, because atoms are rearranged and not created or destroyed, C8 explaining changes in mass when a gas is made or escapes, and measuring mass in closed and open systems.

Exercise P  Do you remember?

1
FWhat do we call a substance made of only one type of atom? Write the word.
2
FWhat is 100 minus 45? Write the number.
3
FWhat is the symbol for carbon? Write the symbol.
4
FWhich gas turns limewater milky? Write the name.
5
FWhich change turns a liquid into a gas? Write the word.
10.1

The law of conservation of mass

Year 7
C8: mass is conserved in a chemical reaction, because the atoms are rearranged and not created or destroyed
Video lesson: The law of conservation of massComing soon
Story. Ali says, “When a reaction happens, some of the mass just disappears.” His teacher asks him to weigh a sealed bag before and after a reaction. The masses are the same. “So the mass was there all along,” says Ali. “Yes,” says his teacher, “it was only rearranged.”

In a chemical reaction, the atoms of the reactants are rearranged to make the products. No atoms are created or destroyed.

So, in a closed system where nothing can enter or leave, the total mass of the products is equal to the total mass of the reactants. This is the law of conservation of mass.

total mass of reactants = total mass of products (in a closed system)

Key ideas

  • Atoms are rearranged in a chemical reaction; they are not created or destroyed.
  • In a closed system, the total mass before equals the total mass after.
  • Conservation of mass is a law that applies to every chemical reaction.
  • If the mass seems to change, look for a gas that has escaped or joined the reaction.
Worked example 10.1.1
In a closed system, the reactants have a total mass of 30 g. What is the total mass of the products? Write the number of grams.

Method. Use conservation of mass.

  1. Read the reactants. The reactants have a total mass of 30 g.
    30 g
  2. Apply the law. In a closed system, the products have the same total mass.
    same total mass
  3. Write the answer.
    30 g

Check. Mass is conserved in a closed system. ✓

Worked example 10.1.2
Two reactants have masses of 12 g and 8 g. What is the total mass of the reactants? Write the number of grams.

Method. Add the reactant masses.

  1. Add. The total is 12 + 8.
    12 + 8 = 20
  2. Apply the law. So the products in a closed system also weigh 20 g.
    20 g
  3. Write the answer.
    20 g

Check. The products also weigh 20 g. ✓

Worked example 10.1.3
Does mass disappear in a closed chemical reaction? Write yes or no.

Method. Use the law.

  1. Use the law. Atoms are rearranged, not destroyed, so the mass stays the same.
    rearranged
  2. Decide. So mass does not disappear, and the answer is no.
    no
  3. Write the answer.
    no

Check. Mass is conserved. ✓

Watch out

Watch out: conservation of mass is about the total mass. The mass of one substance can change while the total does not.

Exam tip

Exam tip. For a conservation question, say the total mass of the reactants equals the total mass of the products, in a closed system.

Try it (with your teacher, or a grown-up at home). With a grown-up, put a little dry sand in a sealed plastic bag and weigh it on kitchen scales. Then weigh the sealed bag again with no changes. Say whether the mass was the same, and why.

Exercise 10A  The law of conservation of mass

1
FWhat is the name of the law that says mass is conserved in a reaction? Write the words.
2
FIn a reaction, are atoms created or destroyed? Write yes or no.
3
FIn a closed system, is the total mass of the products equal to the total mass of the reactants? Write yes or no.
4
FWhat is the total mass of a set of reactants, if it is 75 g? Write the number of grams for the products in a closed system.
5
FWhat is conserved in a chemical reaction? Write one word.
6
FBen says mass disappears when a reaction happens in a closed flask. Is Ben right? Write yes or no.
7
PSA reaction has reactants of 30 g and 20 g in a closed system. What is the total mass of the products? Write the number of grams.
8
PSIn a closed system, the products weigh 80 g. What is the total mass of the reactants? Write the number of grams.
9
PSThree reactants weigh 5 g, 7 g and 8 g in a closed system. What is the total mass of the products? Write the number of grams.
10
PSAli says that because a gas is made in a closed flask, the total mass of the products must be more. Is Ali right? Write yes or no.
11
PSWorking scientifically: name one thing a pupil must keep the same in a test of conservation of mass. Accept any one: the same sealed bag, the same reactants, the same scales.
12
RExplain why mass is conserved in a chemical reaction. Write one sentence.
13
RChallenge: explain why the law of conservation of mass only applies to a closed system, not an open one. Write one sentence.
10.2

Closed and open systems

Year 7
C8: measuring mass in a closed system and in an open system, and explaining a change in mass when a gas is made or escapes
Video lesson: Closed and open systemsComing soon
Story. Priya puts marble chips and acid in an open flask on the balance. The mass goes down. “The law says mass is conserved, so what happened?” she asks. Her teacher says, “Look at the flask. A gas was made, and it escaped into the air. That gas still has mass, but we did not weigh it.”

A closed system is one where no substances can enter or leave, for example a sealed bag. An open system is one where a gas can escape into the air, or a gas from the air can join the reaction.

When a gas is made in an open system and escapes, the mass measured falls. The law still holds; the escaped gas has just not been weighed.

open: gas can escapegas escapessealed: nothing escapesballoon
An open flask (gas can escape) and a sealed flask with a balloon (nothing escapes).

The table gives the masses before and after three experiments, in grams.

ExperimentMass before (g)Mass after (g)What happened
marble chips and acid, open flask120116the gas escaped, so the mass fell
marble chips and acid, sealed bag120120nothing escaped, so the mass stayed the same
magnesium burnt in air, open crucible610oxygen from the air joined, so the mass rose

Key ideas

  • A closed system keeps all its substances, so its mass is conserved.
  • An open system can lose mass if a gas escapes.
  • An open system can gain mass if a gas from the air joins the reaction.
  • The mass of the escaped gas is the mass lost from the flask.
Worked example 10.2.1
In the open flask, what was the mass before the reaction? Write the number of grams.

Method. Read the table.

  1. Read the table. The open flask had a mass of 120 g before the reaction.
    120 g
  2. Write the answer. So the mass before was 120 g.
    120
  3. Write the answer.
    120 g

Check. The mass after was 116 g. ✓

Worked example 10.2.2
How much mass was lost from the open flask? Write the number of grams.

Method. Subtract the mass after from the mass before.

  1. Read the masses. The mass before was 120 g and the mass after was 116 g.
    120 and 116
  2. Subtract. The mass lost is 120 minus 116.
    120 - 116 = 4
  3. Write the answer.
    4 g

Check. 116 + 4 = 120. ✓

Worked example 10.2.3
In the sealed bag, did the mass change? Write yes or no.

Method. Compare the masses.

  1. Compare. The sealed bag weighed 120 g before, and 120 g after.
    120 and 120
  2. Decide. So the mass did not change, and the answer is no.
    no
  3. Write the answer.
    no

Check. Nothing escaped from the sealed bag. ✓

Watch out

Watch out: a fall in mass in an open flask does not break conservation of mass. Ask what escaped; the gas still has mass, but it has left the flask.

Exam tip

Exam tip. When a mass falls in an open system, name the gas that escaped, and say that it was not weighed.

Try it (with your teacher, or a grown-up at home). With a grown-up, put a spoonful of bicarbonate of soda and a little vinegar into a plastic cup, and weigh the cup before and after it fizzes. Then do the same with the cup covered by a balloon, or in a sealed bag. Say which set-up showed the mass change.

Exercise 10B  Closed and open systems

1
FWhat is the name of a system where nothing can enter or leave? Write the two words.
2
FIs a flask with its lid off an open system? Write yes or no.
3
FIn the open flask, what gas escaped? Write the name.
4
FWhat was the mass of the sealed bag after the reaction? Write the number of grams.
5
FIn the sealed bag, does the gas stay in the bag? Write yes or no.
6
PSWhat is the change in mass in the open flask? Write the number of grams, with a minus sign.
7
PSWhat is the change in mass in the sealed bag? Write the number of grams.
8
PSA drink in a flask weighs 250 g. After the cap is taken off, it weighs 246 g. How many grams of gas escaped? Write the number.
9
PSAli says the open flask broke the law of conservation of mass. Is Ali right? Write yes or no.
10
PSWorking scientifically: suggest one change to the open flask that would make it a closed system. Accept any one: put a balloon on the top, or seal the flask with a stopper.
11
RExplain why the mass of the open flask fell in the experiment. Write one sentence.
12
RChallenge: explain why a sealed bag is used in conservation of mass tests. Write one sentence.
10.3

Mass gained from the air

Year 7
C8: when a substance reacts with oxygen from the air, the mass of the product can be greater than the mass of the starting material
Video lesson: Mass gained from the airComing soon
Story. Omar burns a strip of magnesium in an open crucible, as his teacher demonstrates. “The white powder weighs more than the metal,” he says. “Where did the extra mass come from?” His teacher says, “From the oxygen in the air. It joined the magnesium.”

When magnesium burns in air, it reacts with oxygen from the air to make magnesium oxide. The oxygen is a reactant, and it has mass. Because the oxygen joins the reaction, the mass of the product is greater than the mass of the magnesium.

In the experiment, 6 g of magnesium makes 10 g of magnesium oxide. So 10 g minus 6 g, which is 4 g, must be the mass of oxygen that joined.

Key ideas

  • Oxygen from the air is a reactant when a metal burns in air.
  • The product can weigh more than the starting metal, because of the oxygen added.
  • In an open crucible, the oxygen from the air can join the reaction.
  • The total mass is still conserved when the air is counted as part of the system.
Worked example 10.3.1
6 g of magnesium makes 10 g of magnesium oxide. How many grams of oxygen joined the magnesium? Write the number.

Method. Subtract the magnesium from the magnesium oxide.

  1. Read the masses. The magnesium was 6 g and the magnesium oxide is 10 g.
    6 and 10
  2. Subtract. The oxygen that joined is 10 minus 6.
    10 - 6 = 4
  3. Write the answer.
    4 g

Check. 6 + 4 = 10. ✓

Worked example 10.3.2
Does the mass go up or down when magnesium burns in an open crucible? Write the word.

Method. Compare the masses.

  1. Compare. The magnesium oxide weighs 10 g and the magnesium weighed 6 g.
    more
  2. Decide. So the mass goes up.
    up
  3. Write the answer.
    up

Check. Oxygen from the air joins the reaction. ✓

Worked example 10.3.3
Is oxygen a reactant when magnesium burns in air? Write yes or no.

Method. Use the reaction.

  1. Use the reaction. Magnesium reacts with oxygen to make magnesium oxide.
    reacts with oxygen
  2. Decide. So oxygen is a reactant, and the answer is yes.
    yes
  3. Write the answer.
    yes

Check. Oxygen is one of the reactants. ✓

Watch out

Watch out: the magnesium oxide weighs more than the magnesium because it contains oxygen. It does not mean mass was created.

Exam tip

Exam tip. When a metal gains mass in air, say that oxygen from the air joined the reaction.

Try it (with your teacher, or a grown-up at home). Look at a labelled picture of magnesium burning, or of a rusting nail. Say what the extra mass comes from. Burning magnesium is a teacher demonstration only: do not burn anything yourself.

Exercise 10C  Mass gained from the air

1
FWhen magnesium burns in air, what joins it? Write one word.
2
FDoes the mass of magnesium oxide go up or down compared with the magnesium? Write the word.
3
F6 g of magnesium makes 10 g of magnesium oxide. What is the mass of the magnesium oxide? Write the number of grams.
4
FHow many grams of oxygen joined 6 g of magnesium? Write the number.
5
FIs oxygen a reactant in the burning of magnesium? Write yes or no.
6
FDoes oxygen have mass? Write yes or no.
7
PS24 g of magnesium makes 40 g of magnesium oxide. How many grams of oxygen joined? Write the number.
8
PS12 g of magnesium makes 20 g of magnesium oxide. How many grams of oxygen joined? Write the number.
9
PSRusting makes iron heavier, because oxygen joins the iron. Does the mass of iron go up? Write yes or no.
10
PSBen says magnesium lost mass when it burned in air, because the white powder is lighter. Is Ben right? Write yes or no.
11
RExplain why the mass of the magnesium oxide is greater than the mass of the magnesium, in an open crucible. Write one sentence.
12
RChallenge: explain why the total mass is still conserved when magnesium burns in an open crucible, if the air is included. Write one sentence.
10.4

Planning a test of mass

Year 7
C8: measure mass in a closed and an open system; plan a fair test with repeats and a mean; identify an anomaly
Video lesson: Planning a test of massComing soon
Story. Mo plans a test to compare an open flask with a sealed bag. “I want a fair test,” he says. His teacher asks, “What must stay the same, and how many times will you repeat it?”

In a fair test of conservation of mass, only the set-up changes: open or sealed. The same amount of reactants, the same balance and the same method are used each time.

Repeat each test at least three times and find the mean. If one result does not fit the others, check it, and say clearly if it is left out.

mass (g)120open: before116open: after120sealed: before120sealed: after
Bar chart of the mass before and after, in the open flask and the sealed bag.

Key ideas

  • The independent variable is the set-up: open or sealed.
  • Keep the amount of reactants and the balance the same.
  • Repeat each test and find the mean.
  • Check any anomaly before you leave it out.
Worked example 10.4.1
In Mo's test, what is the independent variable? Write the words.

Method. Find the variable that changes on purpose.

  1. Read the plan. Mo changes the set-up, open or sealed, on purpose.
    set-up
  2. Name it. The independent variable is the type of container: open or sealed.
    type of container
  3. Write the answer.
    the type of container

Check. The mass change is the dependent variable. ✓

Worked example 10.4.2
Three open-flask trials lose 4 g, 4 g and 4 g. What is the mean loss? Write the number of grams.

Method. Add the trials and divide by three.

  1. Add. The total loss is 4 + 4 + 4 = 12.
    12
  2. Divide. Divide 12 by 3.
    12 / 3 = 4
  3. Write the answer.
    4 g

Check. Three trials of 4 g give a mean of 4 g. ✓

Worked example 10.4.3
Ruby gets losses of 4 g, 4 g and 9 g. Which trial is the anomaly? Write the number of grams.

Method. Find the reading that does not fit.

  1. Compare. Two trials give 4 g, but 9 g is much bigger.
    9 is bigger
  2. Decide. So the 9 g trial is the anomaly, and it should be checked.
    9 g
  3. Write the answer.
    9 g

Check. Check it before leaving it out. ✓

Watch out

Watch out: do not drop a trial just because it does not match. Check the anomaly, and say why it was left out, if it was.

Exam tip

Exam tip. In a fair test, name the independent variable, the fixed variables, and the repeats you used.

Try it (with your teacher, or a grown-up at home). Plan a test with a grown-up to compare a sealed bag and an open cup, using the same amount of bicarbonate of soda and vinegar. Write the independent variable, two fixed variables, and the number of repeats. Weigh before and after each trial, and find the mean loss in each set-up.

Exercise 10D  Planning a test of mass

1
FIn a test of conservation of mass, what is the independent variable? Write the words.
2
FWhat must stay the same in each trial? Write the words.
3
FWhy are trials repeated? Write the word.
4
FWhat instrument is used to measure mass? Write the word.
5
FShould a pupil use a sealed bag or a bare beaker to show mass is conserved? Write the word: sealed or bare.
6
PSIs it a fair test if the amount of vinegar is different in each trial? Write yes or no.
7
PSMo says a sealed bag gives the same mass before and after, so the test is good. Is Mo right? Write yes or no.
8
PSThree open-flask trials give losses of 4 g, 4 g and 4 g. What is the mean loss? Write the number of grams.
9
PSRuby records losses of 4 g, 5 g and 3 g in the open flask. What is the mean loss? Write the number of grams.
10
PSAli says the 9 g result is an anomaly in an open flask test with results 4, 4 and 9 g. Should he leave it out without checking? Write yes or no.
11
PSWorking scientifically: write a prediction for the sealed bag. Use the words “the mass will stay”. Write one sentence.
12
RExplain why repeats and a mean make a result more reliable. Write one sentence.
13
RChallenge: explain why the balance should be set to zero before the flask is weighed. Write one sentence.
10.5

Finding a missing mass

Year 7
C8: use conservation of mass in a closed system to find a missing mass; the mass of a gas that escaped is the difference
Video lesson: Finding a missing massComing soon
Story. Sam is given the masses of the reactants and the solid product of a reaction. “The gas that escaped was not weighed,” says his teacher. “Work out its mass from the rest, using conservation of mass.”

In a closed system, the total mass of the products equals the total mass of the reactants. If one mass is missing, you can find it:

missing mass = total mass of reactants - total mass of the known products

If the missing mass is a gas that escaped, it is the amount the total mass fell. The same method works for an open system, as long as the escaped mass is the only missing one.

Key ideas

  • Total mass of reactants = total mass of products (closed system).
  • Add the masses of the reactants to find their total.
  • Subtract the known products from the total to find the missing mass.
  • A gas that escapes is the mass that was not weighed.
Worked example 10.5.1
Two reactants weigh 40 g and 25 g. The solid product weighs 55 g, and a gas escapes. What is the mass of the gas? Write the number of grams.

Method. Find the total of the reactants, then subtract the solid product.

  1. Add the reactants. The total of the reactants is 40 + 25 = 65.
    65
  2. Subtract the product. The gas mass is 65 minus 55.
    65 - 55 = 10
  3. Write the answer.
    10 g

Check. 55 + 10 = 65. ✓

Worked example 10.5.2
Reactants weigh 100 g in total. A gas of 12 g escapes, and the solid product is weighed. What is the mass of the solid product? Write the number of grams.

Method. Subtract the gas from the total.

  1. Subtract. The solid product is the total minus the gas, so 100 minus 12.
    100 - 12 = 88
  2. Check. The solid weighs 88 g.
    88
  3. Write the answer.
    88 g

Check. 88 + 12 = 100. ✓

Worked example 10.5.3
In an open flask, the mass before was 120 g and after was 116 g. What is the mass of the gas that escaped? Write the number of grams.

Method. Subtract the mass after from the mass before.

  1. Read the masses. The mass before was 120 g and the mass after was 116 g.
    120 and 116
  2. Subtract. The gas mass is 120 minus 116.
    120 - 116 = 4
  3. Write the answer.
    4 g

Check. 116 + 4 = 120. ✓

Watch out

Watch out: find the total of the reactants first, not the products. Subtract the known products from the total of the reactants.

Exam tip

Exam tip. Write the sum, then the subtraction, then the unit. Check your answer by adding it back.

Try it (with your teacher, or a grown-up at home). With a grown-up, weigh a cup and a small amount of bicarbonate of soda, then add vinegar and weigh again after the fizzing stops. Find the mass of gas that escaped, using the mass before and after. Say whether the gas could have been carbon dioxide.

Exercise 10E  Finding a missing mass

1
FReactants weigh 30 g and 20 g. What is the total mass of the reactants? Write the number of grams.
2
FThree reactants weigh 5 g, 7 g and 8 g. What is their total? Write the number of grams.
3
FIn a closed system, the products weigh 80 g. What is the total mass of the reactants? Write the number of grams.
4
FA reaction has a total product mass of 88 g, and the reactants weigh 100 g. Is any mass missing? Write yes or no.
5
FIn an open flask, the mass before was 120 g and the mass after was 116 g. How much mass was lost? Write the number of grams.
6
PSReactants weigh 40 g and 25 g. The product that is weighed is 55 g. What is the mass of the gas that escaped? Write the number of grams.
7
PSReactants total 100 g. A gas of 12 g escapes. What is the mass of the solid product? Write the number of grams.
8
PSTwo products weigh 15 g and 9 g. What is the total mass of the reactants in a closed system? Write the number of grams.
9
PSA flask of drink weighs 250 g before and 246 g after the cap is taken off. What is the mass of gas that escaped? Write the number of grams.
10
PSBen says the reactants weigh 12 g and 8 g, so the product in a closed system must weigh 18 g. Is Ben right? Write yes or no.
11
PSWorking scientifically: a pupil weighs a sealed bag before and after a reaction, and records 150 g and 150 g. What does this show? Write one sentence.
12
RExplain why the total mass of the reactants is used when finding a missing mass. Write one sentence.
13
RChallenge: a gas of 4 g escapes from a reaction. Explain why the total of the products plus the gas equals the reactants in a closed system. Write one sentence.

Chapter summary

SkillMethod
Conservation of massAtoms are rearranged, not created or destroyed. Total mass before = total mass after, in a closed system.
Open systemA gas that escapes lowers the mass measured; a gas from the air that joins raises it.
OxygenWhen a metal burns in air, oxygen joins it, so the product weighs more.
Fair testChange only the set-up. Repeat and find the mean. Check any anomaly.
Missing massMissing mass = total mass of reactants - total mass of the known products.

Can you…?

  • I can state the law of conservation of mass, and explain why it holds
  • I can say why a mass can seem to fall in an open system
  • I can explain why magnesium gains mass when it burns in air
  • I can plan a fair test of mass with repeats and a mean
  • I can find a missing mass using conservation of mass

Key terms

conservation of mass
the total mass is the same before and after a chemical reaction in a closed system
closed system
a system where nothing can enter or leave
open system
a system where a gas can escape or join the reaction
anomaly
a result that does not fit the others, which must be checked
mean
the total of the readings divided by the number of readings

Exercise 10Z  Check-up

1
What is the name of the law that says mass is conserved in a reaction? Write the words.
2
In a closed system, is the mass of the products equal to the mass of the reactants? Write yes or no.
3
An open flask weighs 120 g before and 116 g after a gas escapes. What is the mass of the gas? Write the number of grams.
4
What gas escapes from the open flask of marble chips and acid? Write the name.
5
When magnesium burns in an open crucible, does the mass go up or down? Write the word.
6
6 g of magnesium makes 10 g of magnesium oxide. How many grams of oxygen joined? Write the number.
7
Reactants weigh 30 g and 20 g in a closed system. What is the total mass of the products? Write the number of grams.
8
A sealed bag weighs 120 g before and 120 g after a reaction. What is the change in mass? Write the number of grams.
9
Name the system where nothing can enter or leave. Write the two words.
10
Explain why a mass can seem to fall in an open flask, even though mass is conserved. Write one sentence.

Total 18 marks. Allow about 18 minutes.

Exercise 10X  Practice test questions

1
FReactants have a total mass of 75 g in a closed system. What is the total mass of the products? Write the number of grams.[1 mark]
2
PSAn open flask weighs 200 g before a gas escapes, and 193 g after. What is the mass of the gas that escaped? Write the number of grams.[2 marks]
3
RExplain why the mass of magnesium oxide is greater than the mass of the magnesium it came from, in an open crucible. Write one sentence.[2 marks]
4
PS24 g of magnesium makes 40 g of magnesium oxide. How many grams of oxygen joined? Write the number of grams.[2 marks]
5
PSReactants total 100 g. A gas of 12 g escapes. What is the mass of the solid product? Write the number of grams.[2 marks]
6
FWhich set-up keeps the mass the same: an open flask or a sealed bag? Write the words.[1 mark]
7
FA pupil says mass disappears in a chemical reaction. Is the pupil right? Write yes or no, and the reason in one word.[2 marks]
8
FWrite the word that describes a system where a gas can escape into the air. Write the word.[1 mark]
9
PSThree reactants weigh 5 g, 7 g and 8 g in a closed system. What is the total mass of the products? Write the number of grams.[2 marks]
10
RExplain why a sealed bag gives a fair test of conservation of mass. Write one sentence.[2 marks]
11
PSA flask of drink weighs 250 g before the cap is taken off, and 246 g after. Write the mass of the gas that escaped, in grams.[1 mark]
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