5.3.1.3Mass changes when a reactant or product is a gas
AQA GCSE Combined Science (8464), Higher tier · Chemistry › Quantitative chemistry › Chemical measurements, conservation of mass and the
Practise Mass changes when a reactant or product is a gas. 11 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Some reactions in open containers seem to break the law of conservation of mass. The measured mass goes up if a gas from the air reacts, and goes down if a gas is produced and escapes. You need to explain these changes using the balanced equation and the particle model.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
State that gases can enter or leaveIn an open container, gases can escape into the air or react from the air.
4
Explain a mass decrease when gas formsThe gas produced escapes into the air, so its mass is no longer measured.
5
Explain why a heated metal gains massOxygen atoms from the air combine with the metal, so the metal oxide has a greater mass than the metal.
6
Use an equation to predict the mass changeLook for (g): a gas reactant means the mass goes up, a gas product means it goes down.
7
Explain mass changes using the particle modelGas particles move quickly and randomly and spread out, so they leave an open container or reach the reactants from the air.
8
Predict the size of a mass change (HT)Use moles, e.g. 100 g (1 mol) of CaCO3 loses 44 g (1 mol) of CO2 when heated.
Notes
Why the mass can seem to change
Mass is always conserved. But in a non-enclosed system (an open container) gases can move in or out without being weighed.
If a reactant is a gas from the air (usually oxygen), the measured mass goes up.
If a product is a gas (such as carbon dioxide or hydrogen), it escapes and the measured mass goes down.
In a sealed container nothing can get in or out, so the mass stays the same.
Mass increases: metals reacting with oxygen
2Mg(s) + O2(g) → 2MgO(s)
Oxygen atoms from the air join onto the magnesium, so the magnesium oxide has a greater mass than the magnesium you started with.
Mass gained = mass of oxygen that reacted.
Mass decreases: gases given off
Thermal decomposition of a metal carbonate: CaCO3(s) → CaO(s) + CO2(g). The carbon dioxide escapes into the air, leaving the metal oxide as the only solid product.
Acid with a carbonate gives off carbon dioxide; acid with a metal such as magnesium gives off hydrogen.
Mass lost = mass of gas that escaped.
Explaining it with particles
Gas particles are far apart and move quickly and randomly in all directions, so a gas spreads out and escapes from an open container.
Oxygen particles in the air collide with the surface of the hot metal and react, becoming part of the solid.
Higher tier: you can predict the size of the change with moles. Heating 100 g (1 mol) of CaCO3 releases 44 g (1 mol) of CO2, so the solid loses 44% of its mass. grade 8+
Cheatsheet
Non-enclosed (open) system: gases can enter or leave, so the measured mass can change
Gas is a reactant (from the air) → measured mass increases
Gas is a product (escapes) → measured mass decreases
Metal + oxygen → metal oxide (mass goes up)
Metal carbonate → metal oxide + carbon dioxide (mass goes down)
Mass of gas = change in mass
Sealed container: mass stays the same
How to answer each type of question
Explain why the mass decreased
2 marks4
Name the gas produced.
Say that it escapes from the open container into the air, so its mass is not measured.
Example. A student added marble chips (calcium carbonate) to hydrochloric acid in an open conical flask on a balance. The mass of the flask and contents decreased during the reaction. Explain why. [2 marks]
Show the model answer
Carbon dioxide gas is produced (1). The gas escapes from the flask into the air, so its mass is no longer measured (1).
Explain why the mass increased
2 to 3 marks5
Say that the substance reacted with oxygen from the air.
Name the product (a metal oxide).
Say that the mass of the oxygen atoms is added to the solid.
Example. A student heated 1.20 g of copper powder in an open crucible. After heating, the mass of the solid was 1.50 g. Explain why the mass increased. [3 marks]
Show the model answer
The copper reacted with oxygen from the air (1) to form copper oxide (1). The oxygen atoms that combined with the copper added to the mass of the solid (1).
Calculate the mass of gas given off
2 to 3 marks6
Subtract the final mass from the starting mass (or the other way round if the mass went up).
Give the unit.
If asked, explain that heating to constant mass shows the reaction is complete.
Example. Zinc carbonate decomposes when heated. ZnCO3(s) → ZnO(s) + CO2(g) A student heated 6.25 g of zinc carbonate in an open tube until the mass stopped changing. The final mass of solid was 4.05 g. (a) Calculate the mass of carbon dioxide given off. [2 marks] (b) Suggest why the student heated the tube until the mass stopped changing. [1 mark]
Show the model answer
(a) 6.25 − 4.05 (1) = 2.20 g (1) (b) To make sure all of the zinc carbonate had decomposed (1).
Predict the mass change from an equation
1 mark each7
Look for gases, shown by (g), in the equation.
Gas on the left comes from the air: mass up. Gas on the right escapes: mass down.
No gases at all: the mass stays the same. Give the reason each time.
Example. Each reaction is carried out in an open container. Predict whether the mass of the container and contents increases, decreases or stays the same. Give a reason for each. (a) 4Na(s) + O2(g) → 2Na2O(s) (b) Mg(s) + H2SO4(aq) → MgSO4(aq) + H2(g) (c) NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) [3 marks]
Show the model answer
(a) Increases: oxygen from the air is a reactant and becomes part of the solid (1). (b) Decreases: hydrogen gas is produced and escapes into the air (1). (c) Stays the same: there are no gases, so nothing enters or leaves (1).
Shortcuts and memory tricks
Gas IN (a reactant from the air) → mass UP. Gas OUT (a product) → mass DOWN.
Scan the equation for (g). No (g) at all means the mass stays the same, even in an open container.
Put a lid on it: in a sealed container the mass never changes.
The change in mass is the mass of the gas. Nothing else needs working out.
Where marks are lost
Saying mass is 'lost' or 'destroyed'. Mass is conserved; the gas is just not being weighed.
Writing 'a gas is produced' without saying that it escapes into the air.
Saying a heated metal gains mass because it 'absorbs heat'. Heat has no mass; oxygen atoms are added.
Forgetting that the oxygen comes from the air when a question only mentions heating a metal.
Subtracting the wrong way round and giving a negative mass of gas.
Exam technique
Use the words 'open' or 'non-enclosed', and name the gas (oxygen, carbon dioxide or hydrogen).
When an equation is given, point to the gas and its state symbol (g) in your answer.
A mass-change explanation needs two points: what the gas is, and whether it enters or leaves.
In practical questions, heating to constant mass shows that the reaction has finished.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Complete the sentence. A system that no substances can enter or leave is called a .................... system.
closed
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) Some iron wool is heated strongly in an open dish on a balance. The iron reacts with oxygen from the air to form iron oxide. What happens to the reading on the balance? Tick (✓) one box.[1]
It increases, because oxygen from the air combines with the iron.
It decreases, because the iron is used up.
It stays the same, because iron oxide is a solid.
It stays the same, because no atoms are made or destroyed.
(b) Complete the sentence. A system that no substances can enter or leave is called a .................... system.[1]
(c) A reaction that produces a gas is carried out in a sealed flask on a balance. Predict what happens to the reading on the balance during the reaction. Explain your answer.[2]
Show the answer and mark scheme
(a)Answer: It increases, because oxygen from the air combines with the iron.
(b)Answer: closed
closed
(c)
the reading stays the same / does not change
the gas cannot escape from the sealed flask, so no mass leaves the system (mass is conserved)
Question 2Medium5 marks
Copper carbonate is a green powder. When it is heated it decomposes to produce black copper oxide and carbon dioxide: CuCO3(s) → CuO(s) + CO2(g) A student heated 12.35 g of copper carbonate in an open test tube until there was no further change. The mass of solid left was 7.95 g.
(a) Calculate the mass of carbon dioxide produced.[1]
(b) Explain why the mass of solid decreased. Use the equation in your answer.[2]
(c) The student repeated the experiment in a sealed container. Predict how the total mass of the container and its contents after heating would compare with the total mass before heating. Give a reason for your answer.[2]
Show the answer and mark scheme
(a)Answer: 4.40 g
4.40 (g)
(b)
carbon dioxide is produced, which is a gas / (g)
the carbon dioxide escapes into the air (so its mass is not included)
(c)
the total mass would stay the same
the carbon dioxide cannot escape / no atoms are lost or made
Question 3Hard6 marks
A teacher ignited some steel wool (mainly iron) inside a sealed flask of air on a balance. The iron reacts with oxygen: 3Fe(s) + 2O2(g) → Fe3O4(s) The flask contained only a small volume of air, so only some of the iron reacted.
(a) The reading on the balance did not change during the reaction, even though the iron gained mass. Explain why.[2]
(b) The experiment was repeated with the flask open to the air. Explain why the reading on the balance increased.[2]
(c) Predict what would happen to the mass of iron oxide formed if the experiment were repeated in a sealed flask of pure oxygen instead of air, using the same mass of steel wool. Give a reason for your answer.[2]
Show the answer and mark scheme
(a)
the flask was sealed, so no substances could enter or leave
the oxygen that combined with the iron was already inside the flask, so the total mass of the flask and its contents stayed the same
(b)
oxygen from the air outside could enter the open flask and react with the iron
the mass of this extra oxygen is added to the solid in the flask, so the total mass on the balance increases
(c)
a greater mass of iron oxide would form
there is more oxygen in the flask, so more (or all) of the iron can react before the oxygen runs out