5.3.1.3Mass changes when a reactant or product is a gas
AQA GCSE Combined Science Foundation (8464), Foundation tier · Chemistry › Quantitative chemistry › Chemical measurements, conservation of mass and the
Practise Mass changes when a reactant or product is a gas. 6 exam-style questions 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.
Key facts
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
Notes
Why the mass can seem to change
diagram
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.
Marble chips and acid: in the open flask the gas escapes and the reading falls; with a bung nothing leaves.
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.
How to answer each type of question
Explain why the mass decreased
2 marksGrade 4
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 answerHide 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 marksGrade 5
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 answerHide 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).
Don’t lose marks
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.
More tips
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.
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.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State that gases can enter or leaveIn an open container, gases can escape into the air or react from the air.
Grade 4
Explain a mass decrease when gas formsThe gas produced escapes into the air, so its mass is no longer measured.
Grade 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.
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