Ghost Gas
AQA GCSE Chemistry 8462 EXAM YEARS 2027-2029 Mass changes when gases are involved
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Ghost Gas: why the balance reading changes when a gas is involved
This free game is part of our AQA GCSE Chemistry (8462) course for exams in EXAM YEARS 2027-2029.
The balance only weighs what is sitting on it
For AQA GCSE Chemistry (spec point 4.3.1.3), some reactions appear to involve a change in mass, but this can usually be explained because a reactant or product is a gas and its mass has not been taken into account. Mass is conserved in every reaction: no atoms are lost or made. The trouble is that you cannot see gases, and a balance only weighs what is actually sitting on it. If a container is open to the air (a non-enclosed system), gas particles can get in from the air or escape into it without anyone noticing.
In Ghost Gas the balance in a night-time lab seems to be haunted: the reading keeps changing on its own. It is not a ghost, it is gas. A particle scope shows every gas molecule as a 3D model, a dashed box shows everything the balance weighs, and the reading changes at the exact moment each molecule crosses that line. Each molecule you see stands for billions of billions of real ones. Move the scope over them to scan them into your gas ledger, and at the end the sum shows where the mass went, for example copper oxide 7.95 g + carbon dioxide 4.40 g = 12.35 g.
Mass goes up: a metal reacting with oxygen
When a metal reacts with oxygen, the mass of the oxide produced is greater than the mass of the metal. In the first level copper powder heats on a heating balance and oxygen molecules drift in from the air. Each one that lands reacts, its oxygen atoms bond to copper atoms in the solid, the orange-brown powder turns black and the reading climbs from 6.35 g to 7.95 g. Look at the equation 2Cu(s) + O2(g) → 2CuO(s): the gas is a reactant from the air, so its mass was never weighed at the start. Later you also burn magnesium, zinc and steel wool.
Mass goes down: thermal decomposition and fizzing flasks
In the thermal decomposition of metal carbonates, carbon dioxide is produced and escapes into the atmosphere, leaving the metal oxide as the only solid product. Green copper carbonate turns black as CO2 molecules rise out of the open crucible, and the reading falls from 12.35 g to 7.95 g. The same happens in solution: sodium carbonate and hydrochloric acid fizz in an open conical flask and the reading drops as the bubbles leave. Magnesium and acid give off hydrogen, which is so light that the reading drops by only 0.02 g, but it still drops.
Then you seal the flask with a rubber bung (an enclosed system). Push the bung in first, then tip the flask: the carbon dioxide bounces around inside and the reading stays at exactly 160.00 g, which proves mass is conserved. Pull the bung out and it escapes. A precipitate reaction with no gas, and burning carbon (gas on both sides of the equation), test whether you really read the state symbols.
How to play Ghost Gas
Read the balanced symbol equation and the setup, then call it: GOES UP, GOES DOWN or STAYS THE SAME (keys 1, 2 and 3). Then move the particle scope with the mouse, your finger or the arrow keys to scan the gas molecules before they drift away or land. There are five levels: Heavier?, Lighter? (with a 3-mark exam answer to build), Seal it, Case files (20 seconds a call, plus an exam calculation) and a 70 second Mass Rush. Every wrong call opens a short explanation with a link back to the lesson. It takes about 8 to 9 minutes.
Exam tip: for any mass change, find the (g), decide if it is a reactant or a product, then write three ideas: name the gas, say its particles joined the solid or escaped into the air, and say mass is still conserved because the gas was not weighed. The mass of gas is the mass before minus the mass after.
Frequently asked questions
Why does a metal get heavier when it is heated in air?
The metal reacts with oxygen from the air. Oxygen atoms bond to the metal atoms, so the solid oxide has more mass than the metal did. The oxygen was not on the balance at the start, so the reading goes up. No atoms are made: the extra mass is the mass of the oxygen that reacted.
Why does a metal carbonate get lighter when it is heated?
It decomposes into a metal oxide and carbon dioxide gas. In an open container the carbon dioxide molecules escape into the air, leaving the metal oxide as the only solid product, so the reading goes down. Mass is still conserved: the carbon dioxide just is not weighed.
What is the difference between an enclosed and a non-enclosed system?
A non-enclosed system is open to the air, so gases can get in or out. An enclosed system is sealed, so nothing can get in or out, and the balance reading stays the same during the reaction.
How do you calculate the mass of gas given off?
Subtract the mass after the reaction from the mass before. For example, 32.50 g - 31.62 g = 0.88 g of carbon dioxide. The container is weighed both times, so its mass cancels out.
Is Ghost Gas free?
Yes. The game is free to play in your browser, with no sign-up. It covers mass changes when a reactant or product is a gas in the AQA GCSE Chemistry course.