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Mass increases when magnesium is heated in air because magnesium atoms combine with oxygen to form magnesium oxide. Free notes and 11 questions answered.
Explaining Mass Changes in Open Systems is part of Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations in GCSE AQA Higher Chemistry.
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Mass increases when magnesium is heated in air because magnesium atoms combine with oxygen to form magnesium oxide. Free notes and 11 questions answered.
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Use this lesson to stay inside Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations while connecting the detail back to the wider Topic 3: Quantitative chemistry topic.
Read the core explanations from Explaining Mass Changes in Open Systems before testing yourself from memory.
You heat copper in air and it gets heavier. You heat limestone and it gets lighter. Has mass been created or destroyed? The answer reveals a crucial difference between what we observe and what actually happens at the particle level.
These puzzling observations happen because we're working with open systems - reactions where gases can escape or enter from the atmosphere. The mass isn't really changing; we're just not accounting for all the particles involved.
When you weigh reactants and products on a balance, you're only measuring what stays in the reaction vessel. If a gas escapes or enters during the reaction, your balance can't detect it - but the gas still has mass.
Picture heating magnesium ribbon in air. The magnesium atoms combine with oxygen molecules from the atmosphere to form magnesium oxide. Your balance shows an increase in mass because oxygen gas (which you didn't weigh initially) has joined the solid magnesium.
Scenario 1: Mass Appears to IncreaseThis happens when a solid or liquid reactant combines with oxygen gas from the air. The oxygen wasn't weighed initially, so the product appears heavier than the starting material.
Example: $2Mg(s) + O_2(g) \rightarrow 2MgO(s)$We weigh the magnesium but not the oxygen gas, so the magnesium oxide seems to have gained mass from nowhere.
Scenario 2: Mass Appears to DecreaseThis happens during thermal decomposition when a gas is produced and escapes. The solid product weighs less than the original compound because part of it has escaped as gas.
Example: $CaCO_3(s) \rightarrow CaO(s) + CO_2(g)$The calcium carbonate breaks down, but the carbon dioxide escapes, leaving only the lighter calcium oxide behind.
The particle model helps us track what's really happening. Every atom that enters the reaction must be accounted for in the products - they don't vanish or appear from nothing.
Worked example: Explaining Mass Change Using Particles
Explain why 10.0g of calcium carbonate produces only 5.6g of solid when heated strongly.
Answer: The mass appears to decrease because 4.4g of CO₂ gas escapes, but total mass is conserved.
The key insight is that particles don't disappear - they just change location. In open systems, some particles move from the reaction vessel into the atmosphere (or vice versa).
Once you understand the particle model, you can predict and calculate mass changes in open systems. The key is identifying which substances are gases and whether they're reactants or products.
Worked example: Predicting Mass Changes
12.0g of magnesium is heated in air. Calculate the mass of magnesium oxide formed. (Mg = 24, O = 16)
Answer: 20.0g of magnesium oxide is formed (an increase of 8.0g due to oxygen from air)
Examiners frequently test your understanding of mass changes in open systems. The key is to identify whether gases are involved and explain using the particle model.
Exam precision note: In Explaining Mass Changes in Open Systems - Mass changes when a reactant or product is a gas - Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
Open systems can appear to gain or lose mass because gases move between the reaction vessel and the surrounding air. Burning magnesium gains mass because oxygen from the air becomes part of the solid product, while thermal decomposition can show a mass decrease because carbon dioxide escapes.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
The balanced equation for magnesium burning is 2Mg(s) + O₂(g) → 2MgO(s). Magnesium combines with oxygen from the air to form magnesium oxide, so the mass appears to increase because the oxygen was not weighed at the start.
An open system is a chemical reaction system where reactants or products can enter or leave the reaction vessel, typically gases that escape to or come from the atmosphere. In open systems mass can appear to change even though it is conserved.
The equation for the thermal decomposition of calcium carbonate is CaCO₃(s) → CaO(s) + CO₂(g). Calcium carbonate breaks down into calcium oxide and carbon dioxide, and the mass appears to decrease because the carbon dioxide gas escapes.
Mass increases when magnesium is heated in air because magnesium atoms combine with oxygen from the air to form magnesium oxide: 2Mg + O₂ → 2MgO. The oxygen gas was not weighed at the start, so the solid product appears heavier.
Mass decreases during thermal decomposition because a gas is produced and escapes. When calcium carbonate is heated, CaCO₃ → CaO + CO₂, the carbon dioxide escapes, leaving only the lighter calcium oxide behind.
Mass is not really lost when a gas escapes from an open system; the gas simply isn't weighed. For example, 10.0g of calcium carbonate leaves 5.6g of calcium oxide because 4.4g of CO₂ escapes, and 5.6g + 4.4g = 10.0g, so total mass is conserved.
To calculate the mass of magnesium oxide formed, find the moles of magnesium, use the 1:1 mole ratio, then use mass = moles × Mr. For 12.0g of magnesium: 12.0 ÷ 24 = 0.50 mol, Mr of MgO = 40, so mass = 0.50 × 40 = 20.0g.
Copper gets heavier when heated in air because it combines with oxygen gas from the air. The oxygen wasn't weighed at the start, so the product appears heavier than the starting metal, even though no mass has been created.
The mass of carbon dioxide given off is the starting mass minus the mass of solid left. When 10.0g of calcium carbonate leaves 5.6g of calcium oxide, the mass of CO₂ produced is 10.0 - 5.6 = 4.4g, which escaped and wasn't weighed.
The particle model explains that particles don't disappear in an open system; they just change location. Every atom that enters a reaction must be accounted for in the products, but some particles move from the reaction vessel into the atmosphere, or from the air into the vessel.
Mass appears to increase when a solid or liquid reacts with a gas from the air, such as oxygen, and appears to decrease when a gas is produced and escapes, as in thermal decomposition. In both cases total mass is conserved once the gas is included.