Combustion and Carbonates

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 2.11 — The combustion of magnesium, hydrogen and sulfur in oxygen
• 2.12 — Formation of carbon dioxide by thermal decomposition of metal carbonates
• 2.13 — Carbon dioxide as a greenhouse gas contributing to climate change

Specification section 2(d) Reactivity series — also taught in The Gases in the Air, The Reactivity Series, Rusting and its Prevention, Redox.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

COMBUSTION is reaction with oxygen, releasing energy. The specification names three substances to know, and they are chosen to make a point: two are elements that burn to give oxides with quite different characters, and the third gives a product everyone drinks.

Statement 2.11 — the three combustions

1. MAGNESIUM burns with a brilliant white flame, so bright it is painful to look at, and leaves a white ash of magnesium oxide. 2Mg + O₂ → 2MgO.
2. Magnesium oxide is a METAL oxide, so it is BASIC — dissolve the ash in water and the solution is alkaline.
3. HYDROGEN burns with a pale blue, almost invisible flame and a squeaky pop, giving water. 2H₂ + O₂ → 2H₂O.
4. Water is neutral, which is worth noting because hydrogen is the only element on this list whose oxide is neither acidic nor basic.
5. SULFUR burns with a pale blue flame, giving a colourless gas with a choking smell. S + O₂ → SO₂.
6. Sulfur dioxide is a NON-METAL oxide, so it is ACIDIC — dissolve it in water and the solution turns litmus red.

That metal-versus-non-metal pattern is the useful generalisation buried in statement 2.11. METAL OXIDES ARE BASIC and NON-METAL OXIDES ARE ACIDIC. It lets you predict the character of an oxide you have never met from the position of the element in the periodic table, and it reappears constantly in the acids and salts lessons later in this topic.

Now carbonates, statement 2.12. Heating a metal carbonate strongly breaks it down into the metal oxide and carbon dioxide, a reaction called THERMAL DECOMPOSITION — decomposition, because one substance becomes two, and thermal because heat is what does it.

Written out, calcium carbonate → calcium oxide + carbon dioxide, or CaCO₃ → CaO + CO₂ on strong heating. This particular decomposition is how quicklime is made industrially, in enormous quantities.

Think of it like the difference between burning a log and shattering a plate

Burning a log requires something else — air — and produces heat. Dropping a plate needs no second substance at all: one thing becomes several because energy was put in. Thermal decomposition is the plate. Only ONE reactant is involved, there is nothing for it to react WITH, and the heat is what supplies the energy to break it apart rather than something released by the reaction. Getting this distinction straight is what stops a learner writing 'the carbonate burns', which implies oxygen is involved and would need it in the equation.

Every metal carbonate on this specification decomposes in the same way — copper(II) carbonate is a satisfying example, changing visibly from green to black as it becomes copper(II) oxide. The carbon dioxide released can be tested by bubbling it through limewater, which turns milky.

Statement 2.13 concerns what that carbon dioxide does in the atmosphere. Radiation from the Sun arrives largely as visible light and passes through the atmosphere to warm the Earth's surface. The warmed surface re-emits energy as INFRARED radiation of a longer wavelength, and this is where carbon dioxide matters: it ABSORBS infrared radiation strongly, and re-emits some of it back towards the surface. The result is that the Earth is warmer than it would be otherwise. That is the GREENHOUSE EFFECT, and it is entirely natural — without it the planet would be far too cold for life.

Why an increase matters when the figure is only 0.04%

1. Burning fossil fuels releases carbon dioxide that had been locked underground for millions of years, and deforestation removes trees that would have absorbed some of it.
2. The concentration of carbon dioxide in the atmosphere has therefore been RISING, and measurably so.
3. More carbon dioxide means more infrared radiation absorbed and re-emitted back towards the surface.
4. So the average global temperature RISES — this is the ENHANCED greenhouse effect, an addition to the natural one rather than a new phenomenon.
5. The consequences include melting ice and rising sea levels, more extreme weather, and shifts in where crops and species can survive.
6. The small percentage is not a reason to expect a small effect: carbon dioxide absorbs infrared very strongly, so a change in a tiny fraction still changes a great deal of absorption.

That last point is worth dwelling on because the arithmetic misleads people. Nitrogen makes up 78% of the air and absorbs essentially no infrared at all; carbon dioxide makes up 0.04% and absorbs it strongly. The proportion of a gas in a mixture tells you nothing about how much work it is doing, and this is one of the clearest cases in chemistry where it would be a mistake to assume otherwise.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

The vocabulary of this page

Combustion

Reaction with oxygen, releasing energy. Two reactants are needed: the substance and oxygen.

Thermal decomposition

The breaking down of ONE substance into two or more by heating. Nothing reacts with it.

Greenhouse effect

The warming of the Earth caused by gases absorbing infrared radiation emitted by the surface and re-emitting some of it back.

Basic oxide

A metal oxide, which reacts with acids and gives an alkaline solution if it dissolves.

Acidic oxide

A non-metal oxide, which gives an acidic solution when it dissolves in water.

Statement 2.11 — the three combustions

2Mg + O₂ → 2MgO

Conditions: Magnesium burning in oxygen, with a brilliant white flame. The white ash is basic.

S + O₂ → SO₂

Conditions: Sulfur burning in oxygen, with a pale blue flame. The choking gas is acidic in water.

CuCO₃ → CuO + CO₂

Conditions: Thermal decomposition on strong heating. Green copper(II) carbonate turns black as copper(II) oxide forms.

Statement 2.13 — the greenhouse effect, in order

1. Radiation from the Sun passes through the atmosphere and warms the Earth's surface.
2. The warmed surface re-emits energy as INFRARED radiation of longer wavelength.
3. Carbon dioxide ABSORBS infrared radiation strongly.
4. It re-emits some of that energy back towards the surface, so the Earth is warmer than it would otherwise be.
5. Burning fossil fuels and deforestation raise the concentration of carbon dioxide, so more infrared is absorbed and the average temperature rises.

Combustion against thermal decomposition

A. COMBUSTION needs TWO reactants — the substance and oxygen — and releases energy. Magnesium burning is combustion.

B. THERMAL DECOMPOSITION needs ONE reactant and energy put IN. A carbonate breaking into an oxide and carbon dioxide is decomposition.

Model answer [3 marks]

Describe what is seen when magnesium burns in oxygen, name the product, and state whether it is acidic or basic. [3]

The magnesium burns with a brilliant white flame that is painful to look at directly, and leaves a white powdery ash. The product is magnesium oxide. Because magnesium is a metal, magnesium oxide is a metal oxide and is therefore basic, so it gives an alkaline solution when dissolved in water.

Model answer [4 marks]

Explain how an increase in the concentration of carbon dioxide leads to a rise in average global temperature. [4]

Radiation from the Sun passes through the atmosphere and warms the Earth's surface, which then re-emits energy as infrared radiation of a longer wavelength. Carbon dioxide absorbs infrared radiation strongly and re-emits some of it back towards the Earth's surface. If the concentration of carbon dioxide increases, more of the infrared radiation leaving the surface is absorbed and re-emitted downwards rather than escaping into space. This means more energy is retained near the surface, so the average global temperature rises.

Not this: Carbon dioxide is only 0.04% of the air, so changing it cannot have much effect.

This: The proportion of a gas says nothing about how much work it does. Nitrogen is 78% of the air and absorbs essentially no infrared; carbon dioxide is 0.04% and absorbs it strongly, so a change in that small fraction changes a great deal of absorption.

Mark-losing trap. METAL oxides are BASIC; NON-METAL oxides are ACIDIC. This predicts oxides you have never met.

Mark-losing trap. Thermal decomposition has ONE reactant. Saying the carbonate 'burns' wrongly implies oxygen.

Mark-losing trap. The greenhouse effect is NATURAL and necessary. What is described here is the ENHANCED effect.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · Name [1 mark] — What is produced when hydrogen burns in oxygen?
2. Grade 7 · Predict [3 marks] — Sulfur dioxide is dissolved in water and universal indicator is added. What colour would you expect, and why?
3. Grade 8 · Explain [3 marks] — A student writes that copper(II) carbonate 'burns' to give copper(II) oxide and carbon dioxide. Explain what is wrong with this description.
4. Grade 9 · Explain [6 marks] — Select every statement that belongs in a full-mark explanation of how rising carbon dioxide concentration causes global temperatures to rise.
5. 9+ · Analyse [6 marks] — A student burns magnesium in air and finds the ash is heavier than the magnesium. They then heat copper(II) carbonate and find the residue is lighter than the carbonate. They conclude that mass is conserved in one reaction but not the other. Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one who worked out what burning actually is, and the one who found what takes carbon dioxide back out again. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Antoine Lavoisier — the one who worked out what burning actually is

Statement 2.11 rests on combustion being a reaction WITH something, and that was Lavoisier's contribution against a theory that said the opposite. Phlogiston theory held that burning released a substance from the fuel, which should make the residue lighter — and yet magnesium and other metals visibly gain mass when they burn. Defenders of phlogiston were reduced to proposing that it had negative weight. Lavoisier weighed sealed vessels before and after, showed the gain came from a component of the air, and named that component oxygen.

• “What is actually happening when something burns?”
• “Why does a metal gain mass while burning?”
• “What did phlogiston theory claim, and why did people believe it?”
• “How did sealing the vessel settle the argument?”
• “Why do metal oxides and non-metal oxides behave so differently?”

Jan Ingenhousz — the one who found what takes carbon dioxide back out again

This page describes carbon dioxide accumulating and mentions deforestation as part of the reason, and Ingenhousz is the person who established what trees are actually doing. In 1779 he showed that plants purify air only in the LIGHT, and that in darkness they do the opposite and spoil it — the green parts, not the whole plant, and only while illuminated. That is the discovery that makes 'deforestation raises carbon dioxide' a mechanism rather than an assertion, and he is the right person to ask how much a forest can really absorb.

• “What do plants do to the air, and when?”
• “Why did it matter that the effect only happens in light?”
• “What happens to the air around a plant at night?”
• “How did you show that only the green parts do it?”
• “Can forests absorb what we are adding to the air?”

Carry on through the course

• Previous lesson: The Gases in the Air
• Next lesson: The Reactivity Series
• Group 1 — the Alkali Metals — Metals soft enough to cut with a knife, which set fire to water — and get worse as you go down.
• Group 7 — the Halogens — A green gas, a brown liquid and a grey solid — and a trend that runs the opposite way to Group 1.
• The Gases in the Air — Four gases, one of which is doing almost all the chemistry — and how to measure it without ever touching it.
• The Reactivity Series — Putting eleven metals in order by watching what each one will and will not do.
• Rusting and its Prevention — Two things are needed to rust iron, and every method of stopping it removes one of them — or cheats.
• All of Chemistry · Chemistry key terms · required practicals · equations · Incandio Science · The 4CH1 specification