Types of reaction · Process
Thermal decomposition
Every reaction so far has joined things together. What kind of reaction takes one substance and gives you two?
Start here
One substance went into the tube. Two came out.
A green powder is heated in a test tube. Nothing is added to it — no acid, no other chemical, and the tube is not open to much air. It turns black, a gas comes off that turns limewater cloudy, and the tube is lighter afterwards.
Every reaction so far had two or more reactants. What has happened here?
The compound has been broken apart by heat. Copper carbonate is copper, carbon and oxygen joined together; heating supplies enough energy to break those joins, and the pieces reassemble as two new substances — black copper oxide, which stays, and carbon dioxide, which leaves. One reactant, two products. Nothing arrived from outside.
Thermal decomposition is one compound being broken down into two or more substances using heat. It is the only reaction type you will meet that runs backwards compared with the others: instead of substances joining, one comes apart.
It also needs the heat continuously. Take the flame away and the reaction stops — which is the opposite of burning, where the reaction supplies its own heat once it has started.
Your turn · the test tube
Heat it, test the gas, then let it cool
In the tube
A green powder. It decomposes at a temperature an ordinary Bunsen burner reaches easily, which is why it is the school version of this reaction.
Calcium carbonate — limestone, chalk and marble are all the same compound. It needs a much higher temperature than the copper one: a roaring blue flame and patience, or an industrial kiln.
Sodium hydrogencarbonate — the raising agent in a cake. It decomposes at oven temperatures, which is the entire reason it is in the recipe.
Heat it
The flame goes under the tube. Nothing happens for a few seconds while the powder heats through.
The flame goes under the lump. Nothing visible happens for a long time — this reaction needs far more heat than the copper carbonate did.
Gentle heating. The white powder starts to fizz and shift as gas works its way out of it.
Watch the solid
The green darkens from the bottom upwards and becomes black. The colour change spreads as the heat does — the part not yet hot enough is still green.
The lump glows and crumbles slightly, and it stays white. There is no colour change to watch, which makes this a good reminder that a colour change is a clue and not a requirement.
It stays white and loses volume. Water vapour condenses in the cool upper part of the tube — this decomposition gives two gases, not one.
Test the gas
The gas is bubbled through limewater, which turns milky. That is the test for carbon dioxide, and only carbon dioxide passes it.
The gas turns limewater milky: carbon dioxide again. This is the only evidence that anything has happened at all.
The gas turns limewater milky, so carbon dioxide is among the products. In a cake, this gas is what makes it rise.
Take the flame away
The flame comes off. The powder stays black as it cools, and it stays black tomorrow.
It cools as a white solid — calcium oxide, quicklime — and it is not the same substance that went in. Drop water on it and it hisses and gets hot.
It cools to a white powder — sodium carbonate — and does not turn back. A cake does not become batter again as it cools either.
Before you take the flame away: what happens as it cools?
Mass in the tube
4.00 g2.58 g4.00 g2.24 g4.00 g2.52 g
Limewater
clearmilky
Heat needed
not yetcontinuouslyflame removed
copper carbonatemakescopper oxide + carbon dioxide
Black copper oxide left in the tube, carbon dioxide gone into the limewater. The tube is lighter by exactly the mass of the gas that left.
calcium carbonatemakescalcium oxide + carbon dioxide
Quicklime in the tube and carbon dioxide gone. This is the reaction that makes cement possible, run in kilns the size of buildings.
sodium hydrogencarbonatemakessodium carbonate + carbon dioxide + water
Three products from one reactant. The carbon dioxide is the point in baking, and the water vapour is why a cake steams as it comes out of the oven.
Three different substances, three word equations, and the same shape every time: one on the left, more than one on the right. None of them needed anything added, all of them needed the flame kept on, and not one of them came back on cooling.
Key fact
Thermal decomposition breaks one compound into two or more substances using heat. One reactant, several products, nothing added — and it does not reverse when it cools.
Five changes · which are decomposition?
Heat is involved in all five. Only some are this reaction
Green copper carbonate is heated and turns black, giving off carbon dioxide.
Thermal decomposition. One compound in, two substances out, using heat and nothing else.
A candle is lit and the wax burns away.
Combustion, not decomposition. Two reactants — wax and oxygen — and the reaction releases heat rather than needing it.
Ice in a beaker is heated and melts to water.
A physical change. One substance in and the same substance out, and it reverses on cooling. No new substance was made at all.
Baking soda in a cake mixture is heated and the cake rises.
Thermal decomposition. The sodium hydrogencarbonate breaks down and the carbon dioxide it releases is trapped in the mixture.
Magnesium ribbon is heated in air and leaves a white powder.
Oxidation — two reactants joining, not one coming apart. The mass goes up here; in a decomposition it goes down.
Think again
“It went black, so the copper carbonate burnt.”
Things that go black in a flame usually have burnt. Commit before you read on.
Burning needs oxygen as a reactant. Do this reaction in a tube with no air in it and it works exactly the same — so nothing was burnt. The black is not soot and it is not charring: it is copper oxide, which is black because that is what copper oxide looks like.
The two reaction types point in opposite directions. In combustion, a substance joins with oxygen and the reaction releases energy. In thermal decomposition, a compound comes apart and the reaction absorbs energy for as long as you supply it. Both involve heat and a Bunsen burner, and that is the whole of their resemblance.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What is thermal decomposition?
Rung 2 · The one that catches people
A tube of copper carbonate is weighed, heated until the colour change is complete, and weighed again. What has happened to the mass, and why?
Rung 3 · Explain
A student heats copper carbonate and says "it burnt, that is why it went black". Give two pieces of evidence from this lesson that show they are wrong, and say what the black solid actually is.
Rung 4 · Take it somewhere new
Cement is made by decomposing limestone in kilns. Explain why this process releases carbon dioxide even if the kiln were heated with electricity from wind turbines, and why that makes the emissions hard to remove.
Key note
Thermal decomposition breaks one compound down into two or more substances using heat. One reactant, several products, and nothing added — so it works even where there is no air. It absorbs energy for as long as you supply it, the mass left behind falls because a gas escapes, and it does not reverse on cooling.
Going further
One thermal decomposition is carried out on an industrial scale that dwarfs everything else in this unit. Limestone is heated in kilns to around 900 °C to make calcium oxide — quicklime — which is the basis of cement. The reaction is unavoidable and it releases carbon dioxide from the rock itself, before you count the fuel used to heat the kiln. Cement production is responsible for something like 8% of the world's carbon dioxide emissions, and the chemistry means you cannot fix it by changing the fuel alone.
The same reaction type is why cakes rise, and why an airbag can inflate in thirty milliseconds. Sodium hydrogencarbonate in baking powder decomposes in the oven and the carbon dioxide it releases is trapped as bubbles in the batter. An airbag holds a solid, sodium azide, which decomposes to nitrogen gas fast enough to fill a bag before your head arrives — a reaction chosen precisely because one solid becomes an enormous volume of gas, on command.
Before this lesson
Next in this unit
At GCSE this becomes
- Endothermic reactions and reaction profiles, and thermal decomposition in the extraction of metals and the making of cement.
Where to next
Ask Mr Badmus AI
Still not sure why nothing burnt in the tube?
These are demonstrations, not things to try at home. Take the tube carrying the gas out of the limewater before the flame comes off, or cold liquid is drawn back into the hot test tube and cracks it.
Lesson content © MrBadmusAI.