Thermal decomposition

Heating copper carbonate: follow the evidence

Step through it in order: what goes in, what you see, how you prove the gas, and how you write it down.

1. 1Just one substance goes inPut a little green powder, copper carbonate, into a boiling tube. That's it. Nothing else is mixed with it. Most reactions you've met needed two substances to react together. This one starts with a single compound.
2. 2Heat is the triggerNow heat the tube. The heat isn't a second ingredient. It's the input that makes the copper carbonate start to break apart. A decomposition caused by heating is called thermal decomposition. (Thermal just means to do with heat.)
3. 3Green turns blackWatch the powder: the green turns black. That black solid isn't scorched copper carbonate. It's a new substance called copper oxide. A new substance is your first clue that a chemical reaction has happened.
4. 4A gas escapesAt the same time, a gas is given off. A delivery tube carries it away and dips below the surface of some limewater, so the gas bubbles through. Bubbles tell you there's a gas. They don't tell you which gas. That needs a test.
5. 5The limewater turns cloudyLimewater starts off colourless. If the gas is carbon dioxide, the limewater turns cloudy (milky white). If it isn't, the limewater stays colourless. Here it goes cloudy, so the gas is carbon dioxide.
6. 6Write it downOne reactant in, two products out: copper carbonate —heat→ copper oxide + carbon dioxide. The copper carbonate sits on the left on its own. The word heat sits on the arrow, because heat is what starts the reaction. The two products go on the right.+

Stability

Do all carbonates break down as easily?

Five carbonates were each heated in a boiling tube fitted with a bung and a delivery tube dipping below the surface of limewater. Each was heated gently at first, then strongly, with fresh limewater every time. The colour changes, and how quickly the limewater turned cloudy, were recorded. Sort each carbonate by what happened.

5 of 5 still to sort.

In chemistry, stable means not particularly reactive. The longer a carbonate takes to decompose, the more stable it is.

What you need to know

• Thermal decomposition is a reaction in which one compound breaks down into two or more simpler substances when it is heated. Unlike many reactions, it doesn't need two substances to be mixed together.
• The input that starts a decomposition is often written over the arrow: copper carbonate —heat→ copper oxide + carbon dioxide.
• When a metal carbonate decomposes on heating, it typically forms a metal oxide and carbon dioxide. Green copper carbonate turns into black copper oxide.
• The test for carbon dioxide: bubble the gas through limewater. It turns from colourless to cloudy if the gas is carbon dioxide, and stays colourless if it isn't.

The big picture

Thermal decomposition is a reaction in which one compound breaks down into two or more simpler substances when it is heated. Heat green copper carbonate and it becomes black copper oxide, giving off carbon dioxide, which turns limewater cloudy. In the word equation, heat sits on the arrow because it starts the reaction. Carbonates differ in stability: copper carbonate decomposed very easily, while sodium and potassium carbonate did not decompose at all when tested.

⚠ Watch out

Writing the metal as a product, as in 'copper carbonate —heat→ copper + carbon dioxide'. When a metal carbonate decomposes on heating, the metal ends up in a metal oxide, so the product is copper oxide.

Memory hook

One in, two out, heat on the arrow. And for copper carbonate: green goes in, black comes out, and the limewater goes milky.

Check yourself

Without looking back: write the word equation for heating copper carbonate, with heat in the right place. Which carbonate tested was least stable, and which two were most stable?