Two (or more) become one

The idea

A reaction in which two or more substances (elements or compounds) combine to form a single product is a combination reaction.

Slaking lime

Quicklime (calcium oxide, CaO) reacts vigorously with water. The product is slaked lime (calcium hydroxide, Ca(OH)₂), used to whitewash walls.

How many substances go in, and how many come out?

• Two go in, one comes out
• One goes in, two come out
• Two go in, two come out

Run three reactions

Press the button. Watch the blocks slide together. Count how many reactants and how many products there are.

Many in, one out

A reaction in which two or more substances (elements or compounds) combine to form a single product is a combination reaction.

The reactants can be elements, compounds, or one of each.

• Reaction | Reactants | Product
• Quicklime + water | 2 compounds | Slaked lime
• Burning of coal | 2 elements | Carbon dioxide
• Hydrogen + oxygen | 2 elements | Water

Notes

A reaction in which two or more substances (elements or compounds) combine to form a single product is a combination reaction.

Check yourself

Which one is a combination reaction?

In 2H₂ + O₂ → 2H₂O, how many different products are formed?

Answer: 1

Only water is formed. One product, so it is a combination reaction.

In CaO + H₂O → Ca(OH)₂, are the reactants elements or compounds?

• C + O₂ → CO₂ — correct. Yes! Two substances, one product.
• CaCO₃ → CaO + CO₂. Here one substance breaks into two. That is the opposite of a combination.
• Fe + CuSO₄ → FeSO₄ + Cu. Two products come out here, so it is not a combination.
• Both are elements. CaO has calcium and oxygen; H₂O has hydrogen and oxygen. Each is a compound.
• Both are compounds — correct. Yes! CaO and H₂O are each made of two elements. The reactants may be elements or compounds.
• One element and one compound. Both CaO and H₂O are compounds here.