Build It Up: Combination Reactions
Combine elements and compounds into new substances.
What this lesson covers
Why it matters
Watch iron filings glow and fuse into a single black magnetite solid. Two elements vanish to create one new compound.
Predict first
What defines a direct combination reaction?
In synthesis, many become one. The atoms rearrange to build a larger, single molecule or formula unit.
- One reactant breaks into two or more products
- Two or more reactants combine to form a single product — correct
- Two compounds swap partners to form two new compounds
- A solid dissolves in a liquid without changing chemically
What you do
Run all three combinations: burn iron in oxygen, let ammonia gas meet hydrogen chloride, and drop sodium oxide into water. Each time, two substances fuse into ONE new product — the signature of a combination reaction.
Check yourself
When ammonia gas meets hydrogen chloride gas, what is the visible sign of the reaction?
NH₃ + HCl → NH₄Cl. The white smoke or ring is the solid salt formed from two gases.
Why is heat required for the reaction between iron and oxygen?
3Fe + 2O₂ → Fe₃O₄. Heat initiates the vigorous combination that produces black magnetite.
In the reaction Na₂O + H₂O → 2NaOH, what type of reaction is this?
Two reactants (sodium oxide and water) combine to form a single product (sodium hydroxide).
- A white ring of solid ammonium chloride forms — correct
- The gases turn blue
- No change occurs because both are gases
- A black precipitate forms
- To provide activation energy for the combination — correct
- To melt the oxygen into a liquid
- Iron does not react with oxygen at all
- To remove the black coating from the iron
- Decomposition
- Combination — correct
- Displacement
- Neutralization