What happens
A metal above hydrogen in the reactivity series reacts with a dilute acid to give a salt and hydrogen gas.
metal + acid → salt + hydrogen
Word equation
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)
Zn(s) + 2CH3COOH(aq) → (CH3COO)2Zn(aq) + H2(g)
Zinc chloride, zinc sulfate (sulphate in NCERT books) and zinc ethanoate are the salts.
Observations to write: effervescence, the zinc gets smaller and dissolves, the solution stays colourless, the flask feels warm.
Why zinc reacts and copper does not
A more reactive metal displaces a less reactive element from its compound. Zinc is above hydrogen, so it pushes hydrogen out of the acid. Copper is below hydrogen, so there is no reaction with dilute acid.
KNaCaMgAlZnFePbHCuAgAuMost reactive on the left, least reactive on the right. Metals left of H react with dilute acids.
Ionic equation and redox
Zn(s) + 2H+(aq) → Zn2+(aq) + H2(g)
Chloride or sulfate ions do not change. They are spectator ions.
- Zinc loses two electrons: Zn → Zn2+ + 2e−. This is oxidation.
- Hydrogen ions gain electrons: 2H+ + 2e− → H2. This is reduction.
- Both happen together, so the reaction is a redox reaction and a displacement reaction.
Test for hydrogen
Hold a burning splint at the mouth of the gas. Hydrogen burns rapidly with oxygen from the air and gives a squeaky pop.
2H2(g) + O2(g) → 2H2O(l)
Use a burning splint, not a glowing one. A glowing splint relights in oxygen. In this lab the gas is trapped in soap bubbles so the pop is small and safe. Never test large amounts of hydrogen.
Why the flask warms
Energy is transferred to the surroundings, so the temperature of the mixture rises. This is an exothermic reaction. With dilute acid the rise is a few degrees, enough to feel on the glass. The temperature then falls back as the zinc is used up and the flask loses energy to the room.
Strong and weak acids
| Acid | Ionisation | Effect with zinc |
|---|
| HCl | Fully ionised, one H+ per formula unit | Steady bubbling, clear warming |
| H2SO4 | Fully ionised, up to two H+ per formula unit | Faster than HCl of the same concentration |
| CH3COOH | Partly ionised, few H+ ions | Slow bubbling, small temperature rise |
The rate depends on the concentration of H+ ions, not just on the amount of acid. Strong and weak describe ionisation. Concentrated and dilute describe how much acid is in the water. They are different ideas.
What changes the rate
- Concentration: more H+ ions means more collisions with zinc.
- Surface area: zinc powder reacts faster than granules or a strip.
- Temperature: warmer acid means faster, more energetic collisions.
- A few drops of copper(II) sulfate solution speed up the reaction. Zinc displaces copper, and the copper on the zinc helps hydrogen form.
Going further
- Dilute nitric acid does not give hydrogen with most metals. It is a strong oxidising agent and oxidises the hydrogen to water.
- Zinc also reacts with warm sodium hydroxide solution to give hydrogen and sodium zincate. Zinc oxide and aluminium oxide are amphoteric.
- Syllabus links: CBSE class 10 science chapters 1, 2 and 3 (reaction of zinc with acid, acids with metals, reactivity series). IGCSE 0620: acids, bases and salts, reactivity series, exothermic reactions, tests for gases.
Write it like the mark scheme
- Say “effervescence” or “bubbles of a colourless gas”, then name the gas: hydrogen.
- Give the test and the result: “burning splint, squeaky pop”.
- Include state symbols and balance the equation.
- Say “exothermic” and link it to the temperature rise.