Formulae and equations — GCSE Chemistry practice questions

Question 1

A weather balloon is filled with hydrogen or helium and rises through air that contains oxygen.

Which row gives the correct formulae of these three gases?

Accepted Answer

B: O₂ · H₂ · He. Oxygen and hydrogen gases are made of molecules of two atoms, O₂ and H₂. Helium is a noble gas, so it exists as single atoms, He.

Question 2

Limestone chips react with dilute nitric acid in a beaker. Bubbles form and the chips slowly disappear.

CaCO₃ + 2HNO₃ → Ca(NO₃)₂ + H₂O + CO₂

Which row gives the correct state symbols for three of the substances?

Accepted Answer

C: (s) · (aq) · (l). The limestone chips are a solid (s). Calcium nitrate dissolves in the water, so it is aqueous (aq). Water itself is a liquid (l), never (aq).

Question 3

A student writes four ionic equations for displacement reactions.

1. Zn + Cu²⁺ → Zn²⁺ + Cu
2. Fe + 2Ag⁺ → Fe²⁺ + Ag
3. Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂
4. Al + Cu²⁺ → Al³⁺ + Cu

Which equations are fully balanced?

Accepted Answer

B: 1 and 3 only. In 1 and 3 the atoms and the total charges (2+ and 2−) match. Equation 2 has two silver atoms on the left and one on the right. Equation 4 has the same atoms, but the charge is 2+ on the left and 3+ on the right.

	oxygen	hydrogen	helium
A	$\ce{O}$	$\ce{H}$	$\ce{He}$
B	$\ce{O2}$	$\ce{H2}$	$\ce{He}$
C	$\ce{O2}$	$\ce{H2}$	$\ce{He2}$
D	$\ce{O2}$	$\ce{H}$	$\ce{He}$