The BBC Bitesize guide to changes of state contains text, infographics and audio to explain the concepts, plus more chemistry revision notes written to the Edexcel GCSE chemistry specification.

\({\%~of~an~element} = \frac{mass~of~an~element~in~the~compound}{relative~formula~mass~M_r} \times 100\) There are six atoms of hydrogen in the compound, so the mass of hydrogen is 6 × 1 = 6.

Exam board Edexcel has launched an investigation into how part of an A-level maths paper was leaked online. Blacked-out images of two questions were shared on social media on Thursday afternoon.