From Atoms to Materials

oxygen water An arrow pointing to the right means ‘changes to’ or ‘forms’. hydrogen The chemical formulae for hydrogen, oxygen and water are: • hydrogen: H 2 • oxygen: O 2 • water: H 2 O chemical equation 化學反應式 + Extension chemical formula 化學式 H 2 O 2 H 2 O ✗ unbalanced + 2H 2 O 2 2 H 2 O ✓ balanced + Writing balanced chemical equations The formation of a compound from its constituent elements involves a chemical reaction. We can use a word equation to represent the reaction. Let’s take the formation of water from hydrogen and oxygen as an example. The word equation is written as: We can also write a chemical equation . In a chemical equation, we use chemical formulae* instead of names to represent the reactants and products. E If you count the number of atoms on each side, you may notice something wrong: the numbers of atoms on each side are not equal. Since atoms can neither be created nor destroyed, the numbers of atoms on both sides must be equal. We can revise this equation by adding some numbers: Now if you count the number of atoms, you will find equal numbers of each type of atoms on both sides. Let’s do Skill Training 13.4 to learn how to balance a chemical equation. reactant(s) on the left hand side arrow product(s) on the right hand side We have just balanced a chemical equation! There are two oxygen atoms on the left, but only one oxygen atom on the right! 55 13 From Atoms to Materials © United Prime Educational Publishing (HK) Limited 2024 All rights reserved; no part of this publication may be reproduced, photocopied, recorded or otherwise, without the prior written permission of the Publisher. Sample