Guided lesson

Word & Chemical Equations

4CH1-1.25

3 guided lessons · 27 checks · 1. Principles of Chemistry → Chemical Formulae, Equations, Calculations

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Conservation of mass

A chemical reaction makes new substances, yet no atom disappears: every atom present before the change is still there afterwards, only joined up in different ways. Mass is therefore never lost. That principle is the Law of Conservation of Mass, and it is what allows a chemical equation to balance at all.

What you know already

You already know from Atomic Structure that matter is built from atoms, and that substances have names and formulae that say what is joined to what. This note turns that knowledge into the language of reactions: the word equation that describes change in names, and the balanced symbol equation that carries state symbols and exact atom counts.

Key points of understanding

New substances form in a reaction, but the same atoms are present before and after.
Atoms cannot be created or destroyed, so every atom in the reactants must appear in the products.
The total mass of the reactants always equals the total mass of the products.
This principle is the Law of Conservation of Mass, and it is what makes balancing possible.

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1.Conservation of mass

4CH1-1.25

Key points

New substances form in a reaction, but the same atoms are present before and after.
Atoms cannot be created or destroyed, so every atom in the reactants must appear in the products.
The total mass of the reactants always equals the total mass of the products.
This principle is the Law of Conservation of Mass, and it is what makes balancing possible.

First pass

Nothing disappears in a reaction. Every atom you start with is still there at the end, so the mass you can weigh before equals the mass you can weigh after.

Guided content from the SME authoring corpus — note rn_JCfdFrHsCkrmMbFj, pinned at 78ad6a2073c6 · promoted SyllabAI/syllabai-resources e967e75f1339

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2.How to write word equations

4CH1-1.25

A word equation describes a reaction in full names: reactants on the left, products on the right, an arrow between them read as to form. Sodium hydroxide reacting with hydrochloric acid becomes sodium hydroxide + hydrochloric acid → sodium chloride + water. No formulae are needed yet — only the names, placed on the correct side of the arrow.

3.How to write balanced equations

4CH1-1.25

A symbol equation uses formulae instead of names, and it must balance: the same number of atoms of every element on both sides. Write the diatomic non-metals as molecules — H2, N2, O2, F2, Cl2, Br2, I2 — and add state symbols when they are asked for. Work left to right, one element at a time, and leave elements standing alone until last.