Formulae & Hazards

Formulae & Hazards ⚗️

Chemists write everything in a compact code. A **formula** shows which atoms are in a substance and how many: **H2O** is two hydrogen atoms and one oxygen atom; **CO2** is one carbon and two oxygen. The small **subscript** number tells you how many of the atom just before it. Get comfortable reading formulae and the rest of chemistry falls into place.

Balancing equations ⚖️

In a reaction, atoms are only rearranged, never created or destroyed, so **mass is conserved**. That means a **balanced equation** has the **same number of each atom on both sides**. You balance it by putting big **numbers in front** of the formulae, for example 2H2O means two water molecules. You must **never** change the small subscript numbers, as that would change the substance.

Find the number

An interactive activity.

Which one balances?

Which of these equations is correctly balanced, with the same number of each atom on both sides?

• 2H2 + O2 gives 2H2O
• H2 + O2 gives H2O
• H2 + O2 gives 2H2O
• 2H2 + 2O2 gives 2H2O

Match each state symbol

The ions that watch

An **ionic equation** shows only the ions that actually change. When an acid neutralises an alkali, H+ + OH- gives H2O, ions such as Na+ and Cl- are left out because they do not react. What are these unreacting ions called? (Higher tier.)

• Spectator ions
• Reactant ions
• Catalyst ions
• Product ions

Working safely ⚠️

Chemicals carry **hazard symbols** so you know the risks at a glance: - **Corrosive**: attacks skin, eyes and materials. - **Flammable**: catches fire easily. - **Toxic**: can cause death or serious harm if taken in. - **Oxidising**: provides oxygen so other things burn fiercely. Safe practical work also means wearing eye protection, handling chemicals carefully and following the method.

Reading the warnings

Select ALL THREE statements about hazards and safety that are TRUE.

• The corrosive symbol warns that a substance can attack skin and eyes
• The flammable symbol warns that a substance catches fire easily
• You should wear eye protection when heating chemicals
• The toxic symbol means a substance is safe to taste
• Hazard symbols are only there for decoration
• You should sniff a gas directly to identify it

Chemistry summary

A balanced equation has the same number of each type of _____ on both sides, because mass is conserved. You balance it by putting numbers in _____ of the formulae, never changing the small numbers. State symbols show whether something is a gas, using _____, or dissolved in water, using _____.