Word and balanced symbol equations, conservation of mass, balancing equations, ionic equations, and explaining apparent mass changes in open systems.

What this dot point is asking

OCR wants you to write word and balanced symbol equations, apply the law of conservation of mass, balance equations correctly, write simple ionic equations, and explain why mass can appear to change in an open container even though it is always conserved overall. Equations and conservation of mass run through the whole of chemistry.

Word and symbol equations

For example, the word equation magnesium + oxygen produces magnesium oxide becomes the symbol equation $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$. The arrow means "reacts to form". State symbols ($\text{s}$, $\text{l}$, $\text{g}$, $\text{aq}$) can be added to show the physical states.

Conservation of mass

Balancing equations

To balance a symbol equation:

1. Write the correct formulae for all reactants and products (do not change these).
2. Count the atoms of each element on both sides.
3. Add balancing numbers in front of formulae to make each element equal.
4. Recount to check every element balances.

The golden rule is that you may only change the big numbers in front of formulae, never the small subscript numbers inside a formula, because changing a subscript changes the substance.

Ionic equations

Apparent changes in mass

Although mass is always conserved, it can appear to change in an open container:

• If a reaction gives off a gas (for example a carbonate releasing carbon dioxide), the gas escapes into the air, so the mass of what is left appears to decrease. The total mass, including the gas, is unchanged.
• If a reaction takes in a gas (for example a metal reacting with oxygen from the air), the gas joins the solid, so the mass appears to increase.

In a sealed container, no gas can enter or leave, so the measured mass stays constant.