What equations show

A chemical reaction starts with reactants and forms products. In an equation, reactants are written on the left and products on the right:

A word equation names the substances without using chemical formulae.

For example, if magnesium reacts with oxygen to form magnesium oxide:

The plus sign on the left means magnesium and oxygen are both reactants. The arrow means "react to form". The substance after the arrow is the product.

A symbol equation gives more detail because it uses chemical formulae:

This is not yet a good final equation because the number of oxygen atoms is different on the two sides. The formulae tell us which atoms are present, but the equation still needs balancing.

Formulae and state symbols

A balanced chemical equation uses formulae. In unfamiliar reactions, the question should give the information needed to choose the formulae, so use the formulae exactly as given before you try to balance the equation.

A good route is:

1. Identify the reactants and products.
2. Write the word equation.
3. Replace names with correct formulae.
4. Add state symbols if the question gives physical states or asks for them.
5. Balance the equation by changing numbers in front of formulae.

State symbols show the physical state of each substance:

For example, the change of liquid water to ice is:

For reactions in solution, (aq) matters. Aqueous sodium chloride is written as NaCl(aq), while solid sodium chloride is written as NaCl(s).

State symbols do not balance an equation. They only describe the substances once the formulae and coefficients are correct.

Balancing equations

A balanced equation has the same number of atoms of each element on both sides. This follows conservation of atoms: chemical reactions rearrange atoms, but they do not create or destroy atoms.

For example, in 2H2O, the coefficient 2 means there are two H2O particles. Because each H2O contains two H atoms and one O atom, 2H2O contains four H atoms and two O atoms in total.

The most important balancing rule is simple: change coefficients, not the formulae. If the correct formula is H2O, do not turn it into H2O2 to make the oxygen count work. That would change the substance from water to hydrogen peroxide.

Worked example: balance the reaction between hydrogen and oxygen to form water.

Start with the correct formulae:

Count atoms:

Oxygen is not balanced. Put 2 in front of H2O:

Now oxygen is balanced, but hydrogen is not:

Put 2 in front of H2:

Final check:

The equation is now balanced.

Calculating relative formula mass

Relative formula mass is calculated from relative atomic masses. In an exam, use the Ar values from the Periodic Table or from the question.

For substances made of molecules, the same calculation may be called relative molecular mass. The symbol is still Mr. Relative formula mass is the safer general term because it works for molecular substances such as water and for ionic substances such as sodium chloride.

Relative formula mass has no unit. It is a relative value, not a mass in grams.

Worked example 1: calculate the Mr of water, H2O. Use Ar values H = 1 and O = 16.

Worked example 2: calculate the Mr of calcium hydroxide, Ca(OH)2. Use Ar values Ca = 40, O = 16 and H = 1.

The bracket means there are two OH groups:

Brackets are a common place to lose marks. Multiply everything inside the bracket by the number outside it.

Exam precision

Pearson-style questions often reward equation work in separate pieces: correct formulae, correct balancing, and correct state symbols where asked. Do the work in that order.

For example:

The formulae are correct, but the equation is not balanced. Oxygen has two atoms on the left and one on the right. Balance it with coefficients:

Then add state symbols if the reaction information supports them:

You can also use Mr values as a check that atoms have been conserved. For the balanced equation:

Left side relative mass:

Right side relative mass:

This is only a check in this lesson. The required skill here is writing balanced equations and calculating formula masses.

Common exam traps:

• changing a correct formula instead of adding a coefficient
• forgetting that a coefficient multiplies the whole formula
• using O2 = 16 instead of O2 = 32
• missing brackets in formulae such as Mg(OH)2 or Pb(NO3)2
• adding state symbols when asked, but putting them on the wrong substance

Use that order when a question combines equation writing with a formula-mass calculation.