Writing and balancing equations

Fold a sheet of paper into a plane, or tear it into bits: it is still paper. Burn it, and you get ash, a new substance with new properties. You can't turn the ash back into paper. That is a chemical reaction: new substances are made.

"How can I tell a reaction has happened?" Look for one of these signs:

• a change of state (a solid forms from two gases, for example);
• a change of colour;
• a gas given off (bubbles);
• a change in temperature (the beaker gets hot or cold).

When a magnesium ribbon burns, it gives a dazzling white flame and leaves a white powder, magnesium oxide. Why clean the ribbon with sandpaper first? Lying in air, it slowly gets a thin layer of oxide that gets in the way of burning.

In a reaction, atoms are only rearranged. None are made and none are lost. Think of the letters of LISTEN moved around to spell SILENT: the same six letters, a new word. This is why mass is conserved: the mass of the products equals the mass of the reactants.

We can write the reaction in words, as a word-equation: Magnesium + Oxygen → Magnesium oxide. With formulae it becomes Mg + O₂ → MgO. Count the oxygen atoms: 2 on the left, 1 on the right. This is a skeletal equation: not yet balanced.

To balance it, change only the numbers in front, never the formulae. H₂O can never become H₃O; that would be a different substance. 2Mg + O₂ → 2MgO now has 2 Mg and 2 O on each side.

The hit-and-trial method, step by step, with Fe + H₂O → Fe₃O₄ + H₂:

• Make a small table: how many atoms of each element on the left, and on the right.
• Start with the compound that has the most atoms (Fe₃O₄). It has 4 O, so write 4H₂O.
• Now H: 8 on the left, so write 4H₂ on the right.
• Last, Fe: 3 on the right, so write 3Fe.
• 3Fe + 4H₂O → Fe₃O₄ + 4H₂. Check every element once more.

Say it in your head as you go ("oxygen on the left, times 4"). It stops silly mistakes. And if your first try doesn't work, that is normal: the method is called hit-and-trial because you try again.

Finally, make the equation tell more:

Conditions such as heat, sunlight or a catalyst are written on the arrow.