When the water vapour in the air cools down overnight, it will often _________ and form dew.

What this blank is asking

The sentence describes how dew forms when the air cools overnight. So the missing word should name the phase change from water vapour (gas) to liquid water.

Linking dew to a change of state

Dew is made of tiny liquid water droplets that appear on cool surfaces (grass, cars, windows). Those droplets come from water vapour in the air.

The correct term

When a gas changes into a liquid, the process is called condensation. In verb form, water vapour will condense.

So the sentence becomes: water vapour will often condense and form dew.