# Question #e2119

##### 1 Answer

Pressure had to **decrease** in order for the volume to **increase**.

#### Explanation:

The important thing to notice here is that temperature is being kept **constant**.

Assuming that the *amount* of gas you have remains **constant** as well, i.e. you don't add or remove gas from the container, then you can use the ideal gas law equation to write

*for the first measurement*

*for the second measurement*

If you replace the product **constant**, in one of these two equations you'll get

This is the mathematical expression for Boyle's Law, which states that pressure and volume have an *inverse relationship* when temperature and number of moles (amount of gas) are ket constant.

An inverse relationship means that if one **increases**, the other must **decrease** and vice versa.

Even before doing any calculations, you can use Boyle's Law to predict what will happen to the pressure. If volume **increased** from **50** to **75 L**, then the pressure *musht have decreased* proportionally.

You can confirm this by

The pressure indeed **decreased**, which corresponds to the **increase** in volume.

So, as a conclusion, when the temperature of the gas is constant, i.e. the *average kinetic energy* of the gas molecules remains unchanged, the volume the gas occupies can only **increase** if pressure **decreases**.

Likewise, the pressure of the gas can only **decrease** if the volume of the gas is **increased**.