# Change of volume with nitrogen

##### Demonstration

This shows the volume change as liquid nitrogen changes into a gas.

#### Apparatus and materials

Method 1

A supply of liquid air (or liquid nitrogren)

Chemical balance

Method 2

Trough

Water

Measuring cylinder (must have a volume 750 times greater than that of the test tube)

Rubber tubing

Small test tubes, similar, 2

A supply of liquid air and liquid nitrogen

#### Health & Safety and Technical notes

A special risk assessment is required for this activity. LEA schools and other subscribers can obtain one from the CLEAPSS School Science Service (01895 251496, e-mail science@cleapss.org.uk).

This will require eye protection and non-absorbent leather gloves for setting up, and rubber gloves when collecting the gas.

Liquid nitrogen presents the following particular risks:

a asphyxiation in oxygen-deficient atmospheres

b fire in oxygen-enriched atmospheres

c cold burns, frost bite and hypothermia from the intense cold

d over-pressurisation from the large volume expansion of the liquid

e manual-handling accidents if using 25 $l$ volumes.

With step c once the nitrogen has started boiling, it is not possible to stop it whilst one refills a box or cylinder with water. Therefore it is necessary to have two or more receivers ready.

#### Procedure

Method 1 - Indirect measurement of volume change

a Pour liquid air or nitrogen quickly into the 100 ml measuring flask of known mass.

b Once the jar has been cooled sufficiently, the violent bubbling will cease. Add more liquid to top the level up to the 100 ml mark so that the volume is known.

c Weigh the whole on the balance and find the mass of liquid.

d Calculate the density of liquid nitrogen.

e Measure the density of air (Air is 80% nitrogen. Oxygen and nitrogen have a similar density.)

Method 2 - Direct measurement of the volume change

a Fill the trough and measuring cylinder with water. Invert the measuring cylinder in the trough.

b Find the volume of the test tube. Do this by filling it with water and measuring the volume of water.

c Pour liquid nitrogen into the second, dry test tube. When the liquid has stopped bubbling, top up the test tube and quickly fit the rubber tubing.

d Insert the rubber tube into the open end of the measuring cylinder and collect the nitrogen gas.

e Compare the ratio of the volume of nitrogen gas with the volume of liquid nitrogen of the same mass.

#### Teaching notes

1 Expect a change in volume of about 1 to 750. 10 ml of liquid nitrogen will release several litres of gas very quickly. Catching the gas with take some practice.

The density of liquid air/nitrogen is about 90 g/100 ml which is 900 kg/m3. The density of air/nitrogen is about 1.2 kg/m3. Hence the volume change when liquid nitrogen becomes gaseous nitrogen is 900/1.2 = 750.

2 If there is no liquid air/nitrogen available, then solid carbon dioxide can be allowed to sublime. A small solid lump of carbon dioxide can be put into a measuring tube filled with water, and the volume of the gas measured. It is better to get the carbon dioxide from a solid block because that produced by a cylinder is not compacted enough.

This experiment was safety-checked in November 2006.