Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

The effect of heating common salt and paraffin-wax

Demonstration

These two materials show contrasting effects when heated.

Apparatus and materials

Power supply, low voltage, variable

Demonstration ammeter, 1 A

Rheostat

Small crucible

Pipe-clay triangle

Tripod

Bunsen burner

Common salt

Paraffin wax

Copper wire, stiff

Leads and crocodile clips

Health & Safety and Technical notes


The melting point of sodium chloride is -800°C. Wear eye protection when heating the crucible and salt and use a safety screen to protect observers.

Adjust the circuit resistance so the current flow is no more than about 100 mA, and ensure sensible levels of ventilation to disperse chlorine gas given off.

The quantity of chlorine released should be very low, but be aware of any pupils with asthma.

There will be a tiny amount of metal sodium on the negative electrode that will need care when disposing of the waste.

 

Procedure


a Place a small amount of salt in the bottom of the crucible. Support two stiff copper wires so that they reach the bottom of the crucible and make electrical contact with the salt. Connect up the circuit. 

a circuit

b After noting that solid salt does not conduct electricity, remove the electrodes from the salt (they are excellent thermal conductors and will remove too much energy), heat the crucible strongly until the salt melts.

c Replace the electrodes and adjust the rheostat to show a current of about 1 amp. Remove the burner and allow the salt to cool; current rapidly falls to zero.

d Repeat the experiment using paraffin-wax instead of salt.

 

Teaching notes


1 Sodium chloride behaves as an insulator until it is nearly at its melting point, and then it readily conducts. Once it is melted, it behaves like an electrolyte and sodium and chlorine ions drift in opposite directions to the electrodes. This provides a method for producing pure sodium from sodium chloride. 

2 Paraffin-wax remains an insulator even when it has melted. It is made up of long-chain molecules consisting of carbon and hydrogen atoms. Both ends of the molecule are inactive and so are unlikely to carry an electrical charge.

This experiment was safety-checked in December 2006