Heating substances

Read our standard health & safety guidance

Refer to CLEAPSS Laboratory Handbook, Section 13.2.4 and Student Safety Sheet 92

Eye protection should be worn whenever a substance is heated

Heating substances is always exciting but it is essential to keep the amounts used to a minimum and to use the correct apparatus in the recommended manner.

Although improvised containers can be used, the notes here refer only to the pieces of laboratory apparatus most commonly used for heating substances:
ignition tubes (small test-tubes)
test-tubes
boiling tubes (large test-tubes)
beakers
evaporating basins
crucibles

Heating solids in test–tubes

Wear eye protection.

Only fill to a maximum of 1/5 full.
Use a suitable test-tube holder.
Hold the test-tube at a slight angle (see diagram).
Ensure that the open end of the test-tube isn’t pointing directly at anybody.
Hold the test-tube so that the bottom is just in the tip of the flame.
Always start heating with a small, gentle flame.

Heating liquids in test tubes

Wear eye protection.

Use a boiling tube (wide diameter) and do not fill to more than 1/10 full.
Add an anti-bumping granule to give smoother boiling. Add the granule before starting to heat.
Use a suitable holder.
Hold the tube at an angle so that the top is well away from the flame.
Hold the test-tube so that the bottom is just in the tip of the flame.
Keep the liquid in the tube moving gently.
For flammable liquids, use a water bath.

Heating flammable liquids

Wear eye protection

Use a boiling tube (wide diameter) and do not fill to more than 1/10 full.
Add an anti-bumping granule to give smoother boiling. Add the granule before starting to heat.
DO NOT heat directly over a naked flame.
Stand the tube in a beaker of hot water (e.g. from a kettle or hot tap).

Heating in beakers (and conical flasks)

Beakers should only be filled to 1/3 of their capacity when used for heating liquids. The addition of a few ‘anti-bumping’ granules will ensure smoother boiling.

Heating in evaporating basins

A flat-bottomed evaporating basin can be heated by supporting it on a wire gauze on a tripod. A round-bottomed evaporating basin is very unstable on a wire gauze so should be supported on a pipe-clay triangle when heating.

Evaporating basins should be filled to between 1/3 and 1/2 full.

When evaporating salt solutions, the solution should be heated (with occasional stirring using a glass rod), until solid just appears evenly around the edge of the liquid. The solution can then be left to cool - possibly overnight (labelled with names of the owners, the chemicals and any relevant safety warnings).

Heating in crucibles

A crucible must be heated on a pipe-clay triangle and not on a gauze. Start with a small, gentle flame before gradually increasing the heating rate. Allow plenty of time in the lesson for crucible and contents to cool down.

Health and safety checked, November 2007