Heating Substances

Why this skill matters

Heating is one of the most fundamental laboratory skills. From thermal decomposition to evaporation, from rate experiments to flame tests, safe and effective heating underpins dozens of IGCSE practicals. Paper 5 and 6 questions regularly ask you to describe a heating method, choose between Bunsen burner and water bath, or identify safety precautions.

Method and technique

Bunsen burner heating

1. Check that the gas tap and Bunsen burner tap are closed. Connect the burner to the gas supply with rubber tubing.
2. Close the air hole. Light a match or use a lighter, then turn on the gas. The yellow safety flame appears.
3. Open the air hole gradually to produce a blue flame. The hottest part is just above the tip of the inner blue cone.
4. For strong heating: place the object in the hottest part of the blue flame. For gentle heating: raise the object higher or use a gauze and tripod for even heat distribution.
5. When heating liquids in a test tube: hold the tube at an angle (about 45 degrees), point the opening away from people, and heat gently, moving the tube in and out of the flame. Never heat the bottom of a test tube directly, as this causes bumping (sudden violent boiling).
6. When heating a crucible: place on a pipe-clay triangle on a tripod. Heat strongly with a roaring blue flame.

Water bath heating

1. Fill a beaker about two-thirds full with water. Heat the water to the required temperature using a Bunsen burner or electric hotplate.
2. Place the vessel containing the substance into the hot water.
3. The substance heats gradually and evenly. The maximum temperature is limited to 100 degrees C (the boiling point of water).
4. Monitor temperature with a thermometer if precise control is needed.

Electric hotplate

Used when no naked flame is permitted (near flammable solvents). Place the vessel directly on the hotplate and control the temperature with the dial.

Common errors and how to avoid them

Heating flammable substances with a naked flame: Ethanol and other organic solvents must be heated with a water bath or electric hotplate, never directly with a Bunsen burner. The vapour can ignite.

Pointing the test tube at someone: When liquid boils suddenly, it can shoot out of the tube. Always point the mouth of the tube away from all people.

Not using a gauze: Without a gauze on the tripod, heat is concentrated on one spot. A gauze spreads the heat evenly across the base of the vessel, reducing the risk of cracking the glass.

Overheating during crystallisation: Boiling a solution vigorously produces small, impure crystals. Heat gently and stop when crystals begin to appear at the edge of the solution.

Marking points examiners look for

• Name the heating method and justify the choice (Bunsen burner vs water bath)
• For water bath: explain why (flammable substance, gentle heating, temperature control)
• Point the test tube opening away from people
• Use a gauze on the tripod for even heating of beakers
• Heat gently when instructed (crystallisation, warming solutions)
• State safety precautions: safety glasses, tie back long hair, use tongs for hot apparatus