Question 24
Describe one experiment to show that liquids expand on heating.
The following experiment demonstrates that liquids expand on heating:
- Take an empty bottle with a tight fitting cork with a hole drilled in its middle, a drinking straw, two bricks, a wire gauze and a burner.
- Fill the bottle completely with water. Add few drops of ink to make it coloured.
- Fix the cork in the mouth of the bottle. Pass the drinking straw through the cork. Put some molten wax around the hole to avoid leakage of water.
- Pour some more water in the drinking straw so that water level in the straw can be seen. Mark the water level in the straw as shown in figure.
- Place the bottle on the wire gauze kept over the two bricks. Heat the bottle using a burner as shown in figure.
- Look at the level of water in the straw.
Conclusion — When water is heated more and more, the level of water in the drinking straw rises. This shows that water expands on heating.
ICSE Class 7 Physics — Heat: Complete Study Guide
Heat is a high-weightage chapter in ICSE Class 7 Physics, carrying approximately 14 marks. It covers the distinction between heat and temperature, thermometry, effects of heat (expansion and change of state), and the three modes of heat transfer. This chapter is rich in both conceptual understanding and numerical problem-solving, making it a favourite for exam questions.
The most critical distinction students must grasp is between heat (a form of energy measured in joules) and temperature (the degree of hotness measured in degrees or kelvin). Heat always flows from a body at higher temperature to one at lower temperature. Students learn to use and compare clinical and laboratory thermometers, and master temperature conversion formulas between Celsius, Fahrenheit, and Kelvin scales.
The chapter explores how heat causes solids, liquids, and gases to expand (with the fascinating exception of water's anomalous expansion below 4 degrees Celsius), how substances change state (melting, boiling, evaporation, condensation, sublimation), and how heat is transferred through conduction (solids), convection (fluids), and radiation (no medium needed). Real-life applications like the thermos flask, sea breeze formation, and cooking utensil design make this chapter highly relatable.
| Formula | Details |
|---|---|
| °F = (9/5 × °C) + 32 | Celsius to Fahrenheit |
| °C = 5/9 × (°F − 32) | Fahrenheit to Celsius |
| K = °C + 273 | Celsius to Kelvin |
Must-Know Concepts
- Heat is energy (J); temperature is degree of hotness (°C/°F/K)
- Clinical thermometer: 35-42°C, has kink. Lab thermometer: -10 to 110°C, no kink
- Expansion: gases > liquids > solids. Water anomalous below 4°C
- Conduction (solids), Convection (fluids), Radiation (no medium needed)
- Thermos flask prevents all three modes of heat transfer
Common Mistakes
- Saying "cold flows into your hand" — heat flows FROM your hand to the cold object
- Wrong formula order: °C = 5/9(°F − 32) — subtract 32 FIRST, then multiply
- Confusing evaporation (surface, any temp) with boiling (throughout, fixed temp)
- Saying "wool produces heat" — wool traps air (insulator), preventing YOUR heat from escaping
Exam Tips
- Show full working in temperature conversions — formula, substitution, calculation, answer with unit
- Use comparison tables for heat vs temperature, clinical vs lab thermometer
- Draw labelled diagrams for thermos flask, sea breeze/land breeze
- The -40° question (where C and F are equal) is a common exam question — practise it
What is the difference between heat and temperature?
Heat is a form of energy measured in joules, while temperature is the degree of hotness or coldness measured in °C, °F, or K. Heat depends on mass, material, and temperature. A large bucket of warm water has more heat energy than a small cup of boiling water, even though the cup has higher temperature.
Why does a metal bench feel colder than a wooden bench on a cold morning?
Both are at the same temperature (room temperature). Metal is a good conductor of heat, so it quickly conducts heat away from your body, making it feel cold. Wood is a poor conductor (insulator), so it conducts heat away slowly, feeling less cold. The difference is in the rate of heat transfer, not the actual temperature.
What is anomalous expansion of water?
Most substances contract when cooled. Water contracts normally from 100°C down to 4°C, but between 4°C and 0°C it expands instead of contracting. This means water has maximum density at 4°C. This is why ice (less dense) floats on water, and why fish can survive in frozen ponds — water at the bottom stays at 4°C.
How does a thermos flask work?
A thermos flask minimises heat transfer by all three modes: the vacuum between double walls prevents conduction and convection (no material for heat to travel through), the silvered inner walls reflect radiation (preventing heat from escaping or entering), and the cork stopper acts as an insulator. This keeps hot liquids hot and cold liquids cold.