Question 38
In summer, ice is kept wrapped in a gunny bag. Explain the reason.
The air filled in the fine pores of the gunny bag is insulator of heat. The air does not allow heat from outside to pass through it to the ice thus preventing the ice from melting rapidly. So in summer, ice is kept wrapped in a gunny bag.
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.