Heat Transfer in Nature – Case-based Questions with Answers
Class 7
Science
Chapter 7
Heat Transfer in Nature — 20 Case-Based Questions (NCERT-aligned)
Each case includes focused questions and clear model answers, prepared strictly as per the NCERT syllabus for CBSE Class 7 board exam standard.
How to use: Read the short case, attempt the questions and then compare with the model answers provided. These cases connect concepts — conduction, convection and radiation — to everyday and natural phenomena.
Case 1 — Metal spoon in hot milk
A metal spoon is left in a cup of hot milk. After a short while the handle feels warm.
Q1. Identify the mode of heat transfer that warmed the spoon handle.
A1. Conduction — heat transferred through direct contact along the metal from the hot end immersed in milk to the handle.
Case 2 — Morning fog near a lake
On a calm morning, fog forms above a lake.
Q1. Which heat transfer processes led to fog formation?
A1. Radiative cooling at night cools the surface; moist air cools and water vapour condenses. Convection is limited due to calm conditions; condensation releases latent heat.
Case 3 — Black and white plates in sun
Two plates, one black and one white, are placed in sunlight. The black plate becomes noticeably hotter.
Q1. Explain why the black plate heats more.
A1. Black surface absorbs more solar radiation (higher absorption), converting it to heat — radiation is primary; conduction spreads heat within the plate.
Case 4 — Sea breeze on a beach
During the day, cool wind blows from sea to land near the coast.
Q1. Describe the heat-transfer-driven mechanism behind this breeze.
A1. Land heats faster by radiation, warming air above it which rises (convection); cooler, denser air from sea moves in — this creates a sea breeze.
Case 5 — Thermos keeping soup hot
A thermos flask keeps soup hot for hours.
Q1. How does the thermos reduce all three modes of heat transfer?
A1. Vacuum reduces conduction and convection (no medium); reflective inner surface reduces radiation; tight seals prevent convective air exchange.
Case 6 — Hot air balloon ascent
A hot air balloon rises when the burner warms the air inside the envelope.
Q1. Explain using convection and density concepts.
A1. Heated air becomes less dense, causing buoyancy; warm air rises and lifts the balloon — convection-driven density change causes ascent.
Case 7 — Night cooling of rooftops
A roof cools rapidly at night and the room below becomes colder.
Q1. Which modes of heat transfer explain the cooling and how can it be reduced?
A1. Radiation from roof to sky causes cooling; conduction through roof transfers heat out of house; limited convection at night. Insulation and reflective roofing reduce heat loss.
Case 8 — Vegetated vs bare soil
Two garden patches, one with plants and one bare, show different temperatures on a hot day.
Q1. Why is vegetated soil cooler?
A1. Plants shade soil (less radiation absorption), transpire (evaporation cools), and the soil retains moisture, all reducing heating compared to bare soil.
Case 9 — Lakes freezing from top
In winter, the surface of a lake freezes while deeper water remains liquid.
Q1. Explain why freezing starts at the surface, using heat transfer principles and water properties.
A1. Water is densest at 4°C; colder water stays on top and freezes first. Ice insulates lower layers, and conduction through water is slow; convection is limited once surface freezes.
Case 10 — Cooling by evaporating sweat
After jogging, a person feels cooler as sweat evaporates from skin.
Q1. Which heat-transfer concept explains the cooling?
A1. Evaporation requires latent heat absorbed from skin, removing thermal energy and causing cooling — a combination of evaporation and convective removal of moist air.
Case 11 — Cooker on a wooden stand
A hot cooker sits on a wooden table; the table does not get as hot as the cooker.
Q1. Explain why the wooden table remains relatively cool.
A1. Wood is a poor conductor, so conduction of heat from cooker to table is slow. Air gap and low contact area further reduce heat transfer.
Case 12 — Reflective blankets for rescue
In emergencies, reflective (space) blankets are used to retain body heat.
Q1. How do these blankets reduce heat loss?
A1. Reflective surface reduces radiative heat loss by reflecting body-emitted infrared radiation back; they also trap a layer of air reducing convective losses.
Case 13 — Hot pavement in city
Asphalt roads get very hot on sunny days, increasing urban temperatures.
Q1. Identify factors that make pavement heat up and suggest mitigation.
A1. Dark asphalt has low albedo, absorbing solar radiation (radiation). Low vegetation and heat storage increase urban heat. Mitigation: light-coloured surfaces, tree cover, and permeable pavements to reduce heat absorption.
Case 14 — Thermometer in shade vs sun
A thermometer kept in shade shows a different reading than one in direct sunlight.
Q1. Explain this difference based on heat transfer.
A1. Sunlit thermometer receives direct radiation heating its bulb, giving higher reading. Shaded thermometer receives less solar radiation and reads air temperature more accurately; convection and conduction affect readings too.
Case 15 — Ice melting under black cloth
Ice covered with a black cloth melts faster than ice under a white cloth when exposed to light.
Q1. Explain why using radiation principles.
A1. Black cloth absorbs more light and converts it to heat, increasing local temperature and melting ice faster. White cloth reflects more light, reducing heat absorption.
Case 16 — Warm water rises above cold water
Warm water from a heater rises to the top of a tank while cold water stays below.
Q1. Identify the mode of heat transfer and reason for layering.
A1. Convection — warmer water is less dense and rises; cold, denser water sinks, creating stratification until mixing occurs.
Case 17 — White painted roofs in hot regions
Many buildings in hot climates have white-painted roofs to stay cooler.
Q1. Explain the scientific reason for this practice.
A1. White paint has high reflectivity (albedo), reflecting more solar radiation and reducing heat absorption — lowers roof temperature and reduces cooling needs.
Case 18 — Evaporation from wet clothes
Clothes hung outside dry faster on a windy day than on a still humid day.
Q1. Explain using heat transfer and mass transfer concepts.
A1. Wind increases convective removal of moist air and supplies drier air, enhancing evaporation which requires latent heat (taken from clothes), causing faster drying. High humidity reduces evaporation rate.
Case 19 — Insulated water bottle vs uninsulated
Hot water in an insulated bottle stays warm longer than in an uninsulated one.
Q1. Explain which modes are reduced inside the insulated bottle.
A1. Insulation reduces conduction and convection by trapping air and using low-conductivity materials; reflective layers reduce radiation losses, preserving heat longer.
Case 20 — Coastal and inland temperature differences
A coastal town has milder summers and winters compared to a nearby inland town.
Q1. Explain how heat capacity and convection in oceans contribute to this difference.
A1. Oceans have high heat capacity and store/release heat slowly, while convection currents redistribute heat. These factors moderate coastal temperatures, making them milder compared to inland areas where land heats and cools rapidly.
Note: These case-based questions are concise, exam-focused, and strictly aligned with the NCERT syllabus for CBSE Class 7 Science.