Heat is energy in transit because of a temperature difference. In CDS Science, the chapter on specific and latent heat turns that simple idea into reliable marks. Master just three formulas, the meaning of latent heat, and the principle of calorimetry, and you can solve almost every numerical the examiner throws at you with confidence.
Why this topic earns easy marks
Heat questions appear almost every year in the CDS and OTA General Science paper. They are popular with examiners because they test a single clear idea and reward students who memorise a few definitions and formulas precisely.
Unlike mechanics, the maths here is light. Most sums need nothing more than multiplication and the equation Q = mcΔT. The trick is to read the question carefully and decide whether the substance is merely warming up or actually changing state.
Before calculating, ask: is the temperature changing, or is the substance melting/boiling at a fixed temperature? That single decision tells you whether to use specific heat or latent heat.
Heat is not the same as temperature
Students lose marks by treating heat and temperature as identical. They are not.
- Heat is a form of energy that flows from a hotter body to a colder one. Its SI unit is the joule (J); the older unit is the calorie.
- Temperature measures the average kinetic energy of the particles in a body. Its SI unit is the kelvin (K); we also use the degree Celsius (°C).
A lit matchstick has a very high temperature but carries little heat energy. A bucket of warm water has a lower temperature yet contains far more heat. So a large body at a low temperature can hold more heat than a tiny body at a high temperature.
1 calorie = 4.18 J (often rounded to 4.2 J). 1 kcal = 1000 cal = 4180 J. The relation between scales: T(K) = T(°C) + 273.
Specific heat capacity defined
The specific heat capacity of a substance is the amount of heat needed to raise the temperature of 1 kg of that substance by 1 °C (or 1 K). It is denoted by c.
Q = m × c × ΔT
where Q = heat supplied (J), m = mass (kg), c = specific heat (J kg−1 K−1), and ΔT = change in temperature (°C or K).
The SI unit of specific heat is J kg−1 K−1. Some standard values worth memorising:
- Water: about 4200 J kg−1 K−1 (the highest among common liquids)
- Ice: about 2100 J kg−1 K−1
- Aluminium: about 900; Iron: about 450; Copper: about 390
Note that ice, water and steam are the same substance but have different specific heats because they are different states of matter.
Heat capacity vs specific heat
Beware a small but frequently tested distinction.
- Specific heat (c) refers to 1 kg of substance. It depends only on the material, not on how much you have.
- Heat capacity (C) refers to the whole object. It is the heat needed to raise the temperature of that particular body by 1 °C.
Heat capacity C = m × c
Unit of heat capacity: J K−1 (no kg). Unit of specific heat: J kg−1 K−1.
So a 2 kg block of iron has twice the heat capacity of a 1 kg block, but both have the same specific heat. Think of specific heat as a property of the material and heat capacity as a property of the actual object in front of you. In calorimetry problems the calorimeter's heat capacity is sometimes given as its water equivalent, which is the mass of water that would absorb the same heat as the vessel for the same rise in temperature. This neat idea lets you treat the metal vessel as if it were an extra bit of water, simplifying the energy balance.
Why water's high specific heat matters
Water has an unusually high specific heat. This single fact explains several everyday observations the examiner loves to ask about.
- Water is used as a coolant in car radiators and power plants because it can absorb a lot of heat with only a small temperature rise.
- Coastal areas have a moderate climate: the sea heats up and cools down slowly, so it keeps nearby land warm in winter and cool in summer.
- Hot-water bottles stay warm for a long time because water releases its stored heat slowly.
If a question asks why land breezes and sea breezes form, or why deserts have extreme day-night temperatures (sand has a low specific heat), the answer traces back to specific heat capacity.
The numbers make the contrast vivid. To raise the temperature of 1 kg of water by 1 °C you need about 4200 J, but to do the same for 1 kg of dry sand you need only around 800 J. So in the same sunshine, sand heats up roughly five times faster than water, and at night it loses that heat just as quickly. This is the root cause of monsoon winds, sea and land breezes, and the harsh climate of inland deserts compared with the gentle weather of port cities.
Latent heat: the hidden energy of change
When a substance changes state, for example ice melting into water, its temperature stays constant even though heat is still being supplied. The heat that goes in without raising temperature is called latent heat (from the Latin for hidden).
This energy is used to break the bonds holding the particles together, not to speed them up. That is why the thermometer reads a steady 0 °C while ice melts and a steady 100 °C while water boils at normal pressure.
Q = m × L
where L is the specific latent heat (J kg−1) and m is the mass undergoing the change of state. There is no ΔT term because temperature does not change.
Two types matter for CDS:
- Latent heat of fusion (solid ↔ liquid). For ice it is about 3.34 × 105 J kg−1 (334 kJ/kg).
- Latent heat of vaporisation (liquid ↔ gas). For water it is about 22.6 × 105 J kg−1 (2260 kJ/kg).
Why steam burns worse than boiling water
A classic CDS question: why does steam at 100 °C cause a far more severe burn than water at 100 °C?
Both are at the same temperature, so temperature is not the answer. The difference is latent heat. When steam touches your skin and condenses into water, it releases its large latent heat of vaporisation (2260 kJ/kg) before the water even begins to cool. Boiling water carries no such extra hidden energy.
1 kg of steam condensing gives out 2260 kJ extra compared with 1 kg of boiling water at the same temperature. That is why steam is used for cooking and heating in pressure systems.
The principle of calorimetry
When two bodies at different temperatures are mixed in an insulated container, heat flows from the hotter to the colder until they reach a common temperature. If no heat escapes:
Heat lost by the hot body = Heat gained by the cold body
m1c1(T1 − T) = m2c2(T − T2)
Here T is the final common temperature. This is just the law of conservation of energy applied to heat. The instrument used to make these measurements is the calorimeter, usually a polished metal vessel inside an insulating jacket to minimise heat loss. Copper is a common choice for the vessel because it has a low specific heat, so it absorbs little of the heat itself and gives a cleaner reading. The outer jacket of wool, felt or an air gap stops convection and conduction carrying heat away to the surroundings.
To set up any mixing sum, list every body, decide whether each is losing or gaining heat, and write one term for each. Equate the total heat lost to the total heat gained, then solve for the single unknown, which is usually the final temperature T.
If a change of state is involved (ice melting, water boiling), you must add the latent heat term (mL) to the energy balance. Forgetting it is the single biggest source of wrong answers in mixing problems.
Worked example: melting and heating ice
How much heat is required to convert 200 g of ice at 0 °C completely into water at 30 °C? Take Lfusion = 3.34 × 105 J kg−1 and cwater = 4200 J kg−1 K−1.
Notice the two distinct stages: first the latent heat to melt (no temperature change), then the specific heat to warm the resulting water. Tackling sums in stages prevents errors.
Mistakes that cost marks
- Mixing units. Convert grams to kilograms before using SI values of c and L, or convert c to per gram. Stay consistent.
- Adding a ΔT to a latent heat step. During melting or boiling, temperature is constant, so use Q = mL only.
- Confusing fusion and vaporisation. Fusion is solid↔liquid; vaporisation is liquid↔gas. Their L values are very different.
- Forgetting the calorimeter. In careful problems the vessel itself absorbs heat (water equivalent); ignore it only when the question says so.
Writing Q = mcΔT for boiling water at a steady 100 °C gives Q = 0 because ΔT = 0, which is wrong. The energy goes into latent heat, so use Q = mL instead.
Evaporation and the cooling effect
Evaporation is the slow change of a liquid into vapour at any temperature from its surface. The fastest, most energetic molecules escape, so the average energy of the remaining liquid falls and the liquid cools. This is why:
- Sweating cools the body: evaporating sweat draws latent heat from the skin.
- Water stays cool in an earthen pot (matka): water seeps through the pores and evaporates, taking heat from the rest.
- You feel cold when spirit or sanitiser is put on your hand: it evaporates quickly, absorbing latent heat.
Evaporation increases with higher temperature, larger surface area, lower humidity and faster wind. These four factors are direct one-mark fillers.
Previous-year question and quick recap
Q. The amount of heat required to change 1 kg of a substance from the solid to the liquid state at its melting point, without any change in temperature, is called its:
Answer: Specific latent heat of fusion. For ice this value is about 3.34 × 105 J kg−1. The temperature stays constant at the melting point throughout the change of state, which is why no ΔT appears in the formula Q = mL.
- Heat is energy (joule); temperature measures average kinetic energy (kelvin).
- Specific heat c: heat to warm 1 kg by 1 °C; water's is high (4200 J kg−1 K−1).
- Q = mcΔT for temperature change; Q = mL for a change of state.
- Latent heat of fusion (ice) ≈ 3.34×105; of vaporisation (water) ≈ 22.6×105 J kg−1.
- Calorimetry: heat lost by hot body = heat gained by cold body.
- Steam scalds worse and evaporation cools, both because of latent heat.
Frequently asked questions
What is the difference between specific heat and latent heat?
Specific heat is the energy needed to change the temperature of 1 kg of a substance by 1 °C, while latent heat is the energy needed to change its state (melt or boil) at constant temperature. Specific heat uses Q = mcΔT; latent heat uses Q = mL.
Why does water have such a high specific heat capacity?
Strong hydrogen bonds between water molecules mean a lot of energy is needed to make them move faster. This high specific heat lets water act as an excellent coolant and gives coastal regions a moderate climate.
Why does temperature stay constant during melting or boiling?
The supplied heat is used as latent heat to break the bonds between particles and change the state, not to increase their kinetic energy. So the thermometer holds steady at 0 °C or 100 °C until the change of state is complete.
Why does steam at 100 °C burn more severely than water at 100 °C?
When steam condenses on the skin it first releases its large latent heat of vaporisation (about 2260 kJ/kg) before cooling, delivering far more energy than boiling water at the same temperature.
What is the principle of calorimetry?
In an insulated system, heat lost by the hotter body equals heat gained by the colder body until they reach a common temperature. It is a direct application of the conservation of energy and is measured using a calorimeter.
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