Physical Science Classes VI-X for School Assistants and TET Paper 2A · Chapter 4

Heat and Temperature, Specific Heat, Latent Heat, Humidity

What to remember

Heat is energy that flows because of a temperature difference (unit joule); temperature tells how hot or cold a body is (unit kelvin or °C). Heat flows from the hotter to the colder body.
Heat needed to change temperature: Q = m c ΔT. Heat needed to change state at constant temperature: Q = m L. Water has a very high specific heat (about 4200 J/kg K) and large latent heats.
Relative humidity shows how close the air is to being saturated with water vapour; it is measured with a hygrometer, and evaporation is faster when humidity is low.

Heat and temperature

Heat is a form of energy. Temperature is the degree of hotness of a body; it is a measure of the average kinetic energy of the molecules. Two bodies at the same temperature have no net heat flow between them (thermal equilibrium). A big bucket of warm water holds more heat than a cup of boiling water, though the cup has the higher temperature.

Units of heat: joule (SI) and calorie. 1 calorie is the heat needed to raise 1 g of water by 1 °C; 1 cal = about 4.18 J. Heat is measured with a calorimeter and temperature with a thermometer.

Temperature scales and thermometers

Scale	Ice point	Steam point	Divisions
Celsius	0 °C	100 °C	100
Fahrenheit	32 °F	212 °F	180
Kelvin (SI)	273 K (approx.)	373 K (approx.)	100

Conversions:

K = °C + 273
°F = (9/5) x °C + 32, or C/5 = (F - 32)/9

Examples: 37 °C = 98.6 °F = 310 K. 100 °C = 212 °F = 373 K. The reading −40 is the same in Celsius and Fahrenheit.

Mercury thermometer: mercury is used because it expands evenly, does not wet glass, is a good conductor, and has a wide liquid range. Clinical thermometer: range about 35 °C to 42 °C (94 °F to 108 °F), has a kink (constriction) that stops the mercury from falling back, so it must be shaken down. It must not be used to measure the temperature of boiling water. Laboratory thermometer: range about −10 °C to 110 °C.

Thermal expansion

Most solids, liquids and gases expand on heating.

Linear expansion in solids: change in length depends on original length and temperature rise. Gases expand most, liquids less, solids least.
Gaps are left in railway tracks and bridges for expansion. The bimetallic strip (two metals joined, such as brass and iron) bends on heating because the two metals expand by different amounts; it is used in thermostats and fire alarms.
Anomalous expansion of water: from 0 °C to 4 °C water contracts on heating. It has its maximum density at 4 °C. This is why lakes freeze from the top, and fish survive in the water below the ice.

Transfer of heat

Mode	Medium needed	Example
Conduction	Solid (particles pass on vibration)	Spoon heated at one end
Convection	Fluid (liquid or gas moves)	Sea breeze; boiling water; hot air rises
Radiation	None (works in vacuum)	Heat of the Sun reaching Earth

Metals are good conductors; wood, air, plastic and wool are poor conductors (insulators). Dark rough surfaces absorb and emit radiation better; shiny surfaces reflect it. White or light clothes are worn in summer. A thermos (vacuum) flask reduces conduction and convection by the vacuum between double walls, and radiation by silvered surfaces.

Specific heat and calorimetry

Specific heat (c) is the heat needed to raise the temperature of 1 kg of a substance by 1 K (or 1 °C). Unit: J/(kg K). Water: about 4200 J/(kg K) or 1 cal/(g °C), one of the highest values. Typical others: ice about 2100, aluminium about 900, copper about 390 J/(kg K). Substances with low specific heat (sand, metals) heat up and cool down quickly.

Heat gained or lost: Q = m c ΔT.

Heat capacity = m c (heat needed to raise the temperature of the whole body by 1 K).

Principle of calorimetry (method of mixtures): with no heat loss, heat lost by the hot body = heat gained by the cold body.

Worked example 1: heat needed to warm 2 kg water by 10 °C: Q = 2 x 4200 x 10 = 84,000 J.

Worked example 2: 100 g of water at 80 °C is mixed with 100 g of water at 20 °C. Final temperature t: 100 (80 − t) = 100 (t − 20), so t = 50 °C.

Why the sea breeze blows: by day land heats up faster than water (lower specific heat), the air above land rises, and cooler air from the sea moves in. At night the land cools faster and the land breeze blows. Because water has a high specific heat, coastal places have a moderate climate.

Latent heat and change of state

Latent heat is the heat absorbed or released during change of state, with no change in temperature. Specific latent heat (L) is the heat per kg: Q = m L.

Quantity	Value for water
Latent heat of fusion (ice to water at 0 °C)	about 336 kJ/kg (80 cal/g)
Latent heat of vaporisation (water to steam at 100 °C)	about 2260 kJ/kg (540 cal/g)

Steam at 100 °C causes a worse burn than boiling water at 100 °C because steam also releases its large latent heat when it condenses. Ice at 0 °C cools a drink better than water at 0 °C because it absorbs latent heat when it melts.

Worked example 3: heat to melt 2 kg ice at 0 °C (L = 336 kJ/kg): Q = 2 x 336 = 672 kJ.

Terms: melting point, boiling point, freezing, condensation, sublimation (solid directly to vapour, e.g. camphor, naphthalene, dry ice). Boiling point rises with pressure (pressure cooker cooks faster) and falls at high altitude, where pressure is lower. Impurities raise the boiling point and lower the freezing point (salt on ice).

Evaporation versus boiling. Evaporation takes place at any temperature, only at the surface, and causes cooling. It is faster with a larger surface area, higher temperature, wind, and low humidity. Boiling happens at a fixed temperature throughout the liquid. A wet cloth, a mud pot (earthen pot, matka) and sweating cool us because evaporation takes latent heat from the surface.

Humidity

Humidity is the amount of water vapour present in the air.

Absolute humidity: mass of water vapour per unit volume of air.
Relative humidity (RH): ratio of the water vapour present to the maximum the air can hold at that temperature, in per cent. Saturated air has RH of 100%.
Dew point: the temperature at which air becomes saturated and vapour starts to condense into dew or fog.
Instruments: hygrometer, psychrometer (wet and dry bulb thermometer).

At high humidity sweat does not evaporate easily, so we feel sticky and uncomfortable. Wet clothes dry slowly on humid days. Warm air can hold more vapour than cool air. Humid weather is common in coastal areas.

Classroom angle

Show that water conducts poorly by heating the top of a test tube with an ice piece held at the bottom.
Show convection with a drop of potassium permanganate crystal in water that is heated.
Rubbing spirit on the hand gives a cooling sensation, showing evaporation cooling.

Exam traps

Heat is energy; temperature is not energy. A large body at lower temperature may hold more heat.
Specific heat is per kg per degree; latent heat is per kg with no temperature change.
Water is densest at 4 °C, not at 0 °C.
Steam at 100 °C is not the same as boiling water at 100 °C as regards burns.
Radiation needs no medium; conduction and convection do.
Clinical thermometer cannot measure very high temperatures.
Relative humidity is a ratio (per cent); absolute humidity is mass per volume.
Evaporation causes cooling; condensation causes warming of the surroundings.

One-liners

1. SI unit of heat is the joule; 1 cal = about 4.18 J.
2. K = °C + 273.
3. F = (9/5) C + 32.
4. Human body temperature is about 37 °C.
5. Water has maximum density at 4 °C.
6. Specific heat of water is about 4200 J/kg K.
7. Latent heat of fusion of ice is about 336 kJ/kg.
8. Latent heat of vaporisation of water is about 2260 kJ/kg.
9. Radiation is the only mode of heat transfer in vacuum.
10. Bimetallic strip is used in thermostats.
11. Dew point is the temperature at which air gets saturated.
12. Hygrometer measures humidity.

Practice questions

The SI unit of temperature is
1. joule
2. degree Celsius
3. degree Fahrenheit
4. kelvin
Answer
D. kelvin
The kelvin is the SI base unit of temperature.
Water has its maximum density at
1. -4 degrees C
2. 4 degrees C
3. 0 degrees C
4. 100 degrees C
Answer
B. 4 degrees C
Water contracts from 0 to 4 degrees C, so it is densest at 4 degrees C.
Normal human body temperature on the Fahrenheit scale is about
1. 37 degrees F
2. 108 degrees F
3. 100 degrees F
4. 98.6 degrees F
Answer
D. 98.6 degrees F
37 degrees C = 98.6 degrees F.
Heat can travel through a vacuum by
1. evaporation
2. conduction
3. radiation
4. convection
Answer
C. radiation
Radiation needs no medium; the Sun's heat reaches Earth this way.
The range of a clinical thermometer is about
1. -10 to 110 degrees C
2. 50 to 150 degrees C
3. 35 to 42 degrees C
4. 0 to 100 degrees C
Answer
C. 35 to 42 degrees C
It is made to read human body temperatures only.
The specific heat of water is about
1. 42000 J/kg K
2. 420 J/kg K
3. 900 J/kg K
4. 4200 J/kg K
Answer
D. 4200 J/kg K
Water: 1 cal/g degree C, i.e. about 4200 J/kg K.
Heat absorbed by ice at 0 degrees C while it melts, without change of temperature, is called
1. heat capacity
2. latent heat of fusion
3. specific heat
4. latent heat of vaporisation
Answer
B. latent heat of fusion
It is the heat of fusion.
Humidity of air is measured with a
1. barometer
2. hygrometer
3. lactometer
4. anemometer
Answer
B. hygrometer
A hygrometer (psychrometer) gives relative humidity.
A bimetallic strip bends on heating because
1. it melts
2. both metals expand equally
3. the two metals expand by different amounts
4. it loses mass
Answer
C. the two metals expand by different amounts
Unequal expansion of the two joined metals bends the strip.
The boiling point of water on the Kelvin scale is about
1. 100 K
2. 473 K
3. 273 K
4. 373 K
Answer
D. 373 K
K = 100 + 273 = 373.
Beach sand becomes hotter than sea water in the afternoon because sand has
1. a lower specific heat
2. a higher specific heat
3. a higher latent heat
4. a lower density only
Answer
A. a lower specific heat
Less heat is needed to raise the temperature of sand by 1 degree.
The temperature at which the Celsius and Fahrenheit readings are the same is
1. 0
2. -40
3. 32
4. 100
Answer
B. -40
C = F = x gives x = (9/5)x + 32, so x = -40.
Which of these is an example of sublimation?
1. Water boiling
2. Dew forming
3. Camphor disappearing on heating
4. Ice melting
Answer
C. Camphor disappearing on heating
Camphor changes directly from solid to vapour.
Heat is transferred in liquids and gases mainly by
1. sublimation
2. radiation
3. conduction
4. convection
Answer
D. convection
The heated fluid moves and carries heat with it.
The dew point is the temperature at which
1. ice melts
2. water is densest
3. water boils
4. air becomes saturated with water vapour
Answer
D. air becomes saturated with water vapour
Cooling to this temperature starts condensation.
Evaporation of a liquid causes
1. cooling of the remaining liquid
2. heating of the liquid
3. no change in temperature
4. increase in density only
Answer
A. cooling of the remaining liquid
The fastest molecules leave, taking latent heat with them.
Gaps are left between rails of a railway track to allow for
1. friction
2. sound
3. thermal expansion
4. light
Answer
C. thermal expansion
Rails expand in summer heat.
A pressure cooker cooks food faster because it
1. reduces the heat needed
2. raises the boiling point of water
3. lowers the boiling point
4. removes the steam
Answer
B. raises the boiling point of water
Higher pressure raises the boiling point, so food cooks at a higher temperature.
The temperature 50 degrees C on the Fahrenheit scale is
1. 82 degrees F
2. 132 degrees F
3. 90 degrees F
4. 122 degrees F
Answer
D. 122 degrees F
F = (9/5) x 50 + 32 = 122.
A temperature of 300 K in degree Celsius is
1. 37 degrees C
2. 27 degrees C
3. 573 degrees C
4. -27 degrees C
Answer
B. 27 degrees C
C = 300 - 273 = 27.
A temperature of 77 degrees F in degree Celsius is
1. 15 degrees C
2. 35 degrees C
3. 25 degrees C
4. 45 degrees C
Answer
C. 25 degrees C
C = (77 - 32) x 5/9 = 25.
The heat needed to raise the temperature of 2 kg of water by 10 degrees C (c = 4200 J/kg K) is
1. 42000 J
2. 84000 J
3. 8400 J
4. 840000 J
Answer
B. 84000 J
Q = m c dT = 2 x 4200 x 10 = 84000 J.
100 g of water at 80 degrees C is mixed with 100 g of water at 20 degrees C. The final temperature, ignoring heat loss, is
1. 100 degrees C
2. 60 degrees C
3. 40 degrees C
4. 50 degrees C
Answer
D. 50 degrees C
Equal masses give the average: (80 + 20)/2 = 50 degrees C.
The heat needed to melt 2 kg of ice at 0 degrees C (L = 336 kJ/kg) is
1. 672 kJ
2. 336 kJ
3. 1344 kJ
4. 168 kJ
Answer
A. 672 kJ
Q = m L = 2 x 336 = 672 kJ.
The heat given out when 1 kg of steam at 100 degrees C condenses to water at 100 degrees C (L = 2260 kJ/kg) is
1. 226 kJ
2. 4200 kJ
3. 2260 kJ
4. 336 kJ
Answer
C. 2260 kJ
Q = m L = 1 x 2260 kJ.
The heat needed to raise 0.5 kg of aluminium (c = 900 J/kg K) by 20 degrees C is
1. 18000 J
2. 9000 J
3. 900 J
4. 4500 J
Answer
B. 9000 J
Q = 0.5 x 900 x 20 = 9000 J.
200 g of water at 90 degrees C is mixed with 300 g of water at 40 degrees C. The final temperature, ignoring heat loss, is
1. 60 degrees C
2. 65 degrees C
3. 50 degrees C
4. 70 degrees C
Answer
A. 60 degrees C
200 (90 - t) = 300 (t - 40) gives 500 t = 30000, t = 60.
The heat needed to melt 500 g of ice at 0 degrees C (80 cal/g) is
1. 400 cal
2. 80000 cal
3. 40000 cal
4. 4000 cal
Answer
C. 40000 cal
Q = 500 x 80 = 40000 cal.
A 1000 W heater supplies all its heat to 1 kg of water (c = 4200 J/kg K). The time needed to raise the temperature by 10 degrees C is
1. 10 s
2. 420 s
3. 4.2 s
4. 42 s
Answer
D. 42 s
Heat = 1 x 4200 x 10 = 42000 J; t = 42000/1000 = 42 s.
Lakes in cold countries freeze at the top first because
1. ice is heavier than water
2. water is densest at 4 degrees C and colder water floats
3. ice conducts heat well
4. water has low specific heat
Answer
B. water is densest at 4 degrees C and colder water floats
Water at 4 degrees C sinks; colder water and ice stay on top.
Steam at 100 degrees C causes a more severe burn than boiling water at 100 degrees C because steam
1. releases a large latent heat on condensing
2. is at a higher temperature
3. is heavier
4. has a lower specific heat
Answer
A. releases a large latent heat on condensing
Condensation releases about 2260 kJ per kg.
A sea breeze blows during the day because
1. land is colder than sea
2. sea heats up faster than land
3. land heats up faster than sea water
4. sea water evaporates fully
Answer
C. land heats up faster than sea water
Warm air above land rises and cool sea air moves in.
Wet clothes dry slowly on a humid day because
1. the air is hotter
2. the clothes get heavier
3. the water boils
4. evaporation is slower in humid air
Answer
D. evaporation is slower in humid air
Air already holding much vapour takes up less.
Ice at 0 degrees C cools a drink better than water at 0 degrees C because ice
1. is lighter
2. absorbs latent heat while melting
3. is a good conductor
4. has higher specific heat
Answer
B. absorbs latent heat while melting
Melting takes 336 kJ per kg without a rise in temperature.
Cooling of water in an earthen pot (matka) is due to
1. conduction
2. radiation only
3. evaporation of water from its pores
4. condensation of air
Answer
C. evaporation of water from its pores
Evaporation takes latent heat from the water.
Which of the statements is/are correct? 1. Heat is a form of energy. 2. Temperature is a form of energy measured in joule.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Temperature measures hotness; heat is the energy.
Which of the statements is/are correct? 1. Evaporation can occur at any temperature. 2. Boiling occurs only at the surface of the liquid.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Boiling occurs throughout the liquid at a fixed temperature.
Which of the statements is/are correct? 1. Water contracts on heating from 0 to 4 degrees C. 2. Water has its minimum density at 4 degrees C.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Water has maximum density at 4 degrees C.
Which of the statements is/are correct? 1. Convection can occur in solids. 2. Radiation can occur in a vacuum.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Convection needs a fluid; radiation needs no medium.
Which of the statements is/are correct? 1. Water has a high specific heat. 2. During the day land heats up faster than the sea.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both facts explain sea breeze and moderate coastal climate.
Which of the statements is/are correct? 1. Relative humidity is a ratio expressed in per cent. 2. Saturated air has a relative humidity of 100%.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both are definitions.
Which of the statements is/are correct? 1. Temperature changes during a change of state at constant pressure. 2. Sublimation is the change of a solid directly into vapour.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Temperature stays constant during a change of state.
Which of the statements is/are correct? 1. Dark rough surfaces absorb radiation better. 2. A clinical thermometer has a constriction to hold the mercury.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both are standard facts.
Which of the pairs is correctly matched?
1. Calorie - unit of power
2. Hygrometer - temperature
3. Latent heat - rise in temperature
4. Bimetallic strip - thermostat
Answer
D. Bimetallic strip - thermostat
Hygrometer measures humidity, latent heat acts at constant temperature, calorie is energy.
Which of the statements is/are correct? 1. Heat flows spontaneously from a colder to a hotter body. 2. Temperature is measured in joule.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
D. Neither 1 nor 2
Heat flows from hot to cold; temperature is in kelvin or degree Celsius.

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1.The specific latent heat of fusion of ethyl alcohol is 100 Jg -1. Find the heat absorbed (in J) by 2.25 g of ethyl alcohol when it melts at its melting point of -114°C.
2.Which of the following has the highest specific heat capacity at room temperature and atmospheric pressure?
3.Find the specific latent heat of vaporisation of 1.25 g of nitrogen (in Jg -1), if it releases 250 Joules of heat when it condenses at its boiling point of 196° C.
4.How much heat should be transferred to the 100 g of an edible oil to raise its temperature by 20°C? (Specific heat of oil 1965 Jkg −1 K −1)
5.How much heat should be transferred to a 100 g block of aluminum (specific heat 900 Jkg −1 K −1) to increase its temperature by 10°C?
6.The average temperature of human body is ______.
7.The ______ across the ends of a resistor is directly proportional to the current through it, provided its temperature remains the same.
8.The speed of sound in air is about 343 m/s at room temperature. What happens to its speed when it travels through water?
9.The SI unit of temperature is/are _________
10.The state of a substance whether it will be solid, liquid or gas is determined by: a. Temperature b. Pressure c. Volume

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