Thermal properties of matter: Class 9 Physics

Key points

Heat is a form of energy that flows from a hotter body to a colder one. It is measured in joules. Temperature tells how hot or cold a body is. It decides the direction of heat flow. Heat flows from higher to lower temperature until both reach the same temperature. A bucket of water at 40 °C has more heat than a cup at 40 °C, though the temperature is the same. Matter is made of particles in constant motion. Temperature is a measure of the average kinetic energy of the particles. When a body is heated, its particles move faster, so its internal energy increases.

A thermometer measures temperature. A liquid-in-glass thermometer uses the expansion of mercury or coloured alcohol in a narrow tube. A thermometer needs two fixed points: the lower fixed point is the melting point of pure ice and the upper fixed point is the boiling point of water at standard pressure.

Common temperature scales are:

• Celsius scale: ice point 0 °C and steam point 100 °C, divided into 100 equal parts.
• Fahrenheit scale: ice point 32 °F and steam point 212 °F, divided into 180 parts.
• Kelvin scale (absolute): the SI scale, with 0 K at absolute zero. The ice point is 273 K, and the steam point is 373 K.

Conversion formulas are T(K) = t(°C) + 273 and F = 1.8 C + 32. Normal body temperature, 37 °C, is 310 K or 98.6 °F. A clinical thermometer has a narrow constriction that stops the mercury from falling back, so the reading can be seen after it is taken out of the mouth. It reads from 35 °C to 42 °C. A kelvin is the same size as a degree Celsius.

Heat capacity of a body is the heat needed to raise its temperature by 1 K. Specific heat capacity c is the heat needed to raise the temperature of 1 kg of a substance by 1 K. Its unit is J/(kg K). The heat gained or lost is Q = mcΔT. Water has a large specific heat capacity, 4200 J/(kg K). So it heats slowly and cools slowly, which is why it is used in car radiators and hot water bottles, and why coastal places have a milder climate than deserts.

When hot and cold substances are mixed, and no heat is lost to the surroundings, heat lost by the hot substance equals heat gained by the cold one. This is the principle of calorimetry (method of mixtures). It is used to find specific heat capacities by experiment.

Matter changes state when it is heated or cooled enough. Melting is solid to liquid at the melting point, and freezing is liquid to solid. Boiling is liquid to gas throughout the liquid at the boiling point, and condensation is gas to liquid. During a change of state the temperature stays constant even though heat is being supplied, because the heat energy is used to break or loosen the bonds between particles, not to speed up the particles.

Latent heat is the heat absorbed or released during a change of state at constant temperature. Q = mL. The specific latent heat of fusion Lf is the heat needed to change 1 kg of solid to liquid at its melting point. For ice it is 3.36 × 10⁵ J/kg. The specific latent heat of vaporisation Lv is the heat needed to change 1 kg of liquid to gas at its boiling point. For water it is 2.26 × 10⁶ J/kg. Steam at 100 °C burns more than boiling water at 100 °C because it also releases the latent heat when it condenses.

Evaporation is the change of a liquid into vapour at any temperature, from the surface only. The faster, more energetic molecules escape, so the liquid left becomes cooler. It is faster when the temperature is higher, the surface area is larger, the air is dry and moving, and the liquid has weaker bonds between particles. Sweating, drying clothes and the cooling of water in an earthen pot are examples. Boiling differs from evaporation because it occurs at one fixed temperature throughout the liquid and is rapid.

Most substances expand when heated and contract when cooled, because the particles vibrate with larger amplitude and move apart slightly. Gases expand most, liquids less and solids least. Expansion in solids is used in thermostats and bimetallic strips, but must also be allowed for in railway lines, bridges and power cables by leaving gaps or using rollers.

Linear expansion of a solid rod: ΔL = αLΔT, where α is the coefficient of linear thermal expansion, in K⁻¹, L is the original length and ΔT is the rise in temperature. Volume expansion of a solid or liquid: ΔV = βVΔT, where β is the coefficient of volume expansion. For a solid, β is about 3α. Liquids expand more than solids, which is why a full container of liquid overflows when heated.

Water behaves unusually between 0 °C and 4 °C. When it is cooled from 4 °C to 0 °C it expands instead of contracting. So water has its maximum density at 4 °C. In a very cold winter, a lake freezes from the top, and the water near the bottom stays at about 4 °C. Ice floats on it, and fish and other aquatic life survive below the ice.

Study tip: Always write which formula you use, Q = mcΔT or Q = mL, and check the unit of c or L in the data. If a question has several stages, such as ice at 0 °C heated to steam, make a separate line for each stage and add the heats at the end. Learn the three values: 4200, 3.36 × 10⁵ and 2.26 × 10⁶.

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Common mistakes

• Using Celsius where kelvin is required, or forgetting that a rise of 1 K equals a rise of 1 °C.
• Saying that temperature rises during melting or boiling. It stays constant while the state changes.
• Confusing heat and temperature, or saying a body contains temperature.
• Using g, kg and joules together in Q = mcΔT without converting to SI.
• Writing that all liquids contract on cooling. Water between 4 °C and 0 °C is an exception.

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