Revision: Heat Science SSLC (English Medium) Class 9 Tamil Nadu Board of Secondary Education

The sum of the potential energy and kinetic energy of a molecule is called its internal energy.

The total internal energy of all the molecules of a substance is called its thermal energy.

Heat is that form of energy which flows from a hot body to a cold body when they are kept in contact.

The measurement of the quantity of heat is called calorimetry.

The kinetic energy due to random motion of the molecules of a substance is known as its heat energy.

One kilo-calorie of heat is the heat energy required to raise the temperature of 1 kg of water from 14.5°C to 15.5°C.

Conduction is the process by which heat flows from the hot end to the cold end of a solid body without any net bodily movement of the particles of the body.

OR

The process by which heat flows from the hot end to the cold end of a solid body without any net bodily movement of the particles of the body is called conduction.

Substances that do not conduct heat easily are called bad conductors of heat.

Solid substances that conduct heat easily are called good conductors of heat.

Convection is the process by which heat is transmitted through a substance from one point to another due to the actual bodily movement of the heated particles of the substance.

OR

The process by which heat is transmitted through a substance from one point to another due to actual bodily movement of the heated particles of the substance is called convection.

The transfer of heat energy from one place to another via emission of EM energy (in a straight line with the speed of light) without heating the intervening medium is called radiation.

OR

The transfer of heat energy from one place to another via emission of EM energy (in a straight line with the speed of light) without heating the intervening medium is called radiation.

Coefficient of Linear expansion is equal to the change in length of a rod of length 1m when its temperature rises by 1°C.

Temperature is a parameter which tells the thermal state of a body (i.e., the degree of hotness or coldness of the body). It determines the direction of flow of heat when two bodies at different temperatures are placed in contact.

The temperature at which the pressure and volume of a gas theoretically reach zero is called absolute zero.

A constant in the ideal gas equation; R = 8.31 J mol⁻¹ K⁻¹.

Extending a graph line beyond the measured data to predict values.

The lowest theoretically possible temperature (0 K = −273.15 °C), where ideal gas molecules have zero kinetic energy.

OR

The lowest attainable temperature, obtained by plotting the relation between pressure of the gas vs its temperature, where all lines for different gases cut the temperature axis at the same point (−273.15°C), is called the absolute zero of temperature.

A hypothetical gas whose molecules have no volume and exert no intermolecular forces; obeys PV = μRT exactly.

The unique temperature & pressure at which solid, liquid, and gas phases of a substance coexist in equilibrium.

OR

The temperature where the solid, liquid, and gas state of a material co-exist in equilibrium, and this occurs only at a unique temperature and pressure, is called the triple point.

The SI absolute temperature scale starting at absolute zero. Written as K (no degree symbol °).

The temperature scale where −273.15°C corresponds to 0 K, i.e., the temperature at which the pressure of a gas would become zero, is called the absolute temperature (0 K).

One kelvin = 1/273.16 of the difference between absolute zero and the triple point of water.

The quantity of heat needed to raise the temperature of the whole body by 1°C (or 1 K) is called heat capacity.

The specific heat capacity of a substance is the amount of heat energy required to raise the temperature of unit mass of that substance through 1°C (or 1 K).

OR

Heat capacity of a body when expressed for the unit mass is called the specific heat capacity of the substance of that body.

OR

The amount of heat energy required to raise the temperature of a unit mass of an object by 1 °C is called the specific heat of that object.

OR

The amount of heat per unit mass absorbed or given out by a substance to change its temperature by one unit (one degree), i.e., 1°C or 1 K, is called specific heat capacity.

The heat capacity of a body is the quantity of heat required to raise its temperature by 1°C. It depends upon the mass and the nature of the body.

Heat capacity or thermal capacity of a body is the quantity of heat needed to raise or lower the temperature of the whole body by 1°C (or 1K).

The heat capacity of a body is the amount of heat energy required to raise its temperature by 1°C or 1K.

• Heat capacity or thermal capacity is defined as the amount of heat energy required to raise the temperature of a body by 1°C. It is denoted by ‘C’.
• C = Q/t, where C’ is the heat capacity, ‘Q’ is the quantity of heat required and ‘f’ is rise in temperature.
• SI unit of heat capacity is J/K. It is also expressed in cal/°C, kcal/°C or J/°C.

The process of change from one state to another at a constant temperature is called the change of phase.

OR

A transition from one state of matter (solid, liquid, or gas) to another is called change of state.

Sublimation is the process in which a solid changes directly into a gas on heating, without passing through the liquid state.

The triple point of water is that point where water in a solid, liquid and gas state co-exists in equilibrium and this occurs only at a unique temperature and a pressure.

Melting or fusion: The process in which a solid is converted to a liquid by absorbing heat is called melting or fusion.

Heat energy is absorbed by a solid during melting and an equal amount of heat energy ‘ is liberated by the liquid during freezing, without any temperature change. It is called the latent heat of fusion.

The physical process that results in the transition of a substance from the solid-state to the liquid state on heating at a fixed temperature is called Melting.

It can be defined as the fixed temperature at which a solid start changing to its liquid state is Melting Point

The change from liquid to gas (or vapour) phase on absorption of heat at a constant temperature, is called vaporisation.

The change from vapour to liquid phase on liberation of heat at a constant temperature is called condensation (or liquefaction) and the particular temperature at which the condensation occurs is called the condensation point of vapour.

The particular temperature at which vaporisation occurs is called the boiling point of liquid.

The change of state from solid to gas directly is called sublimation.

The change of a solid substance directly into a gas or vapour without first changing into a liquid is called sublimation.

OR

The change from solid state to vapour state without passing through the liquid state is called sublimation, and the substance is said to sublime.

The quantity of heat required to convert unit mass of a substance from its solid state to the liquid state, at its melting point, without any change in its temperature, is called its latent heat of fusion (L_f).

OR

The heat energy absorbed at constant temperature during the transformation of solid into liquid is called the latent heat of fusion. The amount of heat energy absorbed at constant temperature by unit mass of a solid to convert into liquid phase is called the specific latent heat of fusion.

The heat energy absorbed (or liberated) in change of phase is not externally manifested by any rise or fall in temperature, it is called the latent heat.

OR

Latent heat is the quantity of heat energy required to change the state of unit mass of a substance from one phase to another, at constant temperature and constant pressure.

OR

The quantity of heat required to convert unit mass of a substance from its liquid state to vapour state, at its boiling point without any change in its temperature is called its latent heat of vapourization (L_v).

Master Conversion Formula:

\[\frac {T_C}{100}\] = \[\frac {(T_{F}-32)}{180}\] = \[\frac {(T_{K}-273.15)}{100}\]

Specific heat capacity c = \[\frac{\text{Heat capacity of body } C'}{\text{Mass of the body } m}\]

or

Specific heat capacity c = \[\frac{Q}{m\times\Delta t}\]

Heat capacity = Q = m × s

• SI Unit: J/K (or equivalently J/°C)
• Dimensional Formula: [ML²T⁻²K⁻¹]

Q = m × L

where,

Q = Heat energy absorbed or released during phase change
m = Mass of the substance undergoing phase change
L = Specific Latent Heat (characteristic of the substance & process)

SI Units = J kg⁻¹

When temperature is constant, the product of pressure and volume of a gas remains constant.

When pressure is constant, the ratio of volume to temperature of a gas remains constant.

When volume is constant, the ratio of pressure to temperature of a gas remains constant.

• Heat is the energy that flows from a hot body to a cold body when they are kept in contact.
• The S.I. unit of heat is joule (J), while calorie and kilocalorie are commonly used units.
• One calorie is approximately equal to 4.2 joule, and kilocalorie is used to measure the energy value of foods.

• The transfer of heat from the hot part to the cold part of an object is called conduction of heat.
• Conduction takes place through solid substances only — it requires a medium.
• Heat travels by molecular collisions: fast-vibrating molecules pass energy to slower neighbours.
• Copper conducts heat faster than aluminium, which conducts faster than steel.
• Conduction of heat through a substance depends on the property of that substance.
• Good conductors: silver, copper, aluminium, brass — all metals.
• Bad conductors: wood, cloth, air, paper — most non-metals.
• Good conductors of heat are also good conductors of electricity, and bad conductors of heat are also bad conductors of electricity.

• Convection occurs only in fluids (liquids and gases) — not in solids.
• In conduction, molecules vibrate but stay in place.
• In convection, molecules physically move from one place to another.
• Heating reduces density → hot fluid rises; cool fluid sinks → a convection current is set up.
• Convection currents transfer heat to the entire mass of the fluid.
• Potassium permanganate makes convection currents visible as magenta-coloured streams.

• When water is heated from the top, its density decreases, and it stays at the top. Since hot water cannot sink, convection does not occur and the bottom remains cool.
• Radiation is the transfer of heat without a medium — through electromagnetic waves.
• Heat from the Sun reaches us through radiation across the vacuum of space.
• All objects above 0 K emit thermal radiation (electromagnetic waves).
• Radiation is a two-step process: thermal energy → EM waves → thermal energy.
• Black or dark surfaces absorb more heat radiation; absorption also depends on the intrinsic properties of the substance.
• An infrared camera uses the radiation emitted by objects to see at night — useful for military surveillance.
• Copper is an excellent conductor; plastic is a bad conductor (insulator).
• Heat readily conducts through metals (copper and steel) but not through non-metals (wood and plastic).

• If no heat flows between two bodies in contact, they are said to be at the same temperature, even though their thermal energies may be different.
• The S.I. unit of temperature is kelvin (K), and absolute zero (0 K) is the temperature at which molecular motion ceases.

• Gases expand linearly with temperature, making them useful for thermometers. This consistent behaviour suggests the existence of a lowest temperature limit.
• Absolute zero (−273.15 °C or 0 K) is the temperature where an ideal gas would have zero pressure. It is the lowest possible temperature.
• The Kelvin scale begins at absolute zero and uses the triple point of water (273.16 K) as a reference point. It is the SI temperature scale.
• The ideal gas equation (PV = μRT) combines all gas laws into a single relationship among pressure, volume, and temperature. It works best for gases at low pressure and high temperature.

• Heat energy absorbed (Q) depends on: mass (m), rise in temperature (Δt), and specific heat capacity (c), i.e., Q ∝ m × Δt × c.
• Heat capacity (C') and specific heat capacity (c) are related by: C′ = m × c.

• A change of state occurs when heat exchange causes a substance to transition between solid, liquid, and gas phases.
• Temperature remains constant during a phase change because heat energy changes molecular arrangement (potential energy), not molecular speed (kinetic energy).
• The heating curve has flat plateaus at the melting point (0 °C) and boiling point (100 °C) for water, with rising slopes in between.

• Formula: Q = mL. Specific latent heat L has SI unit J kg⁻¹.
• Temperature stays constant during any phase change. Heat energy goes into breaking or forming intermolecular bonds, not into raising kinetic energy.
• Latent Heat of Fusion (water): L_f = 3.33 × 10⁵ J kg⁻¹ = 80 cal/g. Heat needed to melt 1 kg of ice at 0°C.
• Latent Heat of Vaporisation (water): L_v = 22.6 × 10⁵ J kg⁻¹ = 540 cal/g. Heat is needed to convert 1 kg of water to steam at 100°C.
• L_v ≫ L_f because vaporisation requires complete molecular separation and work against atmospheric pressure during expansion.
• All latent heat values depend on atmospheric pressure. Standard values quoted at 1 atm. Increasing pressure raises the boiling point (pressure cooker effect).