The three fundamental laws which connect the relation between pressure, volume and temperature are as follows
(i) Boyle's Law 
(ii) Charles's law
(iii) Avogadro's law
Boyle's Law:
When the temperature of a gas is kept constant, the volume of a fixed mass of gas is inversely proportional to its pressure.
∴ P \(\propto\) 1/V

In other words, for an invariable mass of a perfect gas, at constant temperature, the product of its pressure and volume is a constant.
(i.e.) PV = constant
Charles's law (the law of volume):
Charles's law was formulated by a French scientist Jacques Charles. According to this law, When the pressure of gas is kept constant, the volume of a gas is directly proportional to the temperature of the gas.
V \(\propto\) T
\(\text { or } \frac{V}{\mathrm{~T}}=\mathrm{constan} \mathrm{t}\)
Avogadro's law:
Avogadro's law states that at constant pressure and temperature, the volume of a gas is directly proportional to number of atoms or molecules present in it.
i.e. V \(\alpha\) n 
(or) V/ n = constant
(i) Avogadro's number (N_A) is the total number of atoms per mole of the substance.
(ii) It is equal to 6.023 x 10²³ /mol.

Heat energy:
(i) When a hot object is in contact with another cold object, a form of energy flows from the hot object to the cold object, which is known as thermal energy.
(ii) Thus, thermal energy is a form of energy which is transferred between any two bodies due to the difference in their temperatures.
(iii) Thermal energy is also known as 'heat energy' or 'simply heat'.
Characteristics features of heat energy transfer:
(i) Heat always flows from a system at higher temperature to a system at lower temperature.
(ii) The mass of a system is not altered when it is heated or cooled.
(iii) For any exchange of heat, the heat gained by the cold system is equal to heat lost by the hot system.
(iv) Heat gained = Heat lost

The different types of expansion of solid are listed and explained below
(i) Linear expansion
(ii) Superficial expansion
(iii) Cubical expansion
Linear expansion:
(i) When a body is heated or cooled, the length of the body changes due to change in its temperature.
(ii) This expansion is said to be a linear or longitudinal expansion.
(iii) The ration of increase in length of a body per degree celsius rise in temperature to its unit length is called the coefficient of linear expansion. Its SI unit is per Kelvin. The value of coefficient of linear expansion is different for different materials.

(iii) The equation relating the change in length to the change in temperature of a body is given below:
\(\frac { \triangle L }{ { L }_{ 0 } } \) = α_L ΔT
ΔL  -Change in length
L₀ - Original length
ΔT - Change in temperature
α_L - Coefficient of linear expansion.
Superficial expansion:
(i) If there is an increase in the area of a solid object due to heating, then the expansion is called superficial or areal expansion.
(ii) Superficial expansion is determined in terms of coefficient of superficial expansion.
(iii) The ratio of increase in area of the body, per degree rise in temperature to its unit area is called coefficient of superficial expansion.
(iv) Coefficient of superficial expansion is different for different materials. Its SI unit is also per Kelvin.
(v) The equation relating to the change in area to the change in temperature is given below:

\(\frac { \triangle A }{ { A }_{ 0 } } \) =α_A ΔT
ΔA  -Change in area
A₀ - Original area
ΔT - Change in area
α_A - Coefficient of superficial expansion
Cubical expansion:
(i) If there is an increase in the volume of a solid body due to heating, then the expansion is called cubical or volumetric expansion.
(ii) As in the cases of linear and areal expansion, cubical expansion is also expressed in terms of coefficient of cubical expansion.
(iii) The ratio of increase in volume of the body per degree rise in temperature to its unit volume is called coefficient of cubical expansion. This is also measured in K^-1.

(iii) The equation relating to the change in volume to the change in temperature is given below:
\(\frac { \triangle V }{ { V }_{ 0 } } \) = α_v ΔT
ΔV - Change in volume
V₀ - Original volume
ΔT - Change in temperature
α_v - Coefficient of cubical expansion.

Gases are classified as real gases and ideal gases.
Real Gases:
i) If the molecules or atoms of a gases interact with each other with a definite amount of intermolecular or inter atomic force of attraction, then the gases are said to be real gases.
ii) At very high temperature or low pressure, a real gas behaves as an ideal gas because in this condition there is no interatomic or intermolecular force of attraction.
Ideal Gases:
iii) If the atoms or molecules of a gas do not interact with each other, then the gas is said to be an ideal gas or a perfect gas.
iv) Actually, in practice, no gas is ideal. The molecules of any gas will have a certain amount of interaction among them.
v) But, these interactions are weaker when the pressure is low or the temperature is high because the interatomic or intermolecular forces of attraction are weak in ideal gas.
vi)  Hence, a real gas at low pressure or high temperature can be termed as a perfect gas. Ideal gases obey Boyle’s law, Charles’s law and Avogadro's law.
vii)  All these laws state the relationship between various properties of a gas such as pressure (P), volume (V), temperature (T) and number of atoms (n). In a given state of the gas, all these parameters will have a definite set of values.
viii) When there is a change in the state of the gas, any one or more of these parameters change its value.

Real expansion:
i) If a liquid is heated directly without using any container, then the expansion that you observe is termed as real expansion of the liquid.
ii) Coefficient of real expansion is defined as the ratio of the true rise in the volume of the liquid per degree rise in temperature to its unit volume.
iii) The SI unit of coefficient of real expansion is K^-1
Apparent expansion:
i) Heating a liquid without using a container is not possible. Thus, in practice, we can heat any liquid by pouring it in a container.
ii) A part of thermal energy is used in expanding the container and a part is used in expanding the liquid.
iii) Thus, we observed is not the actual or real expansion of the liquid.
iv) The expansion of a liquid apparently observed without considering the expansion of the container is called the apparent expansion of the liquid.
v) Coefficient of apparent expansion is defined as the ratio of the apparent rise in the volume of the liquid per degree rise in temperature to its unit volume.
vi) The SI unit of coefficient of apparent expansion is K^-1.

a) If there is an increase in the area of a solid object due to heating, then the expansion is called superficial or areal expansion.
b) Superficial expansion is determined in terms of coefficient of superficial expansion.
c) The ratio of increase in area of the body per degree rise in temperature to its unit area is called as coefficient of superficial expansion.
d) Coefficient of superficial expansion is different for different materials.
e) The SI unit of Coefficient of superficial expansion is K^-1.
f) The equation relating to the change in area and the change in temperature is given below.

\(\Delta A / A_{o}=\alpha_{A} \Delta T\)
\(\triangle A\) - Change in area (Final area - Initial area)
A_o - Original area
\(\triangle T\) - Change in temperature (Final temperature - Initial temperature)
\(\alpha\)_A - Coefficient of superficial expansion.

i) When a body is heated or cooled, the length of the body changes due to change in its temperature.
ii) Then the expansion is said to be linear or longitudinal expansion.
iii) The ratio of increase in length of the body per degree rise in temperature to its unit length is called as the coefficient of linear expansion.
iv) The SI unit of Coefficient of Linear expansion is K^-1.. The value of coefficient of linear expansion is different for different materials.

5. The equation relating the change in length and the change in temperature of a body is given below:
\(\Delta \mathrm{L} / \mathrm{L}_{0}=\alpha \mathrm{L} \Delta \mathrm{T}\)
 \(\triangle L\) - Change in length (Final Length-Original length)
Lo - Original length
\(\triangle T\) - Change in temperature (Final temperature - Initial temperature)
\(\alpha L\) - Coefficient of linear expansion.

i) Two or more physical systems or bodies are said to be in thermal equilibrium if there is no net flow of thermal energy between the systems.
ii)  Heat energy always flows from one body to the other due to a temperature difference between them.
iii)  If two bodies are said to be in thermal equilibrium, then, they will be at the same temperature.
iv) There will be a transfer of heat energy from the hot body to the cold body until a thermal equilibrium is established between them.

v)  When a cold body is placed in contact with a hot body, some thermal energy is transferred from the hot body to the cold body.
vi)  As a result, there is some rise in the temperature of the cold body and decrease in the temperature of the hot body.
vii) This process will continue until these two bodies attain the same temperature.

Due to difference in conductivity, metals having high conductivity compared to wood. On touch with a finger, heat from the surrounding flows faster to the finger from metals and so one feels the heat. Similarly, when one touches a cold a metal the heat from the finger flows away to the surroundings faster.

A substance that heats up quickly has a low specific heat capacity. Higher the specific heat capacity of a substance, lower is the increase in its temperature.

Watermelon having higher specific heat will enhance its temperature slowly compared to the sandwiches which has lower specific heat.

During driving temperature of the gas increases while its volume remains constant. So according to Charles's law at constant V, P α T. ∴ Pressure of gas increases.