WB Madhyamik Physical science Behaviour of gases

We will discuss the behaviour of gases and how they can be described by different laws and models. We will also learn about some applications of gas laws in our daily life.

• Gases are one of the three states of matter, along with solids and liquids. Gases have no fixed shape or volume, and they can expand or contract to fill any container. Gases are composed of tiny particles called molecules, which are constantly moving in random directions and colliding with each other and the walls of the container.

  The behaviour of gases can be explained by two main models:

• the kinetic molecular theory and
• the ideal gas law.

The kinetic molecular theory is based on the following assumptions:

– The gas molecules are very small compared to the distance between them, and their volume can be ignored.
– The gas molecules are in constant motion and collide with each other and the walls of the container. These collisions are elastic, meaning that no energy is lost or gained during the collisions.
– The gas molecules do not exert any attractive or repulsive forces on each other.
– The average kinetic energy of the gas molecules is proportional to the temperature of the gas.

The kinetic molecular theory can be used to derive some properties of gases, such as pressure, volume, temperature, and density.

• Pressure is defined as the force exerted by the gas molecules per unit area on the walls of the container. Pressure depends on the number of collisions per unit time and the force of each collision. More collisions or stronger collisions result in higher pressure.
• Volume is defined as the space occupied by the gas molecules. Volume depends on the number of gas molecules and their speed. More gas molecules or faster gas molecules result in larger volume.
• Temperature is defined as a measure of the average kinetic energy of the gas molecules. Temperature depends on the speed of the gas molecules. Higher speed means higher temperature.
• Density is defined as the mass of the gas molecules per unit volume. Density depends on the number and mass of the gas molecules and their volume. More or heavier gas molecules or smaller volume result in higher density.

The ideal gas law is a mathematical equation that relates pressure, volume, temperature, and number of moles (a unit to measure amount) of a gas. The ideal gas law can be written as:

PV = nRT

where P is pressure, V is volume, n is number of moles, R is a constant called the universal gas constant, and T is temperature.

The ideal gas law can be used to calculate any one of these variables if we know the other three. For example, if we know the pressure, volume, and temperature of a gas, we can find out how many moles of gas there are.

The ideal gas law assumes that the gas behaves like an ideal gas, meaning that it follows all the assumptions of the kinetic molecular theory. However, in reality, no gas is truly ideal, and there are some deviations from this law at high pressure or low temperature. In these conditions, we need to use more complex models such as van der Waals equation or real gas equation.

• Some applications of gas laws in our daily life are:

– Balloons: Balloons are filled with air or helium gas, which exerts pressure on the walls of the balloon and makes it expand. When we release a balloon, it flies away because the air outside has lower pressure than inside.
– Tyres: Tyres are filled with air or nitrogen gas, which provides cushioning and traction for vehicles. When we drive on a hot road or at high speed, the tyres get heated up and their pressure increases. This can cause them to burst if they are overinflated.
– Refrigerators: Refrigerators use a cycle of compression and expansion of a refrigerant gas to transfer heat from inside to outside. The refrigerant gas is compressed by a compressor and becomes hot and high-pressure. It then passes through a condenser where it releases heat to the surroundings and becomes liquid. It then passes through an expansion valve where it expands and becomes cold and low-pressure. It then passes through an evaporator where it absorbs heat from inside and becomes gas again. This cycle repeats until the desired temperature is reached inside.