## Gases and gas law physics

## Introduction

We knwo all the substances expand on heating. The thermal expanisons of gases is very large in comparison to those solids and liquids. To state the condition of a solid, its temperature, pressure, and volume are required. If anyone among these three is changed, the other two will be affected. so the relationship between these three taking one constant at a time are called gas laws and these laws are reduced to a simple equations called equation of state.

## Dalton’s law of partial pressure

It states that total pressure exerted by a mixture of gases is equal to the sum of the partial pressure of the individual gas when there are no chemical interactions between them.

Fig. Apparatus for demonstrating Dalton’s law of partial pressure.

Consider two bulbs A and B separated by a tap key T. Let the Bulb A has volume V1 containing a gas at pressure p1 and bulb B volume V2 containing second gas at pressure p2. When the tap key is opened. The final volume occupied by each gas is (v1+v2). Experimentally it is found that total pressure exerted by the mixture of gases is

$p=\frac{{p}_{1}{v}_{1}}{{v}_{1}+{v}_{2}}+\frac{{p}_{2}{v}_{2}}{{V}_{1}+{v}_{2}}$…………………………… (equation 1)

if

${p}_{1and{p}_{2}}$the pressure exerted by the first and second gas when they are mixed, then the total pressure of the mixture can be written as.

## Boyle’s law

Boyle’s law states that on temperature keeping constant, the volume of the given mass of a gas is inversely proportional to the pressure. Mathematically, it can be written as

V proportional to

$\frac{1}{p}$or pv = constant ……………………..(i)

If p1 V1 are the initial pressure, and volume, and p2, v2 are the final pressure and volume, then

p1v1= p2v2 = constant

## Charles’s law

Charle’s law states that the volume of the given mass of a gas at constant pressure increases or decreases by a constant fraction of its volume at 0 degree celcious for each degree rise or fall in temperature.

## Important questions and answers

Under what condition do the real gases obey more strictly the gas equation pV=RT? Explain

or

At very low pressure and high temperature, the real gases behave like ideal gases, why?

**ans. **The essential properties of the molecules of an ideal gas are zero volume of the molecules and no mutual force between them. At low pressure, the volume of a gas is large so that the volume of the molecules is negligible as compared to the volume of the gas. At high temperature, The K.E of the molecule is quite large so that effect of intermolecular forces on the motion of the molecules is negligible. Hence at low pressure and high temperature, a real gas is approximately ideal Consequently, the gas equation gives more accurate results.

2. On reducing the volume of a gas at constant temperature, the pressure of a gas increases, why?

**ans. ** On reducing the volume, the number of molecules per unit volume increases. Therefore, a number of molecules collide with the walls per second and hence a large momentum is transferred to the walls per second. If the volume is halved, then the number of molecules per m^3 will be doubled, Therefore, according to the kinetic theory of gases, the pressure will be doubled. This is the explanation of Boyle’s law on the basis of kinetic theory.

3. Air pressure in a car tire increases during driving, why?

**ans. ** Due to the motion of the car, the temperature of the air inside the tire decreases. According to Charles law.

pressure is directly proportional to the pressure inside the tire increases.

4 . Why does the cycle tube burst sometimes in summer?

**ans. **In summer, the temperature of air in a cycle tube increases. Since the volume of air is constant, the pressure of air is directly proportional to the absolute temperature. So the increased pressure of air becomes more than the maximum possible capacity of the tube in summer. Then the cycle tube bursts.

5. Why does food cook faster in a pressure cooker than in an open pot?

**ans. ** When water is heated in a pressure cooker, the pressure of water vapor above the water level increases. This increased pressure causes water to boil at a temperature above 100 degrees Celcius. It means that the same mass of water contains more amount of heat. As a result, the food is cooked in much less time in a pressure cooker.

6. What happens when a metal cylinder containing oxygen is brought to the moon from the earth?

**ans. **There is no atmosphere on the surface of the moon. Outside the cylinder, atmospheric pressure is zero. Thus, the gas inside the cylinder tries to expand with high pressure but it is not strong enough to do so. Also, a pressure of the gas falls with fall in temperature because at the night, the moon is very cold Hence, the oxygen gas can be changed into a liquid state.

7. Why are gas laws not obeyed at low temperature and high pressure?

**ans. ** At low temperature and high pressure, the attraction between molecules becomes appreciable. In addition, the volume of the gas molecules cannot be ignored in comparison to the volume of the gas. Due to these factors, the shows derivation from ideal gas behavior and hence it does not obey gas laws.

## Some other important questions physics grade 11 from gas and gases law

- At what condition Boyle’s law obeys strictly?
- what do you mean by gas constant? What is it’s significance?
- What is the temperature when all molecular motion ceases?
- Explain the concept of absolute zero of temperature?
- Air pressure in a car tire increases during driving, why?
- At very low pressure and high temperature, the real gases behave like ideal gases, why?

## Long answer questions

- Derive the equation of state for an ideal gas.
- State, explain and proof charle’s pressure law.
- State, explain and proof charle’s volume law.
- Explain Boyle’s law and Charle’s law can be combined to give an equation for an ideal gas.
- When gas obeys Boyle’s law then prove that pressure coefficient and volume coefficient are equal.

## Physics practial questions from vernier calliper

also read on wiki for daltons law of partial pressure

### Also read : Chapter heat and temperature

physics grade 11 measurement and dimensions