ICSE Class 9 Chemistry Chapter 02 Study of Gas Laws

Study of Gas Laws

Scope of Syllabus

(i) The behaviour of gases under changes of temperature and pressure; explanation in terms of molecular motion (particles, atoms, molecules); Boyle's Law and Charles' Law; absolute zero; gas equation; simple relevant calculations.

The behaviour of gases under changes of temperature and pressure; explanation in terms of molecular motion (particles, atoms, molecules). Boyle's Law (statement, mathematical form, simple calculations).

Charles' Law; (statement, mathematical form, simple calculations). Absolute zero; Kelvin scale of temperature. Gas equation \(P_1V_1/T_1 = P_2V_2/T_2\); simple relevant calculations based on gas equation.

(ii) Relationship between Kelvin Scale of temperature and Celsius Scale of temperature; Standard temperature and pressure.

Conversion of temperature from Celsius scale to Kelvin scale and vice versa. Standard temperature and pressure. (simple calculations)

Important Points To Remember

1. Anything that has weight and occupies space is called matter.

2. There are three states of matter:

(i) Solid

(ii) Liquid

(iii) Gas.

3. Comparison in the properties of solid, liquid and gas.

Teacher's Note

Think about how a sponge (solid with tiny pores), water (liquid), and air (gas) differ in how tightly their particles are packed - this affects their properties in everyday life.

4. The general characteristics of gases can be explained on the basis of kinetic theory of gases as follows:

(i) Gases have neither definite shape nor definite volume: The molecules in gases are far apart from each other, therefore intermolecular force of attraction is minimum and hence gases occupy the entire space in the container.

(ii) Gases are highly compressible: As the gases have maximum intermolecular space, therefore on compressing the gas the molecules come closer to each other and thereby, decreasing the volume.

(iii) Gases have minimum density: As gases have the smallest mass per unit volume, therefore; they have the minimum density.

(iv) Gases easily undergo diffusion: Gases readily undergo intermixing when kept in contact with each other to form homogeneous mixture as they have maximum intermolecular spaces.

5. The variables used during gas laws are the pressure, temperature and volume.

(i) Units of Temperature

(a) Celsius - °C

(b) Kelvin - K

(c) Normal temperature - 273 K = 0°C

Relationship between Celsius and Kelvin:

\[K = °C + 273\]

For example: Conversion of temperature on the Celsius scale to the Kelvin scale.

(a) 0°C = 0 + 273 = 273 K

(b) -273°C = -273 + 273 = 0 K

(c) 100°C = 100 + 273 = 373 K

(d) 200°C = 200 + 273 = 473 K

(ii) Units of Volume

(a) Millilitre - ml

(b) Cubic centimetre - cm³

(c) Litre - L

Relationship: 1 litre = 1000 ml = 1000 cm³

1 ml = 1 cm³

(iii) Units of Pressure

(a) Atmosphere - atm

(b) cm of Mercury (Hg) - cm Hg

(c) mm of Mercury (Hg) - mm Hg

Relationship between Atmosphere and Mercury:

1 Atmosphere = 76 cm of Hg = 760 mm of Hg

6. Boyle's law states that at constant temperature, the volume of a given mass of dry gas is inversely proportional to pressure.

\[V \propto \frac{1}{P}\]

(at constant T)

V = Volume of the dry gas

P = Pressure on the gas

\[V = k \frac{1}{P}\]

(where k is constant)

\[PV = k\]

Teacher's Note

When you pump up a bicycle tire, you are applying Boyle's Law - as you decrease the volume of air space, the pressure increases, which pushes air into the tire.

Boyle's law equation \[P_1V_1 = P_2V_2\]

P₁ = Initial pressure - V₁ = Initial volume

P₂ = Final pressure - V₂ = Final volume

7. Pressure of an enclosed mass of a dry gas remaining constant, the volume of the gas is directly proportional to absolute temperature. This law is known as Charles' law.

\[V \propto T\]

(at constant P)

V = Volume of the dry gas

T = Temperature of the gas

\[V = kT\]

(where k is constant)

\[\frac{V}{T} = k\]

Charles' law equation \[\frac{V_1}{T_1} = \frac{V_2}{T_2}\]

V₁ = Initial volume - V₂ = Final volume

T₁ = Initial temperature - T₂ = Final temperature

8. The new temperature scale on which the zero is at - 273°C, such that each degree on it is equal to one degree on the Celsius is called the Kelvin scale.

9. Absolute zero is the last or the lowest limit of temperature at which the volume becomes theoretically zero. The temperature for absolute zero is - 273°C.

10. The standard temperature is 0°C or 273 K.

11. The standard pressure is 760 mm of Hg or 76 cm of Hg or 1 atmosphere.

12. By combining Boyle's law and Charles' law the perfect gas equation can be derived as follows:

According to Boyles' law

\[V \propto \frac{1}{P}\]

...(i)

According to Charles' law

\[V \propto T\]

...(ii)

Combining Boyle's law (i) and Charles' law (ii), we get

\[V \propto \frac{1}{P} \times T \Rightarrow V \propto \frac{T}{P}\]

\[V = k \frac{T}{P} \Rightarrow \frac{PV}{T} = k\]

(where k is constant)

Perfect gas equation \[\frac{P_1V_1}{T_1} = \frac{P_2V_2}{T_2}\]

P₁ = Initial pressure - P₂ = Final pressure

V₁ = Initial volume - V₂ = Final volume

T₁ = Initial temperature - T₂ = Final temperature.

Teacher's Note

The combined gas law helps us understand why hot air balloons rise - heating gas increases its volume and decreases its density compared to surrounding air.

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