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For Class 9 Chemistry, this chapter in ICSE Class 9 Chemistry Chapter 02 Study of Gas Laws provides a detailed overview of important concepts. We highly recommend using this text alongside the ICSE Solutions for Class 9 Chemistry to learn the exercise questions provided at the end of the chapter.
Chapter 2 Study of Gas Laws ICSE Book Class Class 9 PDF (2026-27)
Chapter 2
Study of Gas Laws
Latest Syllabus - Scope of Syllabus - Study of Gas Laws
Study of Gas Laws
(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 \(P_1 V_1 / T_1 = P_2 V_2 / T_2\); simple relevant calculations based on gas equation.
(ii) Relationship between Kelvin Scale 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]
A. Introduction
Gases have certain properties which differentiate them from - solids and liquids.
The three states of matter are - solids, liquids and gases.
Gases differ from solids and liquids in the following ways:
1. Gases do not have a - definite shape or volume as in the case of solids and liquids [liquids have no definite shape].
2. The distance between the molecules is - much larger in case of gases, comparatively less in liquids and almost negligible in solids.
3. The force of attraction between the molecules is - negligible in case of gases, comparatively more in liquids and maximum in case of solids.
4. Molecules of a gas move in straight lines, undergo random collision with other molecules and occupy the complete space available in the container unlike solids and liquids.
B. Behaviour and Characteristic Properties of Gases
Behaviour and properties of gases can be explained on the basis of kinetic theory.
| Behaviour and Properties of Gases | Explanation on Basis of Kinetic Theory |
|---|---|
| 1. No Definite Shape or Volume Gases occupy - all the available space i.e. the shape and volume of the container - in which they are filled. | 1. No Definite Shape or Volume Molecules in a gas are far apart - and the forces of attraction between the gas molecules is very weak. Molecules are in continuous motion in the entire available space and their movement is restricted only by the walls of the container. |
| 2. High Compressibility The volume of a given mass of gas can be easily compressed to a much smaller volume by increasing the pressure upon it. | 2. High Compressibility Volume of particles negligible compared to intermolecular distance, hence can be compressed i.e. brought closer to each other. |
| 3. Exert Pressure in All Directions Gases unlike solids and liquids which exert pressure only partially downwards exert equal and uniform pressure on the walls of the container. | 3. Exert Pressure in All Directions Impact of gas molecules with high velocity causes pressure to be exerted on the walls. The same number of molecules strikes a given area on the wall in unit time. |
Teacher's Note
When you inflate a balloon, you can feel the gas pushing outward in all directions equally, which demonstrates why gases exert uniform pressure unlike liquids that flow downward due to gravity.
| Behaviour and Properties of Gases | Explanation on Basis of Kinetic Theory |
|---|---|
| 4. Have Low Densities Gases have much - lower densities compared to solids or liquids. Under ordinary pressure conversion of a given mass of solid or liquid to a gas causes a much greater increase in volume than the original, hence the - density of gases is very low - compared to solids and liquids. | 4. Have Low Densities The intermolecular distance - between the molecules of gases is very large. Hence the number of molecules - per unit volume of a gas is much lower compared to solids and liquids. Gases therefore have very low densities. |
| 5. Diffusibility or Miscibility Gases diffuse readily - and when kept in contact with one another combine completely forming a - homogenous mixture. | 5. Diffusibility or Miscibility The space between the molecules of gases - is very large since the intermolecular distance between the gas molecules is large. Hence the gas molecules have - a tendency of mixing with other gas molecules, their speed of movement depending on the mass and temp. of the gas. |
Teacher's Note
Perfume in a room spreads evenly to all corners quickly because gas molecules move rapidly and have large spaces between them, allowing easy mixing with air molecules.
C. Gas Laws
Behaviour of Gases by Three Variables - Temperature, Pressure and Volume
Gas Laws are Certain Rules which a Gas Follows when Subjected to a Change in - Temperature, Pressure and Volume.
The effect of change of any one of the above parameters affects the other two parameters e.g. volume of a gas undergoes significant change - if its pressure or temperature is slightly changed.
The parameters are described in the table shown below:
| Temperature | Pressure | Volume |
|---|---|---|
| An indicator of the average kinetic energy possessed - by a molecule. | Average force exerted - by bombardment of gas molecules on the walls of a container per unit area of the container or the ratio of the force exerted on the walls to the total area of the container wall. | Gases have indefinite expandability and occupy the volume of the container. |
| Kinetic energy increases - with an increase in temperature. | Volume of the gas - is the space occupied by a fixed mass of gas. | |
| Units Celsius temperature = °C Kelvin temperature = K [K = °C + 273] | Units Atmospheres cm Hg ; mm Hg Torr [Torricellli] 1 atmos.=76 cm Hg = 760 mm Hg 1 mm Hg = 1 torr | 1 Litre = 1 dm³ = 1000 cm³ 1 dm³ = 1000 cm³ = 1000 ml. 1 cm³ = 1 ml ml = millilitre; l = litre, cc = cubic centimetre; dm³ = cubic decimetre. |
Teacher's Note
The Kelvin scale is used in physics and chemistry because it starts at absolute zero where all molecular motion theoretically stops, making it ideal for gas law calculations where ratios of temperatures matter.
C. Gas Laws (Continued)
Relation Between Pressure and Volume of a Gas - When Temperature is Kept Constant.
Pressure is Increased
When the temperature of a fixed mass of gas is kept constant and the - pressure is increased systematically the volume correspondingly decreases.
A gas 'X' occupies 500 ml. when the pressure is 2 atmospheres and - 250 ml. when the pressure is increased to 4 atmospheres.
Pressure is Decreased
When the temperature of a fixed mass of gas is kept constant and the - pressure is decreased systematically the volume correspondingly increases.
If the pressure is decreased to 1 atmosphere the volume of the gas is 1000 ml.
| Experiment No. | Pressure [Atmos.] | Volume [ml.] |
|---|---|---|
| 1. | 2 | 500 ml. |
| 2. | 1 | 1000 ml. |
| 3. | 4 | 250 ml |
Relation Between Temperature and Volume of a Gas - When Pressure is Kept Constant.
Temperature Increased
When the pressure of a fixed mass of gas is kept constant and the - temperature of the gas is increased systematically the volume correspondingly increases.
A gas 'Y' occupies 200 ml. when the temperature is 400 K. If it is heated to 600 K its volume is increased to 300 ml..
Temperature Decreased
When the pressure of a fixed mass of gas is kept constant and the - temperature of the gas is decreased slowly the volume correspondingly decreases.
The gas 'Y' occupies 100 ml. when the temperature is decreased to 200 K.
| Experiment No. | Temperature [in K] | Volume [ml.] |
|---|---|---|
| 1. | 400 K | 200 ml. |
| 2. | 600 K | 300 ml. |
| 3. | 200 K | 100 ml |
Teacher's Note
When you heat air in a hot air balloon at constant pressure, it expands and becomes less dense, allowing the balloon to rise, demonstrating Charles' Law in action.
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ICSE Book Class 9 Chemistry Chapter 2 Study of Gas Laws
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