Class 9 Science

Change of States of Matter

  • Effect of temperature
  • Effect of pressure
  • Unit of temperature

Matters can change from one state to another state. A solid can change into liquid and a liquid can change into gas. A solid turns into liquid on heating and the liquid turns into vapor on further heating.

The change of state of matters mainly depends upon two factors:

  • Temperature
  • Pressure

Effect of Temperature

  • Solids change into liquid with increase in temperature. (Solid → Liquid)
  • A liquid changes into gas with increase in temperature. (Liquid → Gas)
  • Gas changes into liquid with decrease in temperature. (Gas → Liquid)
  • Liquid changes into solid with decrease in temperature. (Liquid → Solid)
  • Example: Ice is solid, water is liquid and vapour is gas.

Ice changes into water with increase in temperature. Water changes into vapor with increase in temperature.

Increase in temperature
Ice → Water → Vapour

Reverse of above changes happens when temperature decreases. Vapor changes into water with decrease in temperature. Water changes into ice with decrease in temperature.

Decrease in temperature
Vapour → Water → Ice

Solid to Liquid

When a solid is heated, its particles get more kinetic energy and, as a result, they start moving rapidly. Heating also increases the space between particles. After a certain rise in temperature, particles move apart to such an extent that the solid turns into liquid.

Liquid to Gas

When a liquid is heated, its particles get more kinetic energy and, as a result, start moving rapidly. Heating also increases the space between particles. After a certain rise in temperature, particles move apart to such an extent that the liquid turns into gas.

The change of solid to liquid is called melting. It is also called fusion. The change of liquid to gas is called vaporization. Melting and vaporization happen because of increase in temperature.

The change of gas into liquid is called condensation. The change of liquid into solid is called freezing. Condensation and freezing happen because of decrease in temperature.

Effect of Pressure

Effect on Solid

There is no effect of pressure on solids.

Solids are non compressible, i.e. solids cannot be compressed as there is no space between their particles to allow compression.

When pressure is increased on a solid, it is deformed and finally broken.

Effect on Liquid

There is no effect of pressure on liquid.

Liquids are non compressible, i.e. liquids cannot be compressed since there is not enough space between their particles to get compressed.

Effect on Gas

The volume of gas decreases with increase in pressure.

Since there is lot of space between the particles, gas is highly compressible. Large volume of gas can be compressed to a small volume. Because of its high compressibility, gas can be compressed in cylinders for easier transportation.

Natural gas is compressed to small volume and packed in cylinders. It is used widely as fuel to running vehicles. Because of compression it is called Compressed Natural Gas or simply known by CNG (Compressed Natural Gas).

Oxygen is compressed and packed in small cylinders, which is used for saving life in hospitals.

Liquefaction of gas by increasing pressure

In general condition, the particles of gas are far from each other. They move independently and rapidly in all directions. There is lot of space between the particles of gas. The force of attraction is negligible between the particles of gas. Due to this, a gas can be compressed to a great extent.

When pressure is applied on a gas, its particles come closer to each other. After applying high pressure, the particles of gas come so close to each other that they start attracting each other. The space between the particles of gas also decreases with increase in pressure.

While applying high pressure a lot of heat is evolved. This heat comes because of the kinetic energy in particles. It is important to keep the temperature low, while applying high pressure to compress a gas. A stage comes when extremely high pressure allows the particles to get bound to each other. At this stage, the gas turns into liquid.

This process is called liquidation or liquefaction of gas.

Petroleum Gas is compressed to small volumes. At extremely high pressure it turns into liquid, and then is packed into cylinders. Now, you know why the gas cylinder in your kitchen is called LPG (Liquefied Petroleum Gas) cylinder.

Oxygen, hydrogen, nitrogen, helium, etc. are compressed to turn them into liquid forms. Liquefied oxygen is used as propellant in spacecraft rockets, in industries and for medical purposes. Liquefied oxygen is of blue color.

It is interesting to note that a gas can also turn into liquid, On further increase in pressure. You may heard about dry-ice. It is also known as Cardice. When carbon dioxide gas is subjected to extremely high pressure, it turns into solid. Dry ice is used in restaurants and also in cryogenics.

Cryogenics is a branch of physics. It deals with effect and production of extremely low temperature.

Once pressure is reduced, a liquefied gas turns into gas, i.e. to gaseous state. When dry ice aka solid carbon dioxide is left in open, it instantly turns into gas without undergoing the liquid state.

Unit of Temperature

Temperature is measured in Degree Celcius, Fahrenheit and Kelvin.


Before the end of 20th century Fahrenheit was used as unit to measure the temperature. The scale of Fahrenheit was named after, the physicist Daniel Gabriel Fahrenheit. Fahrenheit is written as °F and read as degree Fahrenheit.

  • 32°F (Thrity two degree Fahernheit) has been defined as the freezing point of water.
  • 212°F (Two hundred tweleve degree Fahrenheit) has been defined as the boiling point of water.

Degree Celsius

Now-a-days, this unit of temperature is more in use. Celsius is named after the Swedish Astronomer Anders Celsius, who developed a similar scale. Untill 1948 Celsius was known as Centigrade. The word centigrade comes after the Latin word Centum. Degree Celsius is written as °C and read as degree Celsius.

  • 0°C (zero degree Celsius) has been defined as the freezing point of water.
  • 100°C (hundred degree Celsius) has been defined as the boiling point of water.


Kelvin is the SI unit of temperature. The symbol of Kelvin is K. The Kelvin scale is named after the physicist William Thomson, 1st Baron Kelvin.

The scale of Kelvin is used widely because of its many advantages over degree Celsius. In Kelvin, temperatures are usually written in positive.

  • 273 K is defined as the freezing point of water or melting point of ice.
  • 373 K is defined as the boiling point of water.
  • At Kelvin scale -273 K is considered as the absolute zero temperature.

Conversion Formulae

Degree Celsius → Degree Fahrenheit → Kelvin

Celsius to Fahrenheit: `[\ ^0F]=[\ ^0C]xx9/5+32`

Fahrenheit to Celsius: `[\ ^0C]=([\ ^0F]-32)xx5/9`

Celsius to Kelvin: `[K]=[\ ^0C]+273`

Kelvin to Celsius: `[\ ^0C]=[K]-273`


See Answer

1: (a) Temperature, 2: (b) Pressure, 3: (b) Zero, 4: (d) 373, 5: (c) 273, 6: (b) Kelvin, 7: (d) Pascal, 8: (a) Add 273, 9: (b) Subtract 273, 10: (d) 32

Matter in Our Surroundings

Anything that has both mass and volume is called matter. You can also say that anything which has mass and which occupies space is called matter.

Is Matter Around Us Pure?

Elements and compounds are pure substances. All other substances are mixtures which means they are not pure substances.

Atoms and Molecules

Read about law of conservation of mass, law of constant proportions and Dalton's atomic theory.

Structure of Atoms

Atom is made of three particles; electron, proton and neutron. These particles are called fundamental particles of an atom or sub atomic particles.

Cell: The Fundamental Unit of Life

A cell is capable of independent existence and can carry out all the functions which are necessary for a living being.


A groups of cells which is meant for a specific task is called tissue. Tissues are the first step towards division of labour in complex organisms.

Diversity in Living Organisms

Without proper classification, it would be impossible to study millions of organisms which exist on this earth.


If an object changes its position with respect to a reference point with elapse of time, the object is said to be in motion.

Force & Laws of Motion

Force has numerous effects. Force can set a stationary body in motion. Force can stop a moving body.


Earth attracts everything towards it by an unseen force of attraction. This force of attraction is known as gravitation or gravitation pull.

Work & Energy

When force is exerted on an object and object is displaced, work is said to be done. It means work is the product of force and displacement.


Sound is a type of energy. Sound travels in the form of wave from one place to another.

Why Do We Fall Ill

Health is a state of physical, mental and social well being. A condition in which the affected person is unable to carry out normal activities is termed as disease.

Natural Resources

Resources which are obtained from nature are called natural resources. Examples: Air, water, soil, wood, etc.

Improvement in Food Production

Food security is said to exist when all people, at all times, have physical and economic access to sufficient, safe and nutritious food to meet their dietary needs and food preferences for an active and healthy life.