Class 9 Science Chapter 1: Matter in Our Surroundings

Matter in Our Surroundings is Chapter 1 of NCERT Class 9 Science. It explains what matter is made of, how its particles behave, the three common states of matter and how one state changes into another through heating, cooling, pressure and evaporation. Watch the three video lessons below, then use these notes, temperature conversions and solved examples to revise.

What you will learn

• The meaning of matter and the evidence that it is made of tiny particles
• The characteristics of particles: spaces between them, constant motion and attraction
• How solids, liquids and gases differ in shape, volume, compressibility and rigidity
• Melting, boiling, sublimation and the idea of latent heat
• Evaporation, the factors that affect it and why it causes cooling

Video Lessons

Matter in Our Surroundings - Part 1

Matter in Our Surroundings - Part 2

Matter in Our Surroundings - Part 3

Key Concepts and Formulas

Physical Nature of Matter

Air, water, a pencil and even the smell of food are all matter. When salt dissolves in water, the water level hardly rises, because salt particles fit into the spaces between water particles.

• Matter: anything that has mass and occupies volume
• Matter is made of extremely small particles; a few crystals of potassium permanganate can colour a large volume of water, showing how tiny and numerous the particles are

Characteristics of Particles of Matter

The behaviour of all matter can be explained by three simple properties of its particles.

• Particles have spaces between them, which is why one substance can mix into another
• Particles are always moving; their kinetic energy increases as temperature rises
• Diffusion: the spontaneous intermixing of particles of two different substances; it is fastest in gases and becomes faster on heating
• Particles attract each other; this force of attraction is strongest in solids and weakest in gases

States of Matter

The three common states differ because of how closely packed their particles are and how strongly they attract one another.

• Solids: definite shape, definite volume, rigid, negligible compressibility (a sponge is still a solid; it compresses because air is trapped in its pores)
• Liquids: definite volume but no fixed shape, they flow and take the shape of the container, and gases like oxygen and CO₂ can dissolve in them
• Gases: no fixed shape or volume, highly compressible (that is why LPG and CNG are stored in cylinders), and they exert pressure on the walls of the container

Change of State and Latent Heat

During a change of state, the temperature stays constant because the heat supplied is used to overcome the forces of attraction between particles.

T(K) = T(°C) + 273   (more precisely 273.15)
Melting point of ice = 0 °C = 273 K
Boiling point of water = 100 °C = 373 K
Heat absorbed: Q = m × L
L (fusion of ice) ≈ 3.34 × 10⁵ J/kg
L (vaporisation of water) ≈ 22.5 × 10⁵ J/kg

• Melting point: temperature at which a solid melts at atmospheric pressure
• Boiling point: temperature at which a liquid boils at atmospheric pressure
• Latent heat of fusion: heat needed to change 1 kg of a solid into liquid at its melting point
• Latent heat of vaporisation: heat needed to change 1 kg of a liquid into gas at its boiling point
• SI unit of temperature is the kelvin (K)
• Sublimation: a solid changing directly into vapour without becoming a liquid (camphor, naphthalene, ammonium chloride, iodine); the reverse is called deposition

Effect of Pressure

Applying pressure pushes gas particles closer together. Increasing pressure and lowering temperature together can turn a gas into a liquid.

1 atm = 1.01 × 10⁵ Pa

• Solid CO₂ (dry ice) changes directly into gas when the pressure is lowered to 1 atmosphere
• SI unit of pressure is the pascal (Pa)

Evaporation and Cooling

Some particles at the surface of a liquid always have enough energy to escape, even below the boiling point. This surface process is called evaporation.

• Evaporation: change of a liquid into vapour at any temperature below its boiling point; it is a surface phenomenon
• Rate of evaporation increases with larger surface area, higher temperature, lower humidity and higher wind speed
• Evaporation causes cooling because the escaping particles take latent heat from the liquid and its surroundings (water in an earthen pot stays cool; acetone on the palm feels cold)
• Cotton clothes are preferred in summer as they absorb sweat and help it evaporate
• Droplets on the outside of a glass of ice water form because water vapour in the air condenses on the cold surface

Solved Examples

Example 1: Convert 47 °C into the Kelvin scale.

T(K) = T(°C) + 273
T = 47 + 273
∴ T = 320 K

Example 2: Convert 250 K into degree Celsius.

T(°C) = T(K) − 273
T = 250 − 273
∴ T = −23 °C

Example 3: What is the physical state of water at 278 K and at 380 K?

278 K = 278 − 273 = 5 °C  (between 0 and 100 °C)
380 K = 380 − 273 = 107 °C  (above 100 °C)
∴ Liquid at 278 K; gas (steam) at 380 K

Example 4: How much heat is needed to melt 2 kg of ice at 0 °C? (Latent heat of fusion = 3.34 × 10⁵ J/kg)

Q = m × L
Q = 2 × 3.34 × 10⁵
∴ Q = 6.68 × 10⁵ J (temperature stays at 0 °C)

Example 5: Arrange oxygen, water and sugar in increasing order of the force of attraction between their particles.

Answer: Gas particles attract least and solid particles most, so the order is oxygen < water < sugar.

Example 6: Why does steam at 100 °C cause more severe burns than boiling water at 100 °C?

Answer: Steam carries extra energy equal to the latent heat of vaporisation (about 22.5 × 10⁵ J per kg), which is released on the skin when it condenses, so steam delivers more heat and burns more badly.

Common Mistakes to Avoid

• Do not forget to add or subtract 273 when converting between °C and K; and never write "degree kelvin", just K.
• Evaporation happens at any temperature, while boiling happens at a fixed temperature throughout the liquid. Do not mix them up in definitions.
• During melting or boiling the temperature does not rise, even though heat is being supplied. Mention latent heat in such answers.
• When listing factors affecting evaporation, state the direction clearly, for example "higher humidity decreases the rate".

Quick Revision

T(K) = T(°C) + 273
Ice melts at 273 K; water boils at 373 K
Q = m × L
Sublimation: solid → gas (camphor, naphthalene, NH₄Cl)

• Matter has mass and occupies space; it is made of tiny moving particles
• Solids: fixed shape and volume; liquids: fixed volume; gases: neither
• Latent heat is absorbed without any change in temperature
• Evaporation is a surface process that causes cooling

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