Everything around you — the air you breathe, the water you drink, the chair you sit on — is matter. It is made of tiny moving particles, and just by changing how fast they move you can turn ice into water and water into steam!
What is matter?
Anything that has mass and occupies space (volume).
Made of particles
Matter is built from extremely tiny particles with empty space between them.
Three states
Solid, liquid and gas — decided by how strongly particles hold together.
States can change
Heating or cooling and pressure can switch matter from one state to another.
Matter is made of particles
Long ago, people in India described matter as being made of five basic elements (the Panch Tatva) — air, earth, fire, sky and water. Modern science agrees on the key point: matter is made of tiny particles. A simple proof is dissolving. When you stir sugar or salt into water, it seems to disappear, yet the water tastes sweet or salty. This shows the sugar broke up into particles so small that they slipped into the spaces between the water particles. The same way, a tiny crystal of potassium permanganate can colour a huge amount of water, proving each particle is unbelievably small and that there are millions of them.
Characteristics of particles of matter
Particles of matter have four important properties. First, they have space between them — this is why one substance can dissolve into another. Second, they are continuously moving, meaning they have kinetic energy that increases on heating. We see this in diffusion: the smell of incense or food spreads across a room because gas particles mix on their own. Third, particles attract each other — a force of attraction holds them together, strong in iron and weak in gases. Fourth, particles are extremely small, far too tiny to see even with an ordinary microscope.
Diffusion and temperature
Diffusion is the spontaneous mixing of particles of one substance into another. It is fastest in gases, slower in liquids, and almost zero in solids. Importantly, diffusion becomes faster on heating, because particles gain kinetic energy and move quicker. A crystal of copper sulphate spreads colour much faster in hot water than in cold water — clear proof that particle movement increases with temperature.
The three states of matter
Matter exists in three common states. In a solid, particles are packed very closely in a fixed, orderly arrangement with strong forces of attraction. So solids have a fixed shape and fixed volume, are rigid and almost cannot be compressed. In a liquid, particles are close but not fixed; they can slide past one another. So liquids have a fixed volume but no fixed shape — they take the shape of their container and can flow. In a gas, particles are very far apart, move fast and randomly, and have very weak attraction. So gases have neither fixed shape nor fixed volume, can be compressed greatly, and exert pressure on the walls of the container as their particles collide with them.
Compressibility, fluidity and density
Because gases have the most empty space, they are highly compressible — this is why we can fill large volumes of gas like LPG and CNG into small cylinders. Liquids and gases can flow, so both are called fluids. Solids generally have the highest density (most mass packed in a small volume), liquids less, and gases the least. The rate of diffusion and the kinetic energy of particles both follow the order gas > liquid > solid.
Change of state by heating: melting and boiling
When a solid is heated, its particles gain energy and vibrate faster until they break free from their fixed positions and the solid turns into a liquid. This is melting (fusion). The temperature at which a solid melts at normal atmospheric pressure is its melting point — for ice it is 273.15 K (0°C). On further heating, the liquid particles move so fast that they escape into the gaseous state. This is boiling, and the temperature is the boiling point — for water it is 373 K (100°C). A surprising fact is that the temperature does not rise while a substance is changing state, even though heat is being supplied.
Latent heat
The heat that is hidden during a change of state is called latent heat. The latent heat of fusion is the heat needed to change 1 kg of a solid into liquid at its melting point without raising the temperature. The latent heat of vaporisation is the heat needed to change 1 kg of liquid into gas at its boiling point. This is why steam causes far more severe burns than boiling water at the same temperature — steam carries extra latent heat of vaporisation, releasing more energy on your skin.
Sublimation and effect of pressure
Some solids, like camphor, dry ice (solid carbon dioxide) and ammonium chloride, change directly into gas without becoming liquid. This is sublimation, and the reverse (gas to solid directly) is deposition. Changing pressure can also change states. By applying high pressure and lowering temperature, a gas can be turned into a liquid; solid CO2 is stored under high pressure and turns straight to gas when released, which is why it is called dry ice.
Evaporation
Evaporation is the change of a liquid into vapour at any temperature below its boiling point, and it happens only at the surface. Some surface particles always have enough energy to escape into the air. Evaporation increases with higher temperature, larger surface area, faster wind speed, and lower humidity. Evaporation causes cooling: the escaping particles take energy (latent heat) from the surroundings, which is why we feel cool after sweating, why water stays cool in an earthen pot, and why we feel cold when we put acetone or petrol on our palm.
- Temperature: Kelvin = degree Celsius + 273 (K = °C + 273)
- Celsius: °C = K − 273
- Melting point of ice = 273.15 K = 0°C
- Boiling point of water = 373 K = 100°C
- States by particle spacing: solid < liquid < gas (compressibility & spacing increase in this order)
- Density order: solid > liquid > gas
- Fluids = liquids + gases (they can flow)
Convert the boiling point of water, 100°C, into the Kelvin scale.
- Write the formula: K = °C + 273.
- Substitute the value: K = 100 + 273.
- Add the numbers: K = 373.
The melting point of a metal is 600 K. Express this temperature on the Celsius scale.
- Write the formula: °C = K − 273.
- Substitute the value: °C = 600 − 273.
- Subtract the numbers: °C = 327.
To remember the states, picture a school: Solids are students standing in a tight, fixed assembly line (no moving). Liquids are students walking in the corridor (close but moving). Gases are students running wild in a huge playground (far apart, fast). For temperature, remember "Add 273 to go up to Kelvin" — Kelvin is the bigger number.
The most common mistake is writing that temperature keeps rising while ice melts or water boils. It does not — during a change of state the temperature stays constant because all the heat supplied becomes latent heat used to break the forces between particles. Also remember evaporation is a surface phenomenon happening at all temperatures, while boiling happens throughout the liquid at a fixed temperature.
Q1. Why does a gas exert pressure on the walls of its container?
Answer: A gas is made of particles that are far apart and move very fast in all directions in a random manner. These fast-moving particles constantly collide with the walls of the container. Each collision pushes against the wall, and the combined effect of millions of such collisions per second creates a continuous force on the walls. This force acting per unit area of the walls is what we call the pressure exerted by the gas.
Q2. Why does the temperature remain constant during the melting of ice even though heat is supplied continuously?
Answer: When ice melts, the heat supplied is not used to raise the temperature. Instead, it is used to overcome the strong forces of attraction holding the particles in the fixed solid arrangement, so they can move freely as a liquid. This hidden heat absorbed without a temperature change is called the latent heat of fusion. Until all the ice has melted, every bit of heat goes into this process, so the thermometer stays at 0°C (273.15 K).
Q3. Why does evaporation cause cooling? Give an everyday example.
Answer: During evaporation, the fastest-moving particles at the surface of a liquid escape into the air. To leave, these particles absorb energy (latent heat of vaporisation) from the remaining liquid and the surroundings. As energy is taken away, the temperature of the surroundings falls, producing a cooling effect. For example, water kept in an earthen pot (matka) stays cool because water seeping through the tiny pores evaporates and draws heat from the water inside. Similarly, we feel cool when sweat evaporates from our skin.
Q4. List the factors that affect the rate of evaporation and explain how each one acts.
Answer: Four main factors affect evaporation. (1) Temperature — a higher temperature gives particles more energy to escape, so evaporation increases. (2) Surface area — a larger exposed surface lets more particles escape, so spreading clothes increases drying. (3) Wind speed — faster wind carries away water vapour, lowering humidity near the surface and speeding evaporation. (4) Humidity — when the air already holds a lot of water vapour, it cannot take much more, so evaporation slows down on humid days.
- ✅ Matter has mass and volume and is made of tiny, moving, attracting particles with space between them.
- ✅ Three states: solid (fixed shape & volume), liquid (fixed volume only), gas (neither).
- ✅ Heating, cooling and pressure change states; temperature stays constant during a change of state due to latent heat.
- ✅ Evaporation is surface cooling at any temperature; K = °C + 273.
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