1. Physical Nature of Matter

Everything in this universe is made up of material which scientists have named "matter". The air we breathe, the food we eat, stones, clouds, stars, plants and animals, even a small drop of water or a particle of sand—every thing is matter.

Two key conditions for anything to be called matter are: it must occupy space (volume) and have mass.

1.1 Matter is Made up of Particles

For a long time, there were two distinct schools of thought regarding the nature of matter. One group believed matter was continuous (like a block of wood), while the other believed it was particulate (made of discrete particles like sand). Today, we know conclusively that matter is particulate.

1.2 How Small are these Particles of Matter?

The particles of matter are incredibly small—beyond our imagination.

2. Characteristics of Particles of Matter

Based on various observations, scientists have established three fundamental characteristics of particles of matter:

2.1 Particles of Matter have Space between Them

As observed in the dissolving of salt, sugar, Dettol, or potassium permanganate, particles of one type of matter get evenly distributed and slip into the spaces between the particles of the other type of matter.

2.2 Particles of Matter are Continuously Moving

Particles of matter possess kinetic energy. As the temperature rises, particles move faster. Therefore, we can say that with an increase in temperature, the kinetic energy of the particles increases.

• Incense Stick: The smell of an unlit incense stick requires you to go close to it. But when lit, the smell reaches you from a distance because the particles gain kinetic energy from the heat and diffuse faster into the air.
• Diffusion: The intermixing of particles of two different types of matter on their own is called diffusion. For example, dropping blue ink into water—it spreads evenly without stirring. Heating accelerates diffusion.

2.3 Particles of Matter Attract Each Other

There is a force of attraction acting between the particles of matter that keeps them together. The strength of this force of attraction varies from one kind of matter to another.

• Solid objects: An iron nail is very hard to break compared to a piece of chalk or a rubber band because the force of attraction between iron particles is extremely strong.
• Water stream: If you try to cut a stream of water from a tap with your fingers, the stream remains together. The water particles attract each other, preventing the stream from breaking permanently.

3. States of Matter

Matter around us exists in three different states—solid, liquid, and gas. These states arise due to the variation in the characteristics of the particles of matter (spacing, kinetic energy, and force of attraction).

3.1 The Solid State

Solids have a definite shape, distinct boundaries, and fixed volumes. They have negligible compressibility.

• Solids have a tendency to maintain their shape when subjected to outside force.
• They may break under force but it is difficult to change their shape, making them rigid.
• Exceptions explained: A rubber band changes shape under force but regains it when the force is removed (elasticity). A sponge has minute holes filled with air; when pressed, the air is expelled, allowing it to be compressed.

3.2 The Liquid State

Liquids have no fixed shape but have a fixed volume. They take up the shape of the container in which they are kept.

• Liquids flow and change shape, so they are not rigid but can be called a fluid.
• Solids, liquids, and gases can all diffuse into liquids. Aquatic animals breathe dissolved oxygen (gas diffused in liquid).
• The rate of diffusion of liquids is higher than that of solids because liquid particles move freely and have greater space between each other.

3.3 The Gaseous State

Gases have neither a fixed shape nor a fixed volume. They are highly compressible as compared to solids and liquids.

• The liquefied petroleum gas (LPG) cylinder in our homes and compressed natural gas (CNG) used as fuel are examples of highly compressed gases.
• Due to their high compressibility, large volumes of a gas can be compressed into a small cylinder and transported easily.
• In the gaseous state, the particles move about randomly at high speed, hitting each other and the walls of the container, thereby exerting pressure.

Comparison of the Three States of Matter

Property	Solid	Liquid	Gas
Shape and Volume	Fixed shape and fixed volume	No fixed shape but fixed volume	Neither fixed shape nor fixed volume
Compressibility	Negligible	Low	Highly compressible
Interparticle Space	Minimum (tightly packed)	Moderate	Maximum (very loose)
Interparticle Force of Attraction	Maximum	Moderate	Minimum
Kinetic Energy & Movement	Minimum (vibrate at fixed positions)	Moderate (particles can slide past each other)	Maximum (move freely at high speeds)

4. Can Matter Change its State?

Matter can change from one state to another. Water is the most common example, existing as ice (solid), water (liquid), and water vapour (gas). This change is primarily driven by changes in temperature and pressure.

4.1 Effect of Change of Temperature

When we heat a solid, the kinetic energy of its particles increases. The particles vibrate with greater speed, overcoming the forces of attraction, and the solid begins to melt.

• Melting Point: The minimum temperature at which a solid melts to become a liquid at the atmospheric pressure is called its melting point. The melting point of ice is 273.15 K (0°C).
• Latent Heat of Fusion: When a solid melts, its temperature remains constant even though heat is continuously supplied. This hidden heat used to overcome the intermolecular forces is called the latent heat of fusion. It is the amount of heat energy required to change 1 kg of a solid into a liquid at atmospheric pressure at its melting point.
• Boiling Point: The temperature at which a liquid starts boiling at the atmospheric pressure. For water, it is 373 K (100°C). Boiling is a bulk phenomenon.
• Latent Heat of Vaporisation: The hidden heat energy required to change 1 kg of a liquid to a gas at atmospheric pressure at its boiling point. Particles in steam (water vapour) at 373 K have more energy than water at the same temperature because they have absorbed this extra latent heat.

4.2 Effect of Change of Pressure

Applying pressure and reducing temperature can liquefy gases.

• By increasing pressure, the particles of a gas are brought closer together, increasing the intermolecular force of attraction.
• Solid Carbon Dioxide (Dry Ice): It is stored under high pressure. If the pressure is reduced to 1 atmosphere, it gets converted directly to gaseous state without becoming liquid. This is why it is called dry ice.

5. Evaporation

We do not always need to heat a liquid to its boiling point for it to change into a gas. Water left uncovered slowly changes into vapour at room temperature. This phenomenon of change of a liquid into vapours at any temperature below its boiling point is called evaporation.

Unlike boiling, which is a bulk phenomenon, evaporation is a surface phenomenon. Particles at the surface possess higher kinetic energy and are able to break away from the forces of attraction of other particles and escape into the air.

5.1 Factors Affecting Evaporation

The rate of evaporation increases with:

1. An increase of surface area: Spreading wet clothes allows them to dry faster because evaporation is a surface phenomenon.
2. An increase of temperature: Higher temperatures provide more kinetic energy to the particles, helping them escape the liquid state.
3. A decrease in humidity: Humidity is the amount of water vapour in the air. If the air is already holding a lot of water (high humidity), the rate of evaporation decreases.
4. An increase in wind speed: Wind blows away the water vapour particles, decreasing the humidity around the surface and speeding up evaporation.

5.2 How does Evaporation Cause Cooling?

In an open vessel, the liquid keeps on evaporating. The particles of liquid absorb energy from the surrounding environment to regain the energy lost during evaporation. This absorption of energy makes the surroundings cold.

6. More to Know (Plasma & Bose-Einstein Condensate)

Beyond the standard three states, scientists now discuss five states of matter.

• Plasma: The state consists of super energetic and super excited particles in the form of ionised gases. The fluorescent tube and neon sign bulbs consist of plasma. Inside a neon sign bulb there is neon gas and inside a fluorescent tube there is helium gas. When electrical energy flows, the gas gets ionised, charging up and glowing brilliantly with a specific colour. The Sun and stars glow because of the presence of plasma, created by very high temperatures.
• Bose-Einstein Condensate (BEC): In 1920, Indian physicist Satyendra Nath Bose made calculations for a fifth state of matter. Building on this, Albert Einstein predicted the BEC. In 2001, three scientists from the USA received the Nobel Prize for achieving this state. BEC is formed by cooling a gas of extremely low density (about one-hundred-thousandth the density of normal air) to super low temperatures (absolute zero).