Matter in Our Surroundings

Look around you. Everything you see, from the air you breathe and the water you drink to the chair you are sitting on, is made of "matter." Since ancient times, human beings have tried to understand their surroundings. Early Indian philosophers classified matter into five basic elements—the Panch Tatva: air, earth, fire, sky, and water. Modern scientists, however, classify matter based on its physical properties and chemical nature. In this chapter, we will focus on the physical nature of matter.

1. Physical Nature of Matter

To understand matter, we must answer two fundamental questions: Is matter continuous like a block of wood, or is it made of tiny particles like sand?

Matter is made up of particles. These particles are unimaginably small. When you dissolve a spoon of salt in a glass of water, the salt particles get into the spaces between the particles of water, proving that matter is not continuous but particulate.

Characteristics of Particles of Matter

1. Particles of matter have space between them: When you make tea, coffee, or lemonade, particles of one type of matter intermix and fit into the spaces between the particles of the other.
2. Particles of matter are continuously moving: If you light an incense stick in a corner of your room, you can smell it from a distance. The particles of incense mix with the air particles. This continuous movement means they possess kinetic energy. As the temperature rises, particles move faster because their kinetic energy increases. The intermixing of particles of two different types of matter on their own is called diffusion.
3. Particles of matter attract each other: If you try to break a piece of chalk, an ice cube, and an iron nail, you'll find it hardest to break the iron nail. This is because a force of attraction acts between the particles of matter, holding them together. The strength of this force varies from one kind of matter to another.

2. 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 (specifically, the spaces between them and the strength of the force of attraction).

The Solid State

Solids have a definite shape, distinct boundaries, and fixed volumes. This means they have negligible compressibility. * In solids, the particles are closely packed together. * The force of attraction between the particles is very strong. * The kinetic energy of the particles is very low; they can only vibrate about their fixed positions. * Example: A rubber band changes shape under force but regains the same shape when the force is removed. If excessive force is applied, it breaks, showing its rigidity.

The Liquid State

Liquids have no fixed shape but have a fixed volume. They take up the shape of the vessel in which they are kept. * Liquids flow and change shape, so they are not rigid but can be called fluids. * The particles in a liquid are packed less tightly than in a solid. * The spaces between the particles are larger, allowing them to move around more freely. * The forces of attraction are strong enough to keep them together but not strong enough to maintain a rigid shape. * Gases from the atmosphere diffuse and dissolve in water (e.g., aquatic plants and animals rely on dissolved oxygen). The rate of diffusion in liquids is higher than in solids.

The Gaseous State

Gases have neither a fixed shape nor a fixed volume. * Gases are highly compressible compared to solids and liquids. For example, large volumes of Liquefied Petroleum Gas (LPG) or Compressed Natural Gas (CNG) are compressed into small cylinders for transport. * The particles in a gas are very loosely packed and have large empty spaces between them. * The kinetic energy is very high, allowing particles to move randomly at high speeds. * Due to this random movement, the particles hit each other and also the walls of the container. The pressure exerted by the gas is because of this force exerted by gas particles per unit area on the walls of the container.

3. Can Matter Change its State?

Yes, water can exist in all three states: as solid (ice), liquid (water), and gas (water vapour). We can change the state of matter by changing the temperature or the pressure.

Effect of Change of Temperature

• Melting (Fusion): When a solid is heated, the kinetic energy of its particles increases. They overcome the forces of attraction and start moving more freely. The solid melts and turns into a liquid. The temperature at which a solid melts to become a liquid at atmospheric pressure is called its melting point. (Melting point of ice is 273.15 K or 0°C).
• Latent Heat of Fusion: The amount of heat energy required to change 1 kg of a solid into a liquid at atmospheric pressure at its melting point. It is called "latent" (hidden) because this heat energy is absorbed without showing any rise in temperature (it's used to break the bonds of attraction).
• Boiling: When a liquid is heated, particles move even faster until they can completely break free from the forces of attraction. The liquid starts turning into gas. The temperature at which a liquid starts boiling at atmospheric pressure is known as its boiling point. (Boiling point of water is 373 K or 100°C).
• Latent Heat of Vaporisation: The heat energy required to change 1 kg of a liquid to a gas at atmospheric pressure at its boiling point.
• Sublimation: A change of state directly from solid to gas without changing into a liquid state is called sublimation (e.g., camphor, ammonium chloride). The reverse process, changing from gas to solid directly, is called deposition.

Effect of Change of Pressure

Applying pressure and reducing temperature can liquefy gases. * Dry Ice: Solid carbon dioxide ($CO_2$) is stored under high pressure. If the pressure is decreased to 1 atmosphere, it converts directly to gaseous state without becoming liquid. That's why it's called dry ice.

4. Evaporation

Do we always need to heat a liquid to its boiling point for it to turn into a gas? No. Water, when left uncovered, slowly changes into vapour. This phenomenon of change of a liquid into vapours at any temperature below its boiling point is called evaporation.

Factors Affecting Evaporation

1. An increase in surface area: Since evaporation is a surface phenomenon, if the surface area is increased, the rate of evaporation increases. (We spread out clothes to dry).
2. An increase in temperature: More particles get enough kinetic energy to go into the vapour state.
3. A decrease in humidity: Humidity is the amount of water vapour present 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: With faster wind, water vapour particles move away with the wind, increasing evaporation. (Clothes dry faster on a windy day).

How Does Evaporation Cause Cooling?

During evaporation, the particles at the surface of the liquid absorb energy from the surroundings to regain the energy lost during evaporation. This absorption of energy makes the surroundings cold. For example, when you pour some acetone (nail polish remover) on your palm, the particles gain energy from your palm and evaporate, causing the palm to feel cool. Similarly, sweating helps our body maintain a cool temperature during hot summer days.

Summary

Matter is composed of continuously moving, incredibly small particles that attract one another. The strength of these interactions dictates whether matter exists as a rigid solid, a flowing liquid, or a compressible gas. By manipulating temperature and pressure, we can force matter to transition between these states, a principle that governs everything from the water cycle in nature to the technology used to transport natural gas. Understanding evaporation helps us explain everyday cooling phenomena and why we wear cotton clothes in the summer.