Matter In Our Surroundings: Class 9 Science NCERT Guide

Understanding Matter and Its Particulate Nature

So, what exactly is matter? The scientific definition is simple: anything that has mass and occupies space (volume) is called matter. But the real magic is in its composition. Matter is not continuous like a smooth block; instead, it is made up of millions and millions of tiny particles. How do we know this? Think about what happens when you dissolve a spoonful of sugar in water. The sugar disappears, but the water tastes sweet. This is because the tiny particles of sugar find their way into the spaces between the particles of water. This process is called diffusion. The key characteristics of these particles of matter are:

1. They are very, very small: So small that we can't see them with our naked eyes.
2. They have space between them: This is called intermolecular space. It's largest in gases and smallest in solids.
3. They are continuously moving: Particles possess kinetic energy, causing them to move or vibrate constantly. This energy increases with temperature, which is why the smell of hot food travels faster than cold food.
4. They attract each other: A force of attraction, called the intermolecular force, exists between particles, holding them together. This force is strongest in solids and weakest in gases.

The Three States of Matter: Solid, Liquid, and Gas

Solids

Solids have a definite shape and a fixed volume. Their particles are tightly packed in a fixed pattern with very strong intermolecular forces and low kinetic energy. They are rigid and generally incompressible. Examples: Ice, wood, stone.

Liquids

Liquids have a fixed volume but no definite shape; they take the shape of their container. Their particles are close together but can move past one another. The intermolecular forces are weaker than in solids, and particles have more kinetic energy. They can flow and are almost incompressible. Examples: Water, milk, oil.

Gases

Gases have neither a definite shape nor a fixed volume; they fill their entire container. Their particles are far apart with very weak intermolecular forces and high kinetic energy. They are highly compressible and can flow easily. Examples: Air, oxygen, steam.

How Matter Changes Its State

1. Effect of Temperature: Melting and Boiling — When you heat a solid (like ice), its particles gain kinetic energy and start vibrating more intensely. At a specific temperature, the melting point, they overcome the forces of attraction and the solid turns into a liquid. The heat energy required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point is called the latent heat of fusion. Similarly, when you heat a liquid, its particles gain more energy. At the boiling point, they have enough energy to break free completely and turn into a gas. The energy required to change 1 kg of a liquid into gas at atmospheric pressure at its boiling point is the latent heat of vaporization.
2. Effect of Pressure: Liquefaction and Solidification — Changing pressure can also alter the state of matter. By applying high pressure and reducing the temperature, we can force gas particles closer together, increasing the intermolecular forces until the gas liquefies. This is how LPG (Liquefied Petroleum Gas) is stored in cylinders. A fascinating example is solid carbon dioxide, also known as dry ice. Under high pressure, CO₂ is solidified. When the pressure is reduced, it turns directly into a gas without becoming a liquid.
3. Sublimation: The Direct Change — Some substances can change directly from a solid state to a gaseous state without passing through the liquid state. This process is called sublimation. The reverse process, where a gas turns directly into a solid, is called deposition. A common example is camphor (kapoor) or naphthalene balls used in cupboards. When heated or left in the open, they slowly disappear as they sublimate into a gas.

Exam Tip: Don't Confuse Evaporation and Boiling

A very common point of confusion for students is the difference between evaporation and boiling. Both processes turn a liquid into a gas, but they are not the same!

• Boiling is a bulk phenomenon. It happens throughout the entire liquid at a specific temperature called the boiling point (e.g., 100°C for water at sea level). You see bubbles forming from within the liquid.
• Evaporation is a surface phenomenon. It can happen at any temperature below the boiling point. Only the particles at the surface of the liquid with enough kinetic energy escape into the air. This is why a wet floor dries up on its own.

Remember, evaporation is faster when:

1. Surface area is larger (water in a plate evaporates faster than in a glass).
2. Temperature is higher (clothes dry faster on a sunny day).
3. Wind speed is higher (wind blows away water vapour, allowing more evaporation).
4. Humidity is lower (dry air can hold more water vapour).

Practice Questions with Solutions

• Q: Why does the temperature of a substance remain constant during its melting point, even though heat is being supplied continuously? A: Step 1: The heat energy supplied during melting is used to overcome the strong forces of attraction between the particles of the solid. Step 2: This energy, known as latent heat of fusion, is absorbed by the particles without increasing their kinetic energy or temperature. Final answer: The temperature remains constant because the supplied heat is used as latent heat to change the state of matter, not to increase kinetic energy.
• Q: Differentiate between evaporation and boiling. A: Step 1: Evaporation is a surface phenomenon that can occur at any temperature below the boiling point. Boiling is a bulk phenomenon that occurs only at a specific boiling point. Step 2: Evaporation is a slow process that causes cooling. Boiling is a rapid process that does not cause cooling. Final answer: Evaporation is a slow surface process at any temperature, while boiling is a rapid bulk process at a fixed temperature.
• Q: Why does the smell of hot sizzling food reach you several metres away, but you have to go close to get the smell from cold food? A: Step 1: The particles of matter are continuously moving, and their kinetic energy increases with temperature. Step 2: Hot food particles have higher kinetic energy, leading to faster diffusion into the air and spreading quickly over a larger distance. Final answer: Hot food particles diffuse faster due to higher kinetic energy at higher temperatures.
• Q: Arrange the following substances in increasing order of force of attraction between their particles: Water, Sugar, Oxygen. A: Step 1: The force of attraction between particles is strongest in solids, intermediate in liquids, and weakest in gases. Step 2: Oxygen is a gas, Water is a liquid, and Sugar is a solid. Final answer: Oxygen < Water < Sugar

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