Matter in Our Surroundings
CBSE Class 9 · Science · Notes, formulas and practice questions
This chapter explains that all matter is made up of tiny, moving, attracting particles, and uses this idea to describe the three states of matter, the changes of state during melting, boiling and sublimation, and why evaporation causes cooling.
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The chapter starts by treating matter as a collection of particles rather than a continuous blob. The key ideas are that the particles are in motion, there is empty space between them, and they attract one another. The size of the spaces and the strength of the forces determine whether a substance behaves as a solid, liquid or gas. This particle picture also explains compressibility, diffusion, and why heating a solid can eventually turn it into a gas. Almost every fact in this chapter is a logical consequence of these three particle properties.
Solids, liquids and gases are distinguished by their shape, volume and compressibility. In solids, particles vibrate at fixed positions, so their shape and volume are definite. In liquids, particles slide past one another, so the liquid takes the shape of its container but keeps a fixed volume. In gases, particles are far apart with very weak attraction, so the gas fills all available space and compresses easily. The particles themselves do not change size when the state changes; only the spaces between them and the strength of interparticle attraction change.
Applying heat raises the kinetic energy of particles. When a solid is heated, melting begins at a fixed temperature, and the heat supplied is used as latent heat of fusion to separate particles without raising the temperature. Similarly, boiling uses latent heat of vaporisation, which is larger because vapour particles must spread out against the external pressure. So during a phase change the temperature stays constant. External pressure also affects the boiling point: lowering pressure lowers the boiling point, which is why water boils at lower temperatures on mountains.
Some substances change directly from solid to gas through sublimation, while evaporation is a surface phenomenon that occurs below the boiling point. Evaporation speeds up when temperature rises, surface area grows, humidity falls, or wind blows away vapour. Because the fastest-moving particles escape first, evaporation leaves behind liquid with lower average kinetic energy, producing cooling. This explains why sweating cools the body, why wet clothes dry in the sun, and why a desert cooler works well in dry air.
Key terms
- Particles of matter
- All matter is composed of extremely tiny particles. These particles have empty spaces between them, are in constant motion, and attract each other. The relative strength of attraction and the size of the spaces decide whether a substance is a solid, liquid or gas.
- States of matter
- The three main states are solid, liquid and gas. Solids have a definite shape and volume, with particles vibrating at fixed positions. Liquids have a definite volume but no definite shape, with particles able to slide past each other. Gases have neither definite shape nor fixed volume, with particles moving freely and occupying all available space.
- Melting point
- The fixed temperature at which a pure solid changes into a liquid at normal atmospheric pressure. At the melting point, solid and liquid coexist, and the temperature does not rise until the entire solid has melted, even if heat is continually supplied.
- Boiling point
- The temperature at which a liquid changes into vapour throughout its bulk at a given external pressure. Bubbles of vapour form inside the liquid at this temperature. The boiling point rises as external pressure increases and falls as pressure decreases.
- Latent heat of fusion
- The amount of heat energy required to convert 1 kg of a solid at its melting point into liquid at the same temperature, with no temperature rise. This energy is used to overcome the attractive forces between particles and is stored as potential energy in the liquid.
- Latent heat of vaporisation
- The amount of heat energy required to convert 1 kg of a liquid at its boiling point into vapour at the same temperature, with no temperature change. It is larger than the latent heat of fusion because vapour particles must be separated far apart against the external pressure.
- Sublimation
- The change of a solid directly into vapour without passing through the liquid state, or the reverse change of vapour directly to solid. Common substances that sublime include camphor, ammonium chloride, iodine and dry ice.
- Evaporation
- The slow process by which a liquid changes into vapour from its surface at any temperature below its boiling point. Evaporation is faster at higher temperature, with a larger exposed surface area, in lower humidity, and when wind blows away the vapour.
- Evaporation causes cooling
- During evaporation, the molecules with the greatest kinetic energy escape from the liquid first. The remaining molecules have a lower average kinetic energy, so the liquid becomes cooler. This is why sweat cools the body and why surfaces feel cool when alcohol evaporates from them.
Formula sheet
| What | Formula | Notes |
|---|---|---|
| Celsius to Kelvin | K = °C + 273 | Add 273 to a Celsius temperature to get the Kelvin (absolute) temperature. For example 27°C is 300 K. |
| Kelvin to Celsius | °C = K − 273 | Subtract 273 from a Kelvin temperature to get degrees Celsius. For example 373 K is 100°C, the boiling point of water. |
Practice questions with answers
1. Why can a gas be compressed easily, while a solid is almost impossible to compress?
In a gas, the particles are far apart and the empty spaces between them are very large, so pressure can push the particles much closer together. In a solid, the particles are already tightly packed with very little empty space, so applying pressure cannot bring them any closer.
2. Define latent heat of fusion.
Latent heat of fusion is the amount of heat energy required to change 1 kg of a solid at its melting point into a liquid at the same temperature. This heat is consumed in overcoming the attractive forces between particles, so it does not cause any rise in temperature.
3. Why does the temperature of a pure substance remain constant when it is boiling even though heat is being supplied?
During boiling, the supplied heat is used to overcome the interparticle forces of attraction so that the particles can escape as vapour. Since temperature measures the average kinetic energy of the particles, and the heat energy is not contributing to increasing their kinetic energy, the temperature remains constant until all the liquid has boiled away.
4. State any two factors that affect the rate of evaporation and explain how each does so.
The rate of evaporation increases with a rise in temperature, because more particles gain enough kinetic energy to escape from the surface. It also increases with a larger exposed surface area, because more particles are present at the surface at the same time and can leave the liquid.
5. Why does evaporation cause cooling? Give one everyday example.
Evaporation happens fastest for molecules at the liquid surface that have the highest kinetic energy. When these molecules escape, the average kinetic energy of the remaining liquid falls, making it cooler. Sweating on a warm day is an everyday example: the sweat evaporates and draws heat from the skin.
6. Convert 27°C to the Kelvin scale. Also express 373 K in degrees Celsius.
Kelvin = Celsius temperature + 273, so 27°C = 27 + 273 = 300 K. For the reverse, Celsius = Kelvin − 273, so 373 K = 373 − 273 = 100°C.
7. A pure solid melts at 15°C and boils at 150°C. Predict its physical state at 10°C and at 25°C.
At 10°C the temperature lies below the melting point of 15°C, so the substance exists as a solid. At 25°C the temperature is between the melting point and the boiling point, so the substance is a liquid.
8. What is sublimation? Name two substances that undergo sublimation.
Sublimation is the process in which a solid changes directly into vapour without first becoming a liquid, or vapour condenses directly back into a solid. Two examples are camphor and ammonium chloride; iodine and dry ice also sublime.
9. Why does food cook faster in a pressure cooker?
In a closed pressure cooker, heating produces steam that raises the pressure above the water. The increased external pressure raises the boiling point of water above 100°C, so the water and steam become hotter than in an open pan. This higher temperature cooks the food faster.
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