Matter in Our Surroundings: Class 9 Study Guide
Everything around us is made of tiny particles that are always moving.
Matter is anything that has mass and takes up space. It is made of tiny particles with spaces between them. The particles are always moving, and they attract each other. Their speed rises with temperature, which is why sugar dissolves faster in hot water.
Matter exists in three common states: solid, liquid and gas.
Comparing the states
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Fixed | Takes the container's shape | Fills the container |
| Volume | Fixed | Fixed | Not fixed |
| Particles | Packed closely, vibrate in place | Close, can slide past | Far apart, move freely |
| Compressible | Hardly | Hardly | Easily |
Changes of state
Heating ice at 0 °C (273 K) melts it into water without raising the temperature until it has all melted. This is its melting point, and the heat used is called the latent heat of fusion. Water boils at 100 °C (373 K) at normal pressure. Some solids, such as camphor and dry ice, change straight to gas. This is sublimation. To change °C to kelvin, add 273.
Particles vibrate faster.
At its melting point.
At its boiling point.
The changes reverse.
Evaporation
Evaporation is a change from liquid to gas below the boiling point, at the surface of the liquid. It happens faster when the surface area is larger, the temperature is higher, the air is drier or the wind is blowing. It causes cooling, which is why sweating cools us and wet clothes dry faster in the sun.
Worked examples and common mistakes
Example: Convert 25 °C to kelvin: 25 + 273 = 298 K. Example: Why does an earthen pot keep water cool? Water seeps out through tiny pores and evaporates, which cools the pot. Common mistakes: thinking that boiling and evaporation are the same, and saying that particles themselves get hot or expand. They move faster and spread further apart.
Check your understanding: model answers
1. 100 − 273 = −173 °C. 2. Its particles move freely and are far apart, so they spread out. 3. For example higher temperature and larger surface area.
✨ Key Takeaways
- Matter is made of tiny moving particles.
- The states differ in how close the particles are.
- Melting and boiling happen at fixed temperatures.
- Evaporation happens at the surface and cools the liquid.
💬 Questions for Discussion & Reflection
- Convert 100 K to °C.
- Why does a gas fill its container?
- Name two factors that speed up evaporation.
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