Matter in Our Surroundings — NCERT Solutions, AI Tutor & Practice

Understanding: Matter is anything that has mass and occupies space (has volume). Everything around us — air, water, food, stones — is made up of matter.

States of matter: Matter exists mainly in three physical states:

1. Solid — has fixed shape and fixed volume. Example: a piece of iron, ice.
2. Liquid — has fixed volume but no fixed shape (takes the shape of its container). Example: water, milk.
3. Gas — has neither fixed shape nor fixed volume. Example: oxygen, air.

Conclusion: Matter is made of tiny particles and can be classified based on how these particles are arranged and how strongly they attract each other.

The particles of matter have the following characteristics:

1. Particles of matter are very small — smaller than we can imagine. When a crystal of potassium permanganate is dissolved in water and diluted repeatedly, the colour still remains, showing millions of tiny particles.

2. Particles of matter have spaces between them — When sugar or salt dissolves in water, the particles fit into the spaces between water particles, so the level does not rise much.

3. Particles of matter are continuously moving — They possess kinetic energy. This explains diffusion, e.g., the smell of an incense stick spreading across a room.

4. Particles of matter attract each other — There is a force of attraction between particles that keeps them together. This force is different in different kinds of matter.

Conclusion: These characteristics explain properties like diffusion, compressibility, and the different states of matter.

Comparison Table:

| Property | Solid | Liquid | Gas | |---|---|---|---| | Shape | Fixed | Not fixed (takes container's shape) | Not fixed | | Volume | Fixed | Fixed | Not fixed | | Compressibility | Negligible | Very slight | High | | Force of attraction | Very strong | Moderate | Very weak | | Spaces between particles | Very small | Larger than solids | Very large | | Kinetic energy of particles | Lowest | Intermediate | Highest | | Fluidity | Cannot flow | Can flow | Can flow |

Conclusion: As we move from solid to liquid to gas, the force of attraction decreases, the spaces between particles increase, and the freedom of movement of particles increases.

Definition: Diffusion is the intermixing of particles of two different types of matter on their own, due to the continuous random motion of particles.

Reason diffusion is fastest in gases:

1. In gases, particles have very high kinetic energy and move very fast. The spaces between particles are very large and the force of attraction is very weak, so gas particles mix rapidly.

2. In liquids, particles move more slowly and have less space and stronger attraction, so diffusion is slower.

3. In solids, particles only vibrate about fixed positions and are held very tightly, so diffusion is extremely slow (almost negligible).

Order of diffusion rate: $\text{Gases} > \text{Liquids} > \text{Solids}$

Conclusion: More kinetic energy and more space between particles → faster diffusion.

Formulas: $\text{Celsius} = \text{Kelvin} - 273$ $\text{Kelvin} = \text{Celsius} + 273$

(a) 300 K to °C: $300 - 273 = 27\,^\circ\text{C}$

(b) 573 K to °C: $573 - 273 = 300\,^\circ\text{C}$

(c) 25 °C to K: $25 + 273 = 298\ \text{K}$

Conclusion: To go from K to °C subtract 273; to go from °C to K add 273.

Melting point: The temperature at which a solid changes into a liquid at atmospheric pressure is called its melting point. For ice, it is $0\,^\circ\text{C}$ (273 K).

Boiling point: The temperature at which a liquid starts changing into vapour at atmospheric pressure throughout its bulk is called its boiling point. For water, it is $100\,^\circ\text{C}$ (373 K).

Latent heat of fusion: It is the amount of heat energy required to change 1 kg of a solid into liquid at its melting point, without any change in temperature.

Explanation: During melting, the supplied heat is used to overcome the forces of attraction between particles rather than raising the temperature. This is why temperature stays constant during the change of state even though heat is being absorbed.

Conclusion: Latent heat is 'hidden' heat that changes the state without changing temperature.

Understanding the situation: When a solid like ice is heated, its temperature rises until it reaches the melting point. After that, even though heating continues, the temperature stays constant until all the ice has melted.

Analysis: During a change of state, the heat energy supplied is not used to increase the kinetic energy (and hence temperature) of the particles. Instead, it is absorbed as latent heat to overcome the forces of attraction between particles and to separate them so they can move into the new state.

Conclusion: Since the energy is used to change the arrangement of particles rather than increase their speed, the temperature remains constant during the change of state. This 'hidden' heat is called latent heat.

Definition: Sublimation is the change of a substance directly from the solid state to the gaseous state (and vice versa) without passing through the liquid state.

$\text{Solid} \xrightarrow{\text{heat}} \text{Gas} \quad(\text{and}\ \text{Gas} \xrightarrow{\text{cool}} \text{Solid})$

Examples of substances that sublime:

1. Camphor
2. Ammonium chloride
3. Naphthalene balls (mothballs)
4. Dry ice (solid carbon dioxide)

Conclusion: In sublimation, the solid gains enough energy to overcome its intermolecular forces completely and directly enters the gaseous state.

Definition: Evaporation is the phenomenon of change of a liquid into vapour at any temperature below its boiling point. It occurs only at the surface of the liquid.

Factors affecting the rate of evaporation:

1. Surface area — Evaporation increases with an increase in surface area (e.g., clothes spread out dry faster).

2. Temperature — Evaporation increases with an increase in temperature, because more particles get enough kinetic energy to escape.

3. Humidity — Evaporation decreases when humidity (amount of water vapour in air) is high, since the air cannot hold more vapour.

4. Wind speed — Evaporation increases with wind speed, as the water vapour particles are carried away, making room for more evaporation.

Conclusion: Higher surface area, higher temperature, lower humidity, and higher wind speed all increase the rate of evaporation.

Understanding: During evaporation, the particles of a liquid that have the highest kinetic energy escape from the surface as vapour.

Analysis: To change from liquid to vapour, the escaping particles need energy (latent heat of vaporisation). They absorb this energy from the surroundings, including the remaining liquid and any surface in contact with it. As a result, the surroundings lose heat and their temperature falls.

Everyday examples:

• We feel cool when we apply acetone or perfume on our skin — it evaporates quickly, absorbing heat from our body.
• Sweating cools our body: as sweat evaporates, it takes heat from the body.
• Water kept in an earthen pot (matka) stays cool because water seeping through the pores evaporates, absorbing heat.

Conclusion: Since evaporation absorbs heat from the surroundings, it produces a cooling effect.

Effect of pressure: When pressure is applied to a gas, its particles are pushed closer together, reducing the spaces between them. High pressure increases the force of attraction and can convert a gas into a liquid.

Effect of temperature: When temperature is decreased, the kinetic energy of gas particles decreases, so they move more slowly. This allows the forces of attraction to pull them closer, helping the gas to liquefy.

Liquefaction of gases: By applying high pressure and low temperature together, the particles of a gas can be brought close enough for the intermolecular forces to hold them in the liquid state.

Example: LPG (liquefied petroleum gas) and CO₂ are stored as liquids in cylinders under high pressure.

Conclusion: Increasing pressure and lowering temperature both favour the conversion of a gas into a liquid.

(a) A gas fills the vessel completely: Gas particles have very high kinetic energy and negligible force of attraction, so they move rapidly and randomly in all directions, spreading out to occupy all the available space in the container.

(b) A gas exerts pressure on the container walls: The fast-moving gas particles continuously collide with the walls of the container. The force exerted by these countless collisions per unit area is felt as the pressure of the gas.

(c) A wooden table is a solid: A wooden table has a fixed shape and a fixed volume, it is rigid, and it cannot be compressed easily. Its particles are closely packed with strong forces of attraction. All these are properties of a solid, so a wooden table is called a solid.

Conclusion: The behaviour of matter in each case is explained by the arrangement, motion, and forces between its particles.