Matter In Our Surroundings Class 9 Notes

Physical Nature and Characteristics of Particles of Matter

Everything in our surroundings is made up of matter—defined as anything that occupies space (has volume) and has mass. Matter is not continuous like a block of wood; it is particulate, consisting of extremely tiny particles.

Characteristics of Particles of Matter:

1. Particles have spaces between them: When substances like salt, sugar, or potassium permanganate are dissolved in water, their particles occupy the intermolecular spaces between water molecules.
2. Particles are continuously moving: Particles possess kinetic energy. As the temperature increases, kinetic energy rises, and particles move faster. This leads to diffusion—the spontaneous intermixing of particles of two different types of matter.
3. Particles attract each other: A force of attraction acts between the particles of matter. This intermolecular force of attraction keeps the particles together and varies in strength depending on the nature of the substance (strongest in solids, weakest in gases).

Glossary of Key Terms

Matter

Anything that occupies space and has mass.

Diffusion

The spontaneous intermixing of particles of two different substances on their own, driven by kinetic energy.

Melting Point

The temperature at which a solid melts to become a liquid at atmospheric pressure.

Boiling Point

The temperature at which a liquid starts boiling and changing into a gas at atmospheric pressure (a bulk phenomenon).

Latent Heat of Fusion

The amount of heat energy required to change 1 kg of solid into a liquid at its melting point at atmospheric pressure.

Latent Heat of Vaporization

The amount of heat energy required to change 1 kg of liquid into a gas at its boiling point at atmospheric pressure.

Sublimation

The transition of a substance directly from the solid state to the gaseous state without changing into a liquid state.

Deposition

The direct transition of a substance from the gaseous state to the solid state without passing through the liquid state.

Evaporation

A surface phenomenon in which a liquid changes into vapour at any temperature below its boiling point.

Comparison of States of Matter

Interconversion of States of Matter

1. Heating a Solid (Melting) — Increasing temperature increases particle kinetic energy. Particles vibrate faster, overcome intermolecular forces, and break free to form a liquid (Fusion).
2. Heating a Liquid (Boiling) — Continued heating supplies enough energy for liquid particles to overcome forces completely. The liquid enters bulk boiling and turns into vapor (Vaporization).
3. Cooling a Gas (Condensation) — Lowering the temperature reduces particle kinetic energy. Particles lose speed and settle closer together, transitioning back from gas to liquid.
4. Cooling a Liquid (Solidification) — Further cooling reduces kinetic energy to a minimum. Particles lock into fixed geometric patterns, turning the liquid into a solid.
5. Effect of Changing Pressure — Applying high pressure while lowering temperature brings gas particles close enough to transition them directly into a liquid state (e.g., LPG or liquid oxygen production).

Must Remember / Exam Checkpoints

• Matter is particulate in nature and made of extremely tiny particles.
• The rate of diffusion increases with temperature because particles gain more kinetic energy and move faster.
• The SI unit of temperature is Kelvin (K). Formula: K = °C + 273 (exactly 273.15).
• During melting or boiling, the temperature of the substance remains constant until the process is complete, as the energy is absorbed as Latent Heat.
• Solid carbon dioxide (CO2) is called 'Dry Ice' because it converts directly from solid to gas at 1 atm without melting.
• Evaporation is a surface phenomenon (occurs only at the surface), whereas boiling is a bulk phenomenon (occurs throughout the entire liquid volume).
• Evaporation always causes a cooling effect because surface particles absorb heat energy from the surroundings to escape.
• Factors accelerating evaporation: Increased surface area, increased temperature, increased wind speed, and decreased humidity.

Worked Mini-Examples & Formula Applications

• {"title":"Temperature Conversion","description":"Convert 300 K to the Celsius scale.\nFormula: t(°C) = T(K) - 273\nCalculation: 300 - 273 = 27°C."}
• {"title":"Earthen Pot (Matka) Cooling","description":"Why does water in an earthen pot stay cool in summer?\nExplanation: Earthen pots have minute pores. Water seeps out and continuously evaporates. The heat required for evaporation is absorbed from the remaining water inside, lowering its temperature."}

Board Exam Traps and Marking Cues

Watch out for the 'Latent Heat' trap! Many students incorrectly state that adding heat always raises the temperature of a system. In exam answers, clearly write that during phase change, the temperature stays constant. State that this thermal energy is hidden as Latent Heat and is used exclusively to break intermolecular bonds.

Marking Cue: For 'Evaporation vs Boiling' questions, always highlight 'Surface phenomenon vs Bulk phenomenon' to secure full marks.

Quick Revision Check

• Why is ice at 273 K more effective in cooling than water at the same temperature? Ice at 273 K has less energy than water at 273 K. When melting, ice absorbs an additional amount of heat equal to the latent heat of fusion from the surroundings, resulting in a more pronounced cooling effect.
• What is sublimation? Give two examples of sublimable substances. Sublimation is the direct transition of a substance from solid to gas without entering the liquid state. Examples include Camphor, Ammonium Chloride, and Naphthalene.
• Why do we feel cold when we apply acetone or perfume to our palm? Acetone and perfume particles have low boiling points. They absorb latent heat of vaporization from our palm to evaporate, leaving the palm feeling cool.
• Why do we wear cotton clothes in summer? Cotton is a good absorber of water. It absorbs body sweat and exposes it to the environment, allowing for rapid evaporation which absorbs body heat and keeps us cool.

Frequently Asked Questions