CBSE Notes Class 9 Science Chapter 1 - Matter In Our Surroundings Class 9 Notes

Everything around us is made of matter: a pencil, a pen, a table, the food we eat, the clothes we wear, and the walls of our homes. Matter is anything that occupies space, has mass, and can be sensed by us. In other words, matter refers to all the substances and materials that make up the universe.

1.0Classification of Matter 

Since ancient times, people have tried to understand the world around them. Early Indian philosophers believed that all matter,  living and nonliving, comprises five basic elements known as the Panchtatva: Air, Water, Earth, Sky, and Fire. Similarly, ancient Greek philosophers developed a comparable classification of matter. Today, scientists classify matter based on its physical properties and chemical nature.

2.0Physical Nature of Matter

Two views existed about matter: continuous (like a solid block) and particulate (like sand).

Activity: Dissolve 2–3 potassium permanganate crystals in 100 mL water.

Observation: The colour remains visible even after dilution, showing that each crystal contains millions of particles that disperse further.

3.0Characteristics of Particles of Matter

• Particles of matter have space between them. For example, when you dissolve sugar in water, you can stir in more sugar because the space between the water molecules allows the sugar particles to fit in.
• Particles of Matter Are Continuously Moving. For instance, perfume spreads throughout a room as the particles move through the air.
• Particles of Matter Attract Each Other: An example is water droplets forming on a cold glass; the water molecules are attracted and stick together to form droplets.

4.0States Of Matter

Look around and observe that matter exists in three distinct states: solid, liquid, and gas. These states result from variations in the characteristics of the particles of matter.

5.0Can Matter Change its State?

Matter can change its physical state by altering temperature, pressure, or both. For example, heating can turn a solid into a liquid (melting) or a liquid into a gas (evaporation), while cooling can reverse these processes. Increasing pressure can also turn gases into liquids or solids.

Effect of Temperature on Matter

Melting (Fusion): As a solid's temperature rises, its particles gain energy, vibrate faster, and eventually break free, turning the solid into a liquid. The temperature remains constant at the melting point as heat is used to overcome particle attraction. This absorbed heat is called the latent heat of fusion.

Boiling (Vaporization): When heat is added to a liquid, particles move faster until they gain enough energy to become a gas. The temperature at which this occurs is the boiling point. Latent heat of vaporization is the energy needed to convert 1 kg of liquid into gas at the boiling point.

When Temperature Decreases

• Gases: Lower temperatures reduce the kinetic energy of gas particles, causing them to condense into a liquid (e.g., clouds forming).
• Liquids: Lowering the temperature causes liquids to solidify (e.g., water freezing into ice).

Special Cases:

• Sublimation: A solid changes directly into a gas without becoming a liquid.
• Deposition: A gas changes directly into a solid without becoming a liquid.

Thus, temperature changes can shift matter between different states.

Effect of Change of Pressure

Applying pressure can compress particles of matter, bringing them closer together and changing their state. For example, CO₂ gas can be solidified by increasing pressure and lowering temperature.

Evaporation occurs at the surface of a liquid when it turns into vapour at temperatures below its boiling point.

Factors Affecting Evaporation:

• Surface Area: A larger surface area increases evaporation.
• Temperature: Higher temperatures speed up evaporation.
• Humidity: Lower humidity levels enhance evaporation.
• Wind Speed: Increased wind speed accelerates evaporation.

6.0Evaporation Cause Cooling

As a liquid evaporates, its particles absorb energy from the surroundings to replace the energy lost during evaporation. This absorption of energy from the surrounding environment results in a cooling effect.