What is Matter ? Chapter 1 Matter in Our Surroundings Class 9 Science

Matter in Our Surroundings: States of Matter, Properties, and Changes

Everything around us that occupies space and has mass is called matter. The air we breathe, the water we drink, the food we eat, the chair we sit on — all are forms of matter. This chapter in CBSE Class 9 Science introduces the fundamental concepts of the physical nature of matter, its classification based on states (solid, liquid, gas), the properties that distinguish these states, and the processes by which matter changes from one state to another. Understanding these basics is essential for all subsequent chapters in chemistry.

Matter is made up of extremely small particles that are not visible to the naked eye. The space between these particles and the strength of the forces between them determine the physical state of matter. In solids, particles are tightly packed in a fixed arrangement with very little space between them — they have strong intermolecular forces, definite shape and volume, and are incompressible. In liquids, particles are less tightly packed and can move around freely — they have definite volume but take the shape of their container, can flow, and are slightly compressible. In gases, particles are far apart with very weak intermolecular forces — they have neither definite shape nor definite volume, fill their container completely, are highly compressible, and move rapidly in all directions. The kinetic energy of particles increases from solid to liquid to gas, which is why temperature affects the state of matter.

Matter can change from one state to another by changing the temperature or pressure. Melting (fusion) is the process where a solid turns into a liquid at its melting point — the temperature at which solid and liquid states coexist in equilibrium. The melting point of ice is 0°C (273 K). During melting, the heat energy supplied (called latent heat of fusion — 334 kJ/kg for ice) does not raise the temperature but is used to overcome intermolecular forces. Boiling (vaporisation) is the process where a liquid turns into gas at its boiling point — for water this is 100°C (373 K) at standard atmospheric pressure. The latent heat of vaporisation of water is 2260 kJ/kg. Sublimation is the direct transition from solid to gas without passing through the liquid state — camphor, naphthalene, ammonium chloride, and dry ice (solid CO₂) are common examples. Evaporation is a surface phenomenon where liquid molecules at the surface gain enough energy to escape into the gas phase at temperatures below the boiling point. Evaporation causes cooling because the faster (higher-energy) molecules escape, leaving behind slower (cooler) molecules — this is why we feel cold when sweat evaporates from our skin. Factors affecting evaporation include: increasing surface area (faster evaporation), increasing temperature (faster), decreasing humidity (faster), and increasing wind speed (faster). The SI unit of temperature is Kelvin (K), where K = °C + 273. The concept of diffusion — the spontaneous mixing of particles of two substances on their own — demonstrates that particles are in constant motion. The rate of diffusion increases with temperature because particles move faster.

• Matter exists in three states: solid (fixed shape/volume, particles tightly packed), liquid (fixed volume, takes container shape), gas (no fixed shape/volume, particles far apart).
• State changes: melting (solid→liquid), boiling (liquid→gas), sublimation (solid→gas), condensation (gas→liquid), freezing (liquid→solid).
• Latent heat is absorbed during melting and boiling without temperature change — it overcomes intermolecular forces.
• Evaporation is a surface phenomenon that causes cooling; faster with larger surface area, higher temperature, lower humidity, and wind.
• Temperature conversion: K = °C + 273; diffusion demonstrates constant particle motion and increases with temperature.