Science

Finish Line & Beyond

Science Class 9th
Matter in Our Surroundings
Characteristics of Particles of Matter States of Matter Effect of Change of Temperature Effect of Change of Pressure

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Introduction Everything in this universe is made up of material which scientists have named “matter”. The air we breathe, the food we eat, stones, clouds, stars, plants and animals, even a small drop of water or a particle of sand– each thing is matter. Early Indian philosophers classified matter in the form of five basic elements –air, earth, fire, sky and water. Modern day scientists have evolved two types of classification of matter based on their physical properties and chemical nature. Characteristics of Particles of Matter 1. PARTICLES OF MATTER HAVE SPACE BETWEEN THEM- when we make tea, coffee or lemonade (nimbu paani ), particles of one type of matter get into the spaces between particles of the other. This shows that there is enough space between particles of matter. Similarly particles of sugar, salt, Dettol, or potassium permanganate get evenly distributed in water. 2. PARTICLES OF MATTER ARE CONTINUOUSLY MOVING- Particles of matter are continuously moving, that is, they possess what we call the kinetic energy. As the temperature rises, particles move faster. So, we can say that with increase in temperature the kinetic energy of the particles also increases. 3. PARTICLES OF MATTER ATTRACT EACH OTHER- Particles of matter have force acting between them. This force keeps the particles together. The strength of this force of attraction varies from one kind of matter to another. States of Matter 1. THE SOLID STATE- all solids have a definite shape, distinct boundaries and fixed volumes, that is, have negligible compressibility. Solids have a tendency to maintain their shape when subjected to outside force. Solids may break under force but it is difficult to change their shape, so they are rigid. 2. THE LIQUID STATE- Liquids have no fixed shape but have a fixed volume. They take up the shape of the container in which they are kept. Liquids flow and change shape, so they are not rigid but can be called fluid. The rate of diffusion of liquids is higher than that of solids. This is due to the fact that in the liquid state, particles move freely and have greater space between each other as compared to particles in the solid state. 3. THE GASEOUS STATE- Gases are highly compressible as compared to solids and liquids. The liquefied petroleum gas (LPG) cylinder that we get in our home for cooking or the oxygen supplied to hospitals in cylinders is compressed gas. In the gaseous state, the particles move about randomly at high speed. Due to this random movement, the particles hit each other and also the walls of the container.

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Matter Can Change its State Water can exist in three states of matter– • Solid, as ice, • Liquid, as the familiar water, and • Gas, as water vapour. EFFECT OF CHANGE OF TEMPERATURE On increasing the temperature of solids, the kinetic energy of the particles increases. Due to the increase in kinetic energy, the particles start vibrating with greater speed. The energy supplied by heat overcomes the forces of attraction between the particles. The particles leave their fixed positions and start moving more freely. A stage is reached when the solid melts and is converted to a liquid. The temperature at which a solid melts to become a liquid at the atmospheric pressure is called its melting point. The melting point of ice is 273.16 K*. The process of melting, that is, change of solid state into liquid state is also known as fusion. Heat Heat

Solid State Solid State
Cool

Liquid State
Cool

Gaseous State

So, we infer that the state of matter can be changed into another state by changing the temperature. EFFECT OF CHANGE OF PRESSURE Increasing or decreasing the pressure can change the state of matter. Applying pressure and reducing temperature can liquefy gases. For an example, Solid carbon dioxide (CO2) is stored under high pressure. Solid CO2 gets converted directly to gaseous state on decrease of pressure to 1 atmosphere* without coming into liquid state. This is the reason that solid carbon dioxide is also known as dry ice. Thus, we can say that pressure and temperature determine the state of a substance, whether it will be solid, liquid or gas. Evaporation Particles of matter are always moving and are never at rest. At a given temperature in any gas, liquid or solid, there are particles with different amounts of kinetic

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Finish Line & Beyond energy. In the case of liquids, a small fraction of particles at the surface, having higher kinetic energy, is able to break away from the forces of attraction of other particles and gets converted into vapour. This phenomenon of change of a liquid into vapours at any temperature below its boiling point is called evaporation. FACTORS AFFECTING EVAPORATION The rate of evaporation increases with– • An increase of surface area: We know that evaporation is a surface phenomenon. If the surface area is increased, the rate of evaporation increases. For example, while putting clothes for drying up we spread them out. • An increase of temperature: With the increase of temperature, more number of particles get enough kinetic energy to go into the vapour state. • A decrease in humidity: Humidity is the amount of water vapour present in air. The air around us cannot hold more than a definite amount of water vapour at a given temperature. If the amount of water in air is already high, the rate of evaporation decreases. • An increase in wind speed: It is a common observation that clothes dry faster on a windy day. With the increase in wind speed, the particles of water vapour move away with the wind, decreasing the amount of water vapour in the surrounding. HOW DOES EVAPORATION CAUSE COOLING? In an open vessel, the liquid keeps on evaporating. The particles of liquid absorb energy from the surrounding to regain the energy lost during evaporation. This absorption of energy from the surroundings make the surroundings cold. After a hot sunny day, people sprinkle water on the roof or open ground because the large latent heat of vaporisation of water helps to cool the hot surface. Why should we wear cotton clothes in summer? During summer, we perspire more because of the mechanism of our body which keeps us cool. We know that during evaporation, the particles at the surface of the liquid gain energy from the surroundings or body surface and change into vapour. The heat energy equal to the latent heat of vaporisation is absorbed from the body leaving the body cool. Cotton, being a good absorber of water helps in absorbing the sweat and exposing it to the atmosphere for easy evaporation.

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Why do we see water droplets on the outer surface of a glass containing icecold water? Let us take some ice-cold water in a tumbler. Very soon we see water droplets on the outer surface of the tumbler. The water vapour present in air, on coming in contact with the cold glass of water, loses energy and gets converted to liquid state, which we see as water droplets.

Summary • Matter is made up of small particles. • The matter around us exists in three states— solid, liquid and gas. • The forces of attraction between the particles are maximum in solids, intermediate in liquids and minimum in gases. • The spaces in between the constituent particles and kinetic energy of the particles are minimum in the case of solids, intermediate in liquids and maximum in gases. • The arrangement of particles is most ordered in the case of solids, in the case of liquids layers of particles can slip and slide over each other while for gases, there is no order, particles just move about randomly. • The states of matter are inter-convertible. The state of matter can be changed by changing temperature or pressure. • Sublimation is the change of gaseous state directly to solid state without going through liquid state, and vice versa. • Boiling is a bulk phenomenon. Particles from the bulk (whole) of the liquid change into vapour state. • Evaporation is a surface phenomenon. Particles from the surface gain enough energy to overcome the forces of attraction present in the liquid and change into the vapour state. • The rate of evaporation depends upon the surface area exposed to the atmosphere, the temperature, the humidity and the wind speed. • Evaporation causes cooling. • Latent heat of vaporisation is the heat energy required to change 1 kg of a liquid to gas at atmospheric pressure at its boiling point.

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• Latent heat of fusion is the amount of heat energy required to change 1 kg of solid into liquid at its melting point.

• Some measurable quantities and their units to remember: Quantity Temperature Length Mass Weight Volume Density Pressure Unit Kelvin Metre Kilogram Newton Cubic metre Kilogram per cubic metre Pascal Symbol K M Kg N m3 kg m–3 Pa

*Note: Kelvin is the SI unit of temperature, 00 C =273.16 K. we take 00 C = 273 K. To change a temperature on the Kelvin scale to the Celsius scale we have to subtract 273 from the given temperature, and to convert a temperature on the Celsius scale to the Kelvin scale we have to add 273 to the given temperature. * Atmosphere (atm) is a unit of measuring pressure exerted by a gas. The unit of pressure is Pascal (Pa): 1 atmosphere = 1.01 × (10 to the power 5) Pa. The pressure of air in atmosphere is called atmospheric pressure. The atmospheric pressure at sea level is 1 atmosphere, and is taken as the normal atmospheric pressure.

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