• All matter occupy space and have mass. In other words, they have both mass and volume.

• Copper sulphate is blue [CuSo4] and Potassium permanganate is pinkish - violet [KMnO4].

• Just a few crystals of potassium permanganate can color a large volume of water [about 1000 L]. So we conclude that there must be millions of tiny particles in just one crystal of potassium permanganate, which keep on dividing themselves into smaller and smaller particles. This shows that there is enough space between particles of matter.

• Particles of matter are continuously moving, that is, they possess what we call 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.

• Particles of matter intermix on their own with each other. They do so by getting into the spaces between the particles. This intermixing of particles of two different types of matter on their own is called diffusion. On heating diffusion becomes faster.

• 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.

• 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 to their shape, so they are rigid.

• A rubber band changes shape under force and regains the same shape when the force is removed. If excessive force is applied, it breaks.

• The shape of each individual sugar or salt crystal remains fixed, whether we take it in our hand, put it in a plate or in a jar.

• A sponge has minute holes, in which air is trapped, when we press it, the air is expelled out and we are able to compress it.

• We observe that liquids have fixed volume. Liquids flow and change shape, so they are not rigid but can be called fluid.

• The gases from the atmosphere diffuse and dissolve in water. These gases, especially oxygen and carbon dioxide, are essential for the survival of aquatic animals and plants.

• The aquatic animals can breathe under water due to the presence of dissolved oxygen in water. Thus, we may conclude that solids, liquids and gases can diffuse into liquids. The rate of diffusion of liquids is higher than that of solids. That 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.

• We have observed that gases are highly compressible as compared to solids and liquids. Due to its high compressibility, large volumes of a gas can be compressed into a small cylinder and transported easily.

• Due to high speed of particles and large space between them, gases show the property of diffusing very fast into other gases.

• 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. The pressure exerted by the gas is because of this force exerted by gas particles per unit area on the walls of the container.

• 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 minimum temperature at which a solids melts to become a liquid at the atmospheric pressure is called its melting point. The melting point of a solid is an indication of the strength of the force of attraction between its particles.

• To change a temperature on the Kelvin scale to the Celsius scale you have to subtract 273 from the given temperature, and to convert a temperature on the Celsius scale you have to add 273 to the given temperature.

• The melting point of ice is 273.15 K. The process of melting, that is, change of solid state into liquid state is also known as fusion. When a solid melts, its temperature remains the same.

• The temperature of the system does not change after the melting point is reached, till all the ice melts. This happens even though we continue to heat the beaker, that is, we continue to supply heat. This heat gets used up in changing the state by overcoming the forces of attraction between the particles. As this heat energy is absorbed by ice without showing any rise in temperature, it is considered that it gets hidden into the contents of the beaker and is known as latent heat. The word latent means hidden.

• The amount of heat energy that is required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point is known as the latent heat of fusion.

• So particles in water at 0 degree C [237 K] have more energy as compared to particles in ice at the same temperature.

• When we supply heat energy to water, particles start moving even faster. At a certain temperature, a point is reached when the particles have enough energy to break free from the forces of attraction of each other. At this temperature the liquid starts changing into gas. The temperature at which a liquid starts boiling at the atmospheric pressure is known as its boiling point. Boiling is bulk phenomenon. Particles from the bulk of the liquid contain enough energy to change into the vapor state.

• For water to change into vapor - 373 K [100 degrees C = 273 + 100 = 373 K]

• Particles in steam, that is, water vapor at 373 K [100 degrees C] have more energy than water at the same temperature. This is because particles in steam have absorbed extra energy in the form of latent heat of vaporization.

• So, we infer that the state of matter can be changed into another state by changing the temperature.

• A change of state directly from solid to gas without changing into liquid state is called sublimation. Eg. Dry ice.

• The direct change of gas to solid without changing into liquid is called deposition. Eg. Frost.

• Gas to liquid - Condensation

• Liquid to gas - Evaporation

• Liquid to solid - Solidification

• The difference in various states of matter is due to the difference in the distances between the constituent particles. Applying pressure and reducing temperature can liquefy gases.

• 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 dry ice.

• Thus, we can say that pressure and temperature determine the state of a substance, whether it be solid, liquid or gas.

• Atmosphere [atm] is a unit of measuring pressure exerted by a gas. The unit of pressure is Pascal [Pa] : 1 atmosphere = 1.01 x 10[5] {raised to 5} Pa. The pressure of air in atmosphere is called atmospheric pressure.

• At a given temperature in any gas, liquid or solid, there are particles with different amounts of kinetic 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 vapor. This phenomenon of change of a liquid into vapors at any temperature below its boiling point is called evaporation.

  The rate of evaporation increases with -

• An increase of surface area :

  If the surface area is increased, the rate of evaporation increases.

• An increase of temperature :

  With the increase of temperature, more number of particles get enough kinetic energy to go into the vapor state.

• A decrease in humidity :

Humidity is the amount of water vapor present in air. The air around us cannot hold more than a definite amount of water vapor at a given temperature. If the amount of water in air is already high, the rate of evaporation decreases.

• An increase in wind speed :

With the increase in wind speed, the particles of water vapor move away with the wind, decreasing the amount of water vapor in the surrounding.

Q. Why should we wear cotton clothes in summer ?

Ans. 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 vapor. The heat energy equal to the latent heat of vaporization 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.

Q. Why do we see water droplets on the outer surface of a glass containing ice cold water ?

Ans. The water vapor present in air, on coming in contact with the cold glass of water, loses energy gets converted to liquid state, which we see as water droplets.

Important Points

• 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 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.