Matter in Our Surroundings (Class - IX)

K = C + 273
C = K – 273
C = 500 – 273 = 227^oC 

People sprinkle water on the roof or a hot surface because the large latent heat of vaporisation of water helps cool the hot surface. 
Latent heat of fusion is the amount of heat energy required to convert 1 kg of solid into liquid at its melting point. Sublimation is the phenomenon of direct conversion of a solid into the gaseous state on heating and vice-versa on cooling without passing through the intervening liquid state. Precipitation involves formation of insoluble products. Thus, these options are incorrect.

The greater the force of attraction, the more is the energy required to overcome this force of attraction in order to convert solid to liquid. Z has the highest melting point, i.e. the forces of attraction are the highest in its particles.

One is not able to cut the stream of running water as particles of water have forces of attraction between them. Water (liquid) particles do not have very large spaces between them. Therefore, the force of attraction between the particles is high. Moreover, particles of every kind of matter are small and they are continuously moving.

Everything in this universe is made up of 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, everything is matter. Ancient Indian philosophers classified matter in the form of five basic elements the “Panch Tatva” - air, earth, fire, sky and water.

Matter consists of small particles, which have spaces between them. Particles of matter are continuously moving and they possess kinetic energy. The kinetic energy of the particles is directly proportional to temperature, i.e. on increasing the temperature, the kinetic energy of the particles increases. 

Melting is the process of conversion of solid to liquid and the amount of heat energy required to convert 1 kg of solid to liquid at atmospheric pressure at its melting point is called latent heat of fusion. When ice at 0^oC melts to form water at 0^oC, it absorbs extra heat in the form of latent heat of fusion, i.e. water at 0^oC has more heat than ice and therefore, ice at 0^oC is more effective in cooling than water at 0^oC. During melting, heat is absorbed and not released because heat is required to overcome the forces of attraction so as to convert solid to liquid. Therefore, option (4) is incorrect. Evaporation is the conversion of liquid to vapours at temperature below its boiling point. Hence, options (1) and (2) are incorrect.

When heat is supplied to a crystalline solid, its temperature increases and hence, kinetic energy of the constituent particles increases. Due to increase in kinetic energy, the particles start vibrating with greater speed. As the temperature is further increased, the energy supplied in the form of heat overcomes the forces of attraction between the particles. At this stage, the particles leave their fixed position and start melting to become liquid. Once the solid starts melting, the temperature of the substance does not rise during the change of state. This is because the heat being supplied is used up in overcoming the forces of attraction between the particles of a solid during the change of state from solid to liquid. Therefore, temperature of the substance remains the same when solid melts to become liquid. The heat is believed to be hidden within the particles and is called latent heat.

Evaporation is the phenomenon of change of liquid into vapours at any temperature below its boiling point. Evaporation is a surface phenomenon, i.e. only the particles on the surface of the liquid get converted into vapours. Thus, greater is the surface area; more is the rate of evaporation. For example, we often spread wet clothes in order to dry them in air. By doing so, the surface area available for evaporation of water increases and hence, the clothes get dried up soon. However, with the decrease in surface area, the rate of evaporation also decreases. Also, the rate of evaporation increases with increase in temperature. This is due to the reason that as the temperature increases, the kinetic energy of the particles of liquid also increases. 

Melting is the process of conversion of solid into liquid and the amount of heat energy required to convert 1 kg of solid to liquid at atmospheric pressure at its melting point is called latent heat of fusion. When ice at 0^oC melts to form water, it absorbs extra heat in the form of latent heat of fusion, i.e. water at 0^oC has more heat than ice. Water exists as ice at -273 K; it will have the lowest energy at this temperature.