States of Matter

The process of conversion of solids to liquid is called phase change. This phase change from solid to liquid requires some energy in the form of temperature.

We can take example of ICE and WATER phase transformation. Ice is frozen form of water. At temperature lower than 0 degree Celsius, particles movement of water molecules completely restricted, results in the frozen form of water called Ice. Now, when Ice is taken out from refrigerator, it absorbs energy from air.  This increase of the temperature of solid Ice results in the increase of kinetic energy of the particles. Due to the increase in kinetic energy, the particles start vibrating with greater speed. The energy/heat supplied by air 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 solid melts to become a liquid at the atmospheric pressure is called its melting point. The process of melting, that is, change of solid state into liquid state is also known as fusion. 

Melting of glaciers due to increase in global temperature is also example of phase change. Burning of fossil fuels, deforestation, volcanic eruption are the main reason for increase in green house gases (which increases the global temperature). This is our planet, and its over responsibility to use more of greener methods to reduce the production of green house gases.

Freezing is the process of conversion of liquid to solid. When we put water into the freezer, it transfer heat from water molecules to the air of the freezers. This means molecules of water releases energy and force of attraction overcomes the movement of liquid particles resulting in the solidification of liquid called freezing. The temperature at which any liquid starts freezing is called freezing point. The freezing point of water is 0 degree Celsius. When the temperature is slight lower than 0 degree Celsius, all of water freezes to ice.

Vaporization is phenomena of conversion of liquid to gas. This can be differentiate into two categories i.e. Boiling and Evaporations. We can discuss boiling first. 

Boiling: On suppling 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 called boiling. The temperature at which a liquid starts boiling at the atmospheric pressure is known as its boiling point. 

Boiling is a bulk phenomenon. Particles from the bulk of the liquid gain enough energy to change into the vapour state.

Evaporation: Evaporation is the surface phenomena. We know that particles are always in constant motion. 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 liquid into vapours at any temperature below its boiling point is called evaporation. This is slow process as compared to the boiling.  

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. 

Condensation is the opposite of vaporization. This is a phenomena of conversion of vapors to liquid. This can be achieved by cooling the vapors. You must have noticed the water droplets outside a cold soda can. This is due to condensation. The water vapors from air comes in contact with the cold surface of Can, transfer energy to the cold surface which results in lowering down the movement of vapor particles, start depositing on it and appears as water droplets.   

 A change of state directly from solid to gas without changing into liquid state is called sublimation. For example, dry ice (solid carbon dioxide) sublimates at room temperature to produce gaseous carbon dioxide. Another example is Iodine (an element) exist in solid state and convert directly to vapors at room temperature.  

The direct change of gas to solid without changing into liquid is called deposition. For example, formation of frost a leaf. Solid frost is formed when a surface, for example a leaf, is at a temperature lower than the freezing point of water and the surrounding air is humid. Water vapor changes directly to ice without first becoming a liquid.

We discussed the effect of temperature on the three states of matter. Now, we can discuss about the effect of pressure on the states of matter.

Increasing pressure changes the state of matter from gas to liquid and also from liquid to solid. However, increasing pressure alone cannot change the state of matter. The space between the particles that make up a substance is what determines its state of matter. When pressure is given to a gas, it compresses into a liquid, and when pressure is applied to a liquid, it solidifies. Pressure has a negligible influence on solids. Solid dry ice is obtained by compressing carbon dioxide gas. When solid dry ice left open to atmospheric pressure, it turns to gaseous Carbon dioxide. Similarly, Liquified petroleum gas (LPG) used in cooking is also compressed liquid version of petroleum gas.