We know, water exists in all three states –
• Solid as ice
• Liquid as water
• Gas as water vapour.
Now questions arise –
(1) How do changes from one state to other state take place?
(2) What happens to the particles of matter during the change of states?
(3) What happens inside the matter during the change of states?
Matter does not always remain in the same state. It can be made to change its state either by changing in temperature or pressure. Pressure and temperature determine the state of a substance i.e. solid, liquid or gas.
(a) Take some ice cubes in a beaker. Put a thermometer in such a way that its bulb is in contact with the ice. Heat the beaker on a low flame. Record the temperature –
• when the ice starts melting.
• when all the ice has converted into water.
(b) Now, put a glass rod in a beaker, stir well during heating. Heat till the water starts boiling. Record the temperature when most of the water has vaporised.
It is observed from the above activity,
Change in temperature can alter the state of matter.
– If a solid is heated to a temperature, it turns / melts into 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 273K or 0°C. The process of melting, that is, change of solid state into liquid state is also known as fusion.
– If a liquid is heated to a temperature, it changes into a gas. The temperature at which a liquid starts boiling at the atmospheric pressure is called its boiling point. The boiling point of water is 373K or 100°C.
(ii) When a solid melts / a liquid boils, the temperature of the system does not change after reaching the melting / boiling point, till all the solid (ice) melts / liquid (water) boils. This happens even though we continue to heat the beaker i.e. continue to supply the heat.
(iii) Kinetic Molecular Theory of Matter –
We know, all matter is composed of particles. The difference in the three states of matter (solid, liquid and gaseous states) is based on the motion of their particles.
– In solids, the particles are held together by strong inter-molecular force of attraction. They are fixed, but can vibrate about their mean position.
– In liquids, the particles can move about, but remain in contact.
– In gases, the particles move about freely. The kinetic theory explains heat as the kinetic energy or energy of motion of the particles. It also explains the inter-conversion of the states of matter. On increasing the temperature of a matter, the kinetic energy of the particles increases. Due to the increase in kinetic energy, the particles start The kinetic theory explains heat as the kinetic energy or energy of motion of the particles. It also explains the inter-conversion of the states of matter. On increasing the temperature of a matter, the kinetic energy of the particles increases. Due to the increase in kinetic energy, the particles start vibrating or moving with greater speed. The energy supplied by heat overcomes the forces of attraction between the particles.
- When a solid is heated, the particles of a solid leave their fixed positions and start moving more freely. At melting point, the solid melts and converts into a liquid. The temperature of the system does not change after reaching the melting point, till all the solid melts. This happens even though we continue to heat the beaker i.e. continue to supply the heat. This heat gets used up in changing the state by overcoming the forces of attraction between the particles and does not show any rise in temperature. Thus, the amount of heat energy needed to change of 1 kg of a solid into liquid at the same temperature (or its melting point) at atmospheric pressure is called the latent heat of fusion (latent means hidden). Due to latent heat of fusion, particles of water at 0°C or 273K have more energy as compared to particles in ice at the same temperature.
- If still more heat is given, the particles of the liquid gain more kinetic energy and start moving faster. At boiling point, the particles have enough energy to break free from the force of attraction of each other and here, liquid starts changing into gas. Thus, the amount of heat energy needed to change of state, from a loosely bound liquid to a freely moving gas at the same temperature (or its boiling point) is called the latent heat of vaporisation. Due to latent heat of vaporisation, particles of water vapour or steam at 100°C or 373K have more energy as compared to particles in water at the same temperature.(iv) If a gas is cooled, the particles lose energy and condense into liquid and further freezes into a solid.Thus, the process of change of one state of matter into other and back to its original state is called inter-conversion of matter.(v) Sublimation – Sometimes a solid change directly into a gas without changing into liquid state or vice versa is called sublimation.
Example – Iodine, solid carbon dioxide (dry ice), camphor, ammonium chloride, naphthalene etc.
- Dry ice and naphthalene sublime at room temperature. This is the reason, naphthalene balls are kept in toilets and also used as mothballs.
- The mixture of iodine and sand can be separated by heating the mixture. Iodine sublimates and recovers again in solid form by cooling the vapours.
By increasing pressure, particles of matter can be brought close together. When we apply pressure on a gas enclosed in a cylinder, it starts compressing and converts into liquid. Thus, by applying pressure and reducing temperature a gas can be liquefied.
Example – Carbon dioxide is a gas under normal conditions of temperature and pressure. It can be liquefied by putting pressure at ordinary temperature. 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, pressure and temperature determine the state of a substance i.e. solid, liquid or gas.