9. Change of State of Matter

Matter commonly exists in three states — solid, liquid, and gas — and can change from one state to another through the addition or removal of heat energy. These transitions are collectively called changes of state or phase transitions, and each has a specific name depending on the states involved.

9.1 Melting and Freezing

Melting (or fusion) is the process by which a solid changes into a liquid on absorbing heat, occurring at a fixed temperature for a pure crystalline substance called its melting point. The reverse process, freezing (or solidification), is the change of a liquid into a solid on the removal of heat, occurring at the freezing point, which for a pure substance is numerically the same temperature as its melting point (for water/ice, this is 0°C at standard atmospheric pressure). During melting or freezing, as noted earlier, the temperature of the substance remains constant while the phase change is in progress, because the heat supplied or removed is entirely latent heat, used to break or form the intermolecular bonds of the solid lattice rather than to change the kinetic energy (and hence temperature) of the molecules.

9.2 Boiling, Evaporation and Condensation

Boiling is the rapid conversion of a liquid into vapour that occurs throughout the bulk of the liquid (not just at its surface) when the liquid is heated to a specific temperature, called its boiling point, at which the vapour pressure of the liquid becomes equal to the surrounding atmospheric pressure. For water at standard (1 atmosphere) atmospheric pressure, the boiling point is 100°C. Evaporation, by contrast, is a slower process of vaporisation that occurs only at the surface of a liquid, at any temperature (not just at the boiling point), and does not require the liquid to be heated to a specific fixed temperature. Evaporation is a surface phenomenon driven by the fact that even at temperatures below the boiling point, some fast-moving molecules near the liquid's surface possess enough kinetic energy to escape into the vapour phase. Because it is the more energetic molecules that escape during evaporation, the average kinetic energy (and hence temperature) of the remaining liquid decreases — this is why evaporation causes cooling, which is the principle behind sweating cooling the human body, the use of earthen (matka) pots to cool water in summer (water seeping through the porous walls evaporates and cools the water inside), and the functioning of evaporative (desert) coolers.

The rate of evaporation is increased by: an increase in temperature, an increase in surface area of the liquid exposed, a decrease in humidity of the surrounding air, and an increase in wind speed over the liquid surface (which carries away vapour molecules and maintains a lower vapour concentration near the surface, promoting further evaporation) — this is precisely why wet clothes dry faster on a hot, dry, windy day when spread out rather than bunched up. Condensation is the reverse process of vaporisation: the change of a vapour or gas into a liquid on cooling (or on removal of heat/latent heat), such as water vapour in the atmosphere condensing into tiny water droplets to form clouds, dew, or mist, or steam condensing into water droplets on a cold surface such as the underside of a lid on a pot of boiling water.

9.3 Sublimation

Sublimation is the direct change of a solid into vapour (gas) on heating, without passing through the intermediate liquid state, and the reverse process (vapour directly forming a solid, also sometimes called deposition) can likewise occur on cooling. Common substances that sublime under ordinary atmospheric conditions include solid carbon dioxide (commonly known as 'dry ice', used for refrigeration and creating stage fog/smoke effects), naphthalene (used in mothballs/camphor balls for repelling insects from stored clothes), iodine, ammonium chloride, and camphor.

9.4 Effect of Pressure on Melting and Boiling Points

The melting point and boiling point of a substance are not absolute, fixed constants of nature independent of conditions — both depend significantly on the surrounding pressure.

• Effect of pressure on boiling point: increasing the external (atmospheric) pressure on a liquid raises its boiling point, while decreasing the pressure lowers the boiling point. This is precisely the principle exploited by a pressure cooker (higher internal pressure raises water's boiling point above 100°C, cooking food faster) and explains why water boils at a lower temperature at high altitudes, such as on mountains (where atmospheric pressure is lower than at sea level) — for example, water may boil at only around 70°C atop very high peaks, which can make ordinary open-pot cooking take much longer or leave food inadequately cooked, which is why pressure cookers are especially valuable at high altitudes.
• Effect of pressure on melting point: for most substances, increasing pressure raises the melting point slightly. However, ice (frozen water) is a notable and important exception: because ice is less dense than liquid water (due to water's anomalous expansion on freezing, discussed earlier), increasing the pressure on ice actually lowers its melting point, causing it to melt at a temperature below 0°C. This phenomenon, called regelation, is classically illustrated by a wire loaded with weights at both ends slowly cutting through a block of ice: the pressure exerted by the wire locally lowers the melting point of the ice beneath it, causing localised melting; the wire sinks into the resulting meltwater, which then refreezes above the wire once the pressure is relieved (since the water there is no longer under the localised high pressure), leaving the ice block intact as a whole even after the wire has passed all the way through it.

PART B: SOUND