Changes in Physical State.pdfVIP

Changes in Physical State Daphny Bian The interconversion of physical states of matter Melting and freezing The temperature at which a pure substance turns to a liquid is called the melting point (m.p.). This always happens at one particular temperature for each pure substance. ? The process is reversed at precisely the same temperature if a liquid is cooled down. It is then called the freezing point (f.p.). ? The melting point and freezing point of any given substance are both the same temperature. ? For example, the melting and freezing point of pure water take place at 0℃. Evaporation ? If a liquid is left with its surface exposed to the air, it evaporates. When liquids change into gases over a range of temperature, the process is called evaporation. ? Evaporation takes place from the surface of the liquid. ? The rate of evaporation depends upon: Temperature: the warmer the liquid, the faster it evaporates. Wind speed: the higher the wind speed, the more evaporation. Surface area: the larger the surface area, the faster the liquid evaporates. Boiling ? Eventually, at a certain temperature, it becomes hot enough to form gas within the liquid and not just at the surface. Bubbles of gas appear inside the liquid. This process is known as boiling. ? It takes place at a specific temperature, at the boiling point (b.p.) for each pure liquid. Condensation (gas-liquid) ? The reverse of evaporation is condensation. Also there is a condensation point (c.p.) for each pure liquid at a specific temperature, ? This is usually brought about by cooling. However, it is possible at normal temperatures to condense a gas into a liquid by increasing the pressure, without cooling. ? The word vapour is used for a gas that can be compressed into liquid without cooling. ? Normally, though, we use the words gas or vapour when talking generally about the gas state. Sublimation ? A few solids – carbon dioxide (dry ice) and iodine, for example – do not melt whe