Name: Separation of Solid-Solid Mixtures
Uploaded: 2019-07-19
Duration: 7 min 13 s
Description: When metals react with non-metals, electrons are transferred from the metal atom to the non-metal atom, forming ions. Let's explore their reaction with hydrogen and chlorine gases

Question

Write the effect of impurities and external pressure on the freezing and boiling points of water. Explain with examples.

Accepted Answer

When impurities are added to water, the boiling point of water increases and the freezing point decreases.

This is because the impurities lower the vapour pressure of water. The boiling point of a liquid is the temperature at which its vapour pressure is equal to the atmospheric pressure. When impurities are added to water, they lower the vapour pressure of water. Therefore, more heat is required to raise the vapour pressure of water to the atmospheric pressure, which results in an increase in boiling point.

When an impurity is added to water, its freezing point decreases. This is because the depression in freezing point increases with an increase in the concentration of solute. Because on adding the solute, vapour pressure becomes lower than that of pure solvent. Hence, freezing occurs at a lower temperature. When ethylene glycol (non-volatile) is added to water, the freezing point of water is lowered. Hence, ethylene glycol is used as an antifreeze. For example, saltwater has a lower freezing point than pure water because salt lowers the freezing point of water. An antifreeze lowers the freezing point of water in car radiators so that it does not freeze in cold weather.

External pressure also affects the boiling and freezing points of water.

With increase in pressure the boiling point of water increases.This is because when more pressure is acting on the surface of a liquid the liquid cannot vapourises easily. That is why in a pressure cooker the increased pressure raises the boiling point of water and food cooks faster. In mountainous terrains (where the altitude is high), the pressure of the atmosphere is relatively lower than the atmospheric pressure at sea level. This is the reason why food cooks at a slower pace in mountainous areas (the lower atmospheric pressure causes water to boil at temperatures below $100 ° C$ ).

Changes in pressure can raise or lower the freezing point of a substance. Generally, pressures lower than one atmosphere, lower the temperature at which a substance freezes. But for water, a higher pressure gives a lower freezing point. For water, at low pressures, vapour directly turns to ice without becoming a liquid. The value of freezing point of a substance is always same as that of the melting point.

Write the effect of impurities and external pressure on the freezing and boiling points of water. Explain with examples.

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