Matter : Change of State

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Substances can change state from one state to another. Just like how a puddle of water disappears a few hours after a rain, it is because the water changed its state from liquid to gas. When heat is lost, steam condenses to become water while water freezes to become ice.

The process by which a gas changes to become a liquid is known as condensation. When steam/water vapour in the air is cooled (e.g. when they are in contact with cool surfaces) heat energy is lost to their surroundings. Particles in them vibrate slower, forces of attraction are formed between them and they condense into water droplets. Condensation is one way of separating impurities from water and purifying it. Condensation can take place at any temperature.

The process by which a liquid becomes a solid is known as freezing. Water loses heat energy to the surroundings when cooled down. Particles’ movement slows down; they take fixed positions as stronger forces form between them. Water now freezes into ice right at 0°C.

The process by which a liquid gains heat to become gas is known as evaporation. Do note that impurities in water, such as salt, will lower the melting point and raise the boiling point of water.

The differences between boiling and evaporation of water are as follows:

1. Boiling only occurs at the boiling point, which is 100°C*. Evaporation occurs at any temperature below the boiling point of water.
2. Boiling occurs throughout the liquid while evaporation only occurs at the surface of the liquid.
3. Boiling occurs very quickly while evaporation is a long process which occurs slowly.

*Do note that impurities in water, such as salt, will lower the melting point and raise the boiling point of water.

There are 5 key factors that affect the rate of evaporation:

1. Surrounding temperature. Higher surrounding temperate results in a faster rate of evaporation.
2. A temperature of the object. The higher temperature of the object itself causes faster rates of evaporation. Water gains more heat energy to evaporate when it is hotter.
3. Wind. The presence of wind results in a faster rate of evaporation. The stronger the wind, the faster the rate of evaporation.
4. Exposed surface area. Water spread over the larger area has a larger exposed surface area (more water can escape. Greater exposed a surface area of water = faster rate of evaporation.
5. Humidity, which refers to the amount of water vapour in the air. It is harder for water to evaporate if the air is humid and saturated as this means there is a lot of water vapour in the air. Increase in water vapour in the air = increase in humidity. Lower humidity = faster rate of evaporation as this means the surrounding air can take in more water vapor.

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