P5 Cycles in Water

Water exists in 3 states — solid, liquid, and gaseous states.

Solid	Liquid	Gas
Ice	Water droplets	Water vapour
Hail	Clouds	Steam
Icicles	Dew
Snow	Mist
	Fog

Water in the solid and liquid states is visible, while water in the gaseous state is invisible.

Water can change from one state to another when it gains or loses heat.

Gains Heat →	Solid → Liquid → Gas
Loses Heat →	Gas → Liquid → Solid

Melting:

• Melting is the process of changing from the solid state to the liquid state
• Occurs when water in the solid state (ice) gains heat
• The melting point of ice is 0°C
• When ice melts at room temperature, its temperature remains unchanged at 0°C until all the ice has melted completely
• After all ice has melted, the water's temperature increases until it reaches room temperature

Freezing:

• Freezing is the process of changing from the liquid state to the solid state
• Occurs when water loses heat to become ice
• The freezing point of water is 0°C
• When water freezes in a freezer, its temperature remains unchanged at 0°C until all the water has frozen
• After all water has frozen, the ice's temperature decreases until it reaches the freezer's temperature

Condensation:

• Condensation is the process of changing from the gaseous state to the liquid state
• Occurs when water in the gaseous state (water vapour or steam) loses heat
• Condensation occurs at any temperature
• When water vapour comes into contact with a cooler surface, it loses heat to the surface and changes into water

Boiling:

• Boiling is the process of changing from the liquid state to the gaseous state at a fixed temperature
• Occurs when water gains heat to become steam
• The boiling point of water is 100°C
• When water boils, its temperature remains unchanged at 100°C until all the water becomes steam
• The steam loses heat to the cooler surrounding air and condenses to form tiny water droplets that appear as mist

Evaporation:

• Evaporation is the process of changing from the liquid state to the gaseous state at any temperature below the boiling point
• Occurs when water gains heat to become water vapour
• Evaporation occurs only at the water surface

The rate of evaporation refers to how quickly water evaporates.

The rate of evaporation depends on temperature, wind, exposed surface area, and humidity.

Factor	Effect on Rate of Evaporation
Temperature	The higher the surrounding temperature, the higher the rate of evaporation. A higher temperature causes water to change into water vapour quickly.
Wind	The stronger the wind, the higher the rate of evaporation. The wind carries the water vapour away from the water surface and more water can evaporate into water vapour to occupy the space.
Exposed Surface Area	The larger the exposed surface area, the higher the rate of evaporation. A bigger exposed surface area allows more water to evaporate into water vapour.
Humidity	The lower the humidity, the higher the rate of evaporation. When the humidity is low, it is easier for water to evaporate into water vapour.

The lenses of a pair of spectacles become misty after the user walks out from an air-conditioned room.

• The lenses lost heat to the cooler surrounding air in the air-conditioned room and became cooler
• When the user walked out, the warmer water vapour in the surrounding air outside the room came into contact with and lost heat to the cooler surface of the lenses, condensing to form water droplets

The evaporation of a liquid can be used to cool its surroundings or a surface.

• As a liquid gains heat from its surroundings to evaporate, the surroundings lose heat and cool down

• Water exists in 3 states: solid (ice), liquid (water), gaseous (steam/water vapour)
• Solid: definite shape + volume | Liquid: no definite shape, definite volume | Gas: no definite shape or volume
• Melting: solid → liquid (gains heat) at 0°C
• Freezing: liquid → solid (loses heat) at 0°C
• Boiling: liquid → gaseous (gains heat) at 100°C — throughout the water
• Evaporation: liquid → gaseous (gains heat) at any temperature below boiling point — only at surface
• Condensation: gaseous → liquid (loses heat) at any temperature
• During phase changes, temperature remains unchanged
• Rate of evaporation increases with: higher temperature, stronger wind, larger surface area, lower humidity