💦 Water and Changes of State
1. Water can exist in three different states, depending on the surrounding temperature.
2. The three states of water are solid state (e.g. ice), liquid state (e.g. tap water) and gaseous state (e.g. steam).
3. Depending on whether water gains or loses heat, it can change its state.
| State | Examples |
|---|---|
| Solid | Ice, Hail, Icicles, Snow |
| Liquid | Water droplets, Clouds, Dew, Mist, Fog |
| Gas | Water vapour, Steam |
1. Melting is the process by which a solid gains heat from the surroundings and becomes a liquid at a fixed temperature.
2. The melting point of pure ice is 0°C.
- When ice melts at room temperature, its temperature remains the same at 0°C until all the ice has completely melted.
- When all the ice has melted into water, the temperature of the water increases until it reaches room temperature.
1. Boiling is the process by which a liquid gains heat from the surroundings and becomes a gas at a fixed temperature.
2. The boiling point of pure water is 100°C.
- When the temperature of the water reaches 100°C (boiling point of water), the water starts to boil.
- When water boils, the temperature remains the same at 100°C until all the water becomes steam.
1. Adding impurities such as salt or sugar to water lowers the melting point of ice or freezing point of water to below 0°C and raises the boiling point of water to above 100°C.
| Segment | State of the substance |
|---|---|
| A → B (rising) | Solid |
| B → C (flat, melting point) | Solid + Liquid |
| C → D (rising) | Liquid |
| D → E (flat, boiling point) | Liquid + Gas |
| E → F (rising) | Gas |
The temperature remains constant at its melting point / boiling point.
1. Evaporation takes place when water (liquid state) gains heat from its surroundings and turns into water vapour (gaseous state).
2. Evaporation can take place at any temperature between the melting point and boiling point.
| Boiling | Evaporation | |
|---|---|---|
| Similarity: The liquid gains heat to undergo a change in state from a liquid to a gas. | ||
| Differences | Occurs throughout the liquid | Occurs only at the surface of the liquid |
| Occurs only at a fixed temperature (boiling point) | Occurs over a range of temperatures (as long as the substance is in liquid state) | |
1. The rate of evaporation is how fast water evaporates to form water vapour.
- Wind speed
- Exposed surface area
- Temperature
2. Temperature of water or surrounding air: As the temperature of the water or the surrounding air increases, the rate of evaporation of water increases.
3. Wind speed / Presence of wind: Wind removes water vapour near the surface of the water more quickly, allowing more evaporation to occur. The presence of wind increases the rate of evaporation of water. As the wind speed increases, the rate of evaporation of water increases.
4. Exposed surface area of water: Evaporation occurs only at the surface of water. As the exposed surface area of the water increases, the rate of evaporation of water increases.
5. Humidity: The amount of water vapour in the air. As the humidity of air decreases, the rate of evaporation of water increases.
1. 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.
1. Freezing is the process by which a liquid loses heat to the surroundings and becomes a solid at a fixed temperature.
2. The freezing point of pure water is 0°C (same as melting point).
- When the temperature of the water reaches 0°C (freezing point of water), the water starts to freeze into ice.
- The temperature of water remains constant at 0°C until all the water has frozen into ice.
- When all the water has frozen, the temperature of the ice decreases until it reaches the temperature of the freezer.
1. Condensation is the process by which a gas loses heat to the surroundings and becomes a liquid.
2. Condensation takes place at any temperature.
3. The hot water vapour/steam rises and comes into contact with a cooler surface, it loses heat and condenses to form water droplets.
| Segment | State of the substance |
|---|---|
| A → B (falling) | Liquid |
| B → C (flat, freezing point) | Liquid + Solid |
| C → D (falling) | Solid |
| D → E (flat) | Solid |
The temperature remains constant at its freezing point.
Answer template:
Warmer water vapour from ___ came into contact with and lost heat to the cooler surface of ___, condensing to form water droplets.
1. Factor affecting the rate of condensation — temperature difference: The greater the temperature difference between the water vapour and the surface for condensation, the greater the rate of condensation of the water vapour to form more water droplets.
- The heat from the Sun causes water from the ground and water bodies to gain heat and evaporate.
- The water vapour rises to the sky. It loses heat to the surrounding air and condenses to form tiny water droplets.
- The water droplets gather to form clouds.
- When the water droplets in the clouds get too heavy, they fall as rain and return back to the Earth.
| Condensation | Evaporation | |
|---|---|---|
| Similarity: Both undergo a change in state. | ||
| Differences | Gaseous state to liquid state | Liquid state to gaseous state |
| Heat loss | Heat gain | |
| Action | Examples |
|---|---|
| Reduce | Brushing teeth with a cup of water instead of a running tap; Taking short showers (instead of full baths) |
| Reuse | Watering plants with water used to wash rice or vegetables; Washing the floor and toilet with used water or collected rainwater |
| Recycle | Treating waste water in water treatment plant to make it drinkable |