Water

                              WATER                                     

1. All living things are made up largely of water. Water makes up about 70% to 90% of the different kinds of bodies.

2. Without water there will be no life. Water is essential (needed) to lilfe. All forms of life, plants, animals and microorganisms depend on water for their survival.

3. Water covers more than two-thirds of the Earth’s surface. Most of the Earth’s water is found in oceans and very little water is present as fresh water in rivers and lakes.

4. Pure water is colourless, odourless and tasteless.

The changing states of water

1. Water exists on earth in three states depending on the temperature of its surrounding, depending on how hot or cold it is:

1. Solid

    Ice (when water freezes, ice is formed)

    Snow

    Icebergs (large mass of ice broken of from a glacier and floating in the sea)

    Hail

    Frost

2. Liquid

    As drinking water

    Water in ponds, river

    Rain

3. Gas

    Steam (when water boils, steam is produced)

    Water vapour

2. Temperature is a measure of the degree of hotness or coldness of an object. It is measured in degree Celsius.

3. Water reaches its maximum density at 4^oC; ie the same mass of water will have the smaller volume at this temperature. This is known as water anomaly.

4. What is density?

   Mass

Density = ----------   (Mass divided by volume)

                                      Volume

THE PROCESSES OF WATER

1. When water gain or loses heat (temperature changes), it changes its state.

2. There are five processes of water:
• Freezing
• Melting
• Boiling
• Evaporation
• Condensation

Note:

Þ   Pure substances are substances without any impurities added to it

Þ   Impurities are something that is not of the same substance. When a substance is mixes with an impurity, it is known as a mixture.

FREEZING

1. When water loses heat to its surroundings, its temperature drops.

2. When the temperature of water falls to 0^oC (or when water is cooled to 0^oC), liquid water changes to ice. This process is called freezing.

3. Freezing is a heat loss process.

4. When water changes from liquid to solid state, we say that it freezes.

5. The temperature at which a liquid freezes is called the freezing point. The freezing point of pure water is 0^oC. 

6. Freezing point = Melting point (only for all pure substances)

7. When water freezes, its volume increases, this makes ice less dense than water, causing ice to float.

8. Freezing point differs from one substance to another. Also, if there are impurities in the substance, the freezing point will be lower.
1. E.g. if sugar is added to water and the liquid is cooled, the freezing point of the ‘impure water’ is lowered to about -5^o C.

MELTING

1. When ice gains heat from its surroundings, it changes to the liquid state.

2. Melting is the process when a solid changes into a liquid.

3. The melting point of ice is 0^oC.

4. The melting point of pure substances is fixed.

5. The melting point of all substances is different from one another.

6. Melting points change when a substance is mixed with impurities.

7. The melting point will be lower if there are impurities in them, the greater the amount of impurities, the lower the melting point will be.

8. Hence melting points are very useful in determining the purity of a sample. Impure sample have lower melting points than pure solids.

BOILING

1. When we heat water to a temperature of 100^o C, many air bubbles form in the water. These bubbles contain water in the gaseous state, which will escape from the liquid water into the air around it.

2. At 100^o C, water changes from a liquid to a gas. This process is known as boiling.

3. The temperature at which a liquid boils is known as its boiling point.

4. The boiling point of pure water is 100^o C.

5. Under normal conditions, the highest temperature that pure water can reach is 100^o C. Any additional heat gained by the water after that is used to convert it into its gaseous state (steam). It’s temperature does not increase further.

6. The boiling point of a pure substance is fixed. However, boiling points differ from one substance to another.

7. Boiling points are changed when there are impurities in the substance.

8. The boiling point will be higher if there are impurities in them. The greater the amount of impurities, the higher the boiling point.

9. Air pressure also affects the boiling point.

1. The higher the pressure, the higher the boiling point. Water can boil at 120^o C to 130^o C in a pressure cooker.
2. The lower the pressure, the lower the boiling point. E.g. at the top of a very high mountain, water can boil at a temperature as low as 90^o C because of very low air pressure at high heights.

EVAPORATION

1. Evaporation is a process by which a liquid changes into gas.

2. Evaporation takes place all the time and at any temperature, even at very cold places like the North and South Pole.

3. Evaporation takes place when water changes to a gas below its boiling point.

4. During evaporation, a liquid absorbs heat from the surrounding. Hence, evaporation has a cooling effect. For example, our skin feels cooler when our sweat evaporated.

5. How slowly or quickly evaporation takes place is known as the rate of evaporation.

1. The rate of evaporation is HIGH if evaporation takes place quickly (ie things dry up faster)
2. The rate of evaporation is LOW if evaporation takes place slowly.

6. The rate of evaporation refers to how fast liquid water changes to water vapour.

7. The rate of evaporation depends on the following factors:

1. Temperature:

                                               i.     The higher the temperature, the faster the rate of evaporation.

                                             ii.     An increase in temperature causes water to evaporate faster.

                                            iii.     E.g. clothes dry very fast on a hot and sunny day.

2. Wind:

                                               i.     The stronger the wind, the faster the rate of evaporation.

                                             ii.     On a windy day, clothes dry faster.

3. Humidity:

                                               i.     Water evaporates faster when the air is less humid.

                                             ii.     Humidity is the amount of water vapour in the air.

Þ   High humidity means that there is a lot of water vapour in the air causing the air to be moist.

Þ   Low humidity means that there is less water vapour in the air, causing the air to be dry.

Þ   When the amount of water vapour in the air increases, humidity also increases.

Þ   When the amount of water vapour in the air decreases, the humidity also decreases.

                                            iii.     Singapore is very humid so our perspiration does not evaporate very quickly.

4. Exposed surface area:

                                               i.     The larger the exposed surface area, the faster the rate of evaporation.

                                             ii.     E.g. water in a large plate will evaporate faster than water in a small cup.

Evaporation versus Boiling

Characteristics	Evaporation	Boiling
When does it occur?	Occurs all the time and at any temperature.	Occurs only at a liquid’s boiling point.
Where does it occur?	Takes place ONLY at the liquid’s surface	Boiling can take place throughout the liquid.
Speed	Process of evaporation is very slow	Boiling happens very fast.

CONDENSATION

1. When water vapour or steam are cooled and loses heat, the gas becomes a liquid.

2. The change from the gaseous state to the liquid state is called condensation.

3. Water vapour condenses into liquid water whenever it touches a surface that is relatively cooler.

4. Examples of condensation in daily lives:

1. When a glass of ice water is left at room temperature, the water vapour in the surrounding air touches the cold outer surface of the glass and condenses into water droplets.
2. When you place a metal spoon over a kettle of boiling water, droplets of water can be seen on the spoon.
3. Dew form on the leaves of plants in the morning because condensation of water vapour into water droplets when they touch the cooler leaf surfaces.
4. Daddy’s spectacles turn misty when he steps out of a air conditioned car because water vapour from the surrounding condenses on the cold surface of the lens.
5. Sometimes, when the weather is very cold, the water vapour in the air condenses to form millions of tiny water droplets. The air becomes misty and unclear. This phenomenon is called a fog.

                                               i.     Fog: condensed water vapour in the air at or near ground level.