© 2009 Marshall Cavendish International (Singapore) Private Limited

Questions to think about… What are the three states of water? How does one state of water change into another? What affects the rate of evaporation? © 2009 Marshall Cavendish International (Singapore) Private Limited

The three states of water Water exists in three states – solids, liquids or gases. When water gains or loses heat, it changes from one state to another. When water gains heat, it becomes hotter and when it loses heat, it becomes colder. How hot or cold water is, is called its temperature. © 2009 Marshall Cavendish International (Singapore) Private Limited

Temperature The unit of measurement for temperature is degrees Celsius or o C. The thermometer is an instrument for measuring temperature. a laboratory thermometer The level of liquid in the thermometer rises or falls as the temperature changes. To read the temperature on a thermometer, look at the level of liquid and read the marking nearest to it. © 2009 Marshall Cavendish International (Singapore) Private Limited

Temperature What is the temperature shown on this lab thermometer? © 2009 Marshall Cavendish International (Singapore) Private Limited

Changing states of water Freezing o Ice is the solid state of water. It has a definite shape and volume. o Water changes into ice when it is placed in a cold place such as a freezer. o In the freezer, water loses heat and its temperature slowly drops to 0 o C. o At this temperature, water changes from a liquid to solid. o This change of state is called freezing. o Thus, 0 o C is the freezing point of water. © 2009 Marshall Cavendish International (Singapore) Private Limited

Changing states of water Freezing o At 0 o C, water freezes to become ice. water ice loses heat © 2009 Marshall Cavendish International (Singapore) Private Limited

Changing states of water Melting o When ice is just taken out of the freezer, the temperature of ice is lower than 0 o C. o When the ice is placed in a warmer place, it begins to take in heat from the surroundings. o The temperature of the ice rises slowly to 0 o C. o At this temperature, ice starts to change from a solid into a liquid. o This is known as melting. © 2009 Marshall Cavendish International (Singapore) Private Limited

Changing states of water Melting o During this process, the ice continues to gain heat as it melts to become water. o However, the temperature of the water remains at 0 o C. o The heat taken in during melting is used to change the solid ice to liquid water. o At 0 o C, ice changes to water. This is called the melting point of ice. © 2009 Marshall Cavendish International (Singapore) Private Limited

Changing states of water Melting o The melting point of ice is 0 o C. ice water gains heat © 2009 Marshall Cavendish International (Singapore) Private Limited

Changing states of water Boiling o When water gains heat, its temperature increases. o When its temperature reaches 100 o C, water starts to boil and change to steam. o The temperature of 100 o C is called the boiling point of water. © 2009 Marshall Cavendish International (Singapore) Private Limited

Changing states of water Boiling o At 100 o C, water boils to become steam. water steam gains heat © 2009 Marshall Cavendish International (Singapore) Private Limited

Changing states of water Evaporation o When water on the clothes’ surface gains heat from the surroundings, it changes into a gas called water vapour. o Like steam, water has no definite shape and volume. o Water vapour is invisible and escapes into the air. o Slowly, all the water changes into water vapour and the clothes become dry. Drying wet clothes… © 2009 Marshall Cavendish International (Singapore) Private Limited

Evaporation o This change of state from a liquid to a gas is called evaporation. o Evaporation occurs all the time and at any temperature. o Even when water is boiling at 100 o C, evaporation is occurring as well. Changing states of water gains heat water water vapour © 2009 Marshall Cavendish International (Singapore) Private Limited

Changing states of matter Condensation Have you ever noticed water droplets on the surface of a glass of cold water? Where do the water droplets come from? © 2009 Marshall Cavendish International (Singapore) Private Limited

Changing states of matter Condensation o The cool surface of the glass cools the air surrounding it. o As a result, the water vapour in the air loses heat and changes to liquid on the surface of the glass. © 2009 Marshall Cavendish International (Singapore) Private Limited

Changing states of matter Condensation o The change of state from a gas to liquid is called condensation. o Like evaporation, this can take place at any temperature. loses heat water steam © 2009 Marshall Cavendish International (Singapore) Private Limited

Changing states of matter Condensation o The spout of a kettle of boiling water seems to be giving out white ‘clouds’. o These are not steam. o Steam is found in the space between the ‘clouds’ and the spout. o You cannot see steam because it is invisible. white ‘cloud’ steam © 2009 Marshall Cavendish International (Singapore) Private Limited

Changing states of matter Condensation o The white ‘clouds’ are actually tiny droplets of water floating in the air. o These are formed when steam loses heat and changes back to water as it cools. o This change of state of steam from a gas to a liquid is also called condensation. white ‘cloud’ steam © 2009 Marshall Cavendish International (Singapore) Private Limited

Changing states of matter Condensation white ‘cloud’ steam loses heat water steam © 2009 Marshall Cavendish International (Singapore) Private Limited

Rate of evaporation The rate of evaporation is how fast water changes to water vapour. Evaporation can slow down or occur faster depending on some factors – temperature, wind, area of exposed surface and humidity. © 2009 Marshall Cavendish International (Singapore) Private Limited

Rate of evaporation Temperature o An increase in temperature causes water on a surface to change to water vapour faster. o The higher the temperature, the higher the rate of evaporation. A higher temperature will cause the water in the puddle to evaporate into water vapour faster. © 2009 Marshall Cavendish International (Singapore) Private Limited

Rate of evaporation Wind o When water evaporates, water vapour gathers above the surface of water. o If wind is present, the water vapour will be carried away, making space for more water vapour to rise. o This increases the rate of evaporation. o The stronger the wind, the higher the rate of evaporation. © 2009 Marshall Cavendish International (Singapore) Private Limited

The stronger the wind, the higher the rate of evaporation, and the faster the fish will dry. © 2009 Marshall Cavendish International (Singapore) Private Limited

Rate of evaporation Area of exposed surface o Water vapour evaporates into the air from the exposed surface of water. o The larger the area of the exposed surface, the greater the amount of water vapour produced. o Hence, the rate of evaporation is higher. © 2009 Marshall Cavendish International (Singapore) Private Limited

The larger the area of exposed surface, the higher the rate of evaporation, and the faster these dyed clothes will dry. © 2009 Marshall Cavendish International (Singapore) Private Limited

Rate of evaporation Humidity o Humidity is the amount of water vapour in the air. o When the air is filled with a lot of water vapour, humidity is high. o This makes it more difficult for water to evaporate. o When air is dry, humidity is low. o Low humidity makes it easier for water to evaporate. o Hence, the rate of evaporation will be higher when the humidity is lower. © 2009 Marshall Cavendish International (Singapore) Private Limited

The rainforest is a very humid place. Thus, water evaporates slowly. © 2009 Marshall Cavendish International (Singapore) Private Limited

The desert is a very dry place. Since humidity is low, water evaporates quickly. © 2009 Marshall Cavendish International (Singapore) Private Limited