CHEMISTRY DIGITAL TEXT CLASS:8 CREATED BY SANIYA S.M REG NO:18221980010 PHYSICAL SCIENCE

WATER

CONTENTS

1. Water 2. Boiling point of water 3. Heat capacity of water 4. Anomalous expansion of water 5. Freezing of water 6. Surface tension of water 7. Let us assess

WATER Water is one of the natural resources, which are found in an adequate amount. It is an essential source for the existence of life on the planet earth. It is widely used for various purposes such as drinking, washing, bathing, cleaning, cooking, irrigation, and other industrial and domestic uses. Sources of Water There are various sources of water. About 97% of the water on the Earth’s surface is covered with water. The three main sources of water are: • Rainwater. • Groundwater – This includes water bodies like Wells and Springs. • Surface water – This includes different water bodies like Reservoirs, Rivers, Streams, Ponds, Lakes and Tanks.

Boiling point of water The temperature at which liquid boils at normal atmospheric pressure is called its Boiling Point. Under this condition, addition of heat results in the transformation of the liquid into its vapour without raising the temperature. At any temperature a liquid partly vaporizes into the space above it until the pressure exerted by the vapour reaches a characteristic value called the vapour pressure of the liquid at that temperature. As the temperature is increased, the vapour pressure increases

• At sea level, water boils at 100° C (212° F). At higher altitudes the temperature of the boiling point is lower.

Heat Capacity of Water Specific Heat Capacity is the heat required to raise temperature of the unit mass of a given substance by a given amount. APPLICATIONS • Water as a cooling agent acts excellent as a cooling agent in engines. • Water is used as coolant car radiators. Due to its high specific heat capacity, it can absorb a large amount of heat energy from the engine of the car, but its temperature does not rise too high. • Land has lower heat capacity than sea water. Therefore, in day time, the temperature of the land increases faster than the sea. • Handles of cooking utensils are made of substances with high specific heat capacities so that its temperature won’t become too high even if it absorbs large amount of heat.

Anomalous Expansion of water The anomalous expansion of water is an abnormal property of water whereby it expands instead of contracting when the temperature goes from 4 degree centigrade to zero degree centigrade, and it becomes less dense. • The density is maximum at 4 degree centigrade and decreases below that temperature APPLICATIONS • Aquatic Life: This unique property of water helps aquatic life to survive. With a further drop in temperature, water on top forms ice and becomes a bad conductor of heat. This stops the heat from escaping the water body and helps aquatic life to survive. Hence, aquatic life can survive even when the temperature reaches or falls below. • Bottle Burst: If you put a fully-filled water bottle in the refrigerator and its temperature is below 4 degrees Celsius, then according to the anomalous behavior of water, the water inside the bottle will expand. Therefore, due to no space in the bottle for the expansion of water molecules, water will exert force on the walls of the bottle and it will burst.

FREEZING POINT OF WATER The freezing point of water is the temperature at which water changes from a liquid to a solid. At standard atmospheric pressure, water freezes at a temperature of 0 degrees Celsius (32 degrees Fahrenheit).

SURFACE TENSION OF WATER Surface tension, property of a liquid surface displayed by its acting as if it were a stretched elastic membrane. This phenomenon can be observed in the nearly spherical shape of small drops of liquids and of soap bubbles. Because of this property, certain insects can stand on the surface of water.

LET US ASSESS 1.When water is heated at its boiling point or melting point, its temperature does not change. a. What is meant by boiling point and melting point? b. What are the boiling and freezing points of water? c. Why is there no change in temperature? 2. Heat equal quantity of water and coconut oil in two vessels. a. The temperature of which liquid will be raised rapidly? b. Which has more heat capacity? 3. When water is frozen in glass bottles, it is advised not to fill the bottles completely. Explain the reason.