Samacheer Class 8 Science - Water

The gas which is evolved at cathode: Hydrogen (H 2 ).
The gas which is evolved at anode: Oxygen (O 2 )
The ratio of H 2 and O 2 = 2 : 1.

Dissolved oxygen in water is crucial for the survival of aquatic life. Fish extract dissolved oxygen from water through their gills and use it for respiration. Without sufficient dissolved oxygen, fish and other aquatic organisms cannot survive in water. Dissolved carbon dioxide in water is equally important as it is utilized by aquatic plants during photosynthesis to produce food and oxygen. Additionally, carbon dioxide dissolved in water reacts with limestone and other carbonate rocks to form calcium bicarbonate, which is soluble in water. Marine organisms such as snails, oysters, and corals extract calcium carbonate from calcium bicarbonate present in water to build and maintain their shells and skeletal structures. The balance of these dissolved gases is therefore essential for maintaining healthy aquatic ecosystems and supporting diverse forms of aquatic life.

Temporary hardness of water is caused by the presence of soluble carbonate and bicarbonate salts of calcium and magnesium dissolved in water. These salts are formed when water passes through limestone and chalk deposits. Temporary hardness can be easily removed by boiling the water, as the heat causes the bicarbonates to decompose and form insoluble carbonates that settle out. Permanent hardness of water is caused by the presence of soluble chloride and sulphate salts of calcium and magnesium. These salts remain dissolved in water even after boiling and cannot be removed by simple heating. Permanent hardness requires other methods for removal such as adding washing soda, which converts the chlorides and sulphates into insoluble carbonates, or using distillation and ion-exchange processes.

The specific latent heat of vaporization of water is defined as the amount of heat energy required to convert one unit mass of water from its liquid state to gaseous state (steam) at its boiling point without any change in temperature. When water is heated to 100°C at normal atmospheric pressure, it begins to change into steam. During this phase change, the temperature of water remains constant at 100°C even though heat energy is continuously supplied. This supplied heat energy is not used to increase the temperature but is instead stored within the steam molecules to overcome the intermolecular forces of attraction and allow them to escape as a gas. This stored energy is called the latent heat of vaporization of steam. The latent heat of vaporization is much larger than the latent heat of fusion because more energy is needed to completely separate molecules from the liquid state to the gaseous state compared to the energy needed to change from solid to liquid state.

There are several methods to remove hardness from water depending on the type of hardness present. Boiling is an effective method for removing temporary hardness from water. When water is boiled, the bicarbonate salts of calcium and magnesium decompose to form insoluble carbonates, which settle down and can be separated by filtration. Adding washing soda is used to remove permanent hardness of water. Washing soda reacts with the chloride and sulphate salts of calcium and magnesium present in hard water and converts them into insoluble carbonates. These insoluble carbonates are then removed by filtration, leaving behind soft water. Ion-exchange is another modern method that uses ion-exchange resins to replace the calcium and magnesium ions with sodium or hydrogen ions, thereby converting hard water into soft water. Distillation is the most effective method for removing both temporary and permanent hardness from water. In this process, water is boiled to produce steam, which is then condensed back into liquid form. The water obtained after distillation is called distilled water and is the purest form of water, as all dissolved salts and minerals are left behind during the distillation process.

In conventional water treatment plant, water is subjected to different process. They are:
Sedimentation:
Water from lakes or rivers is collected in large sedimentation tanks.
There, it is allowed to stand undisturbed so that suspended impurities settle down at the bottom of the tank.
Sometimes, a chemical substance such as potash alum is added to water to speed up the process of sedimentation, this is called loading.
The particles of potash alum combine with the suspended impurities and make them settle down at a faster rate.
Filtration:
Water from the sedimentation tanks is then pumped to the filtration tanks.
Filtration tanks contain filter beds made up of gravel, sand, pebbles, activated charcoal and concrete.
Water passes through these layers and becomes free from any remaining dissolved or suspended impurities completely.sterilisation:
The filtered water is treated chemically to remove the remaining germs or bacteria, this process is called sterilisation.
The chemicals that are used in this process are chlorine and ozone.
The process of adding chlorine in adequate amounts to water is called chlorination.
The water from filtration tanks is pumped into chlorination tanks, where chlorine is added to remove harmful bacteria and other germs.
Ozonisation is a process in which water is treated with ozone gas to kill the germs present in it.
The sterilisation of water can also be done by exposing it to air and sunlight.
Oxygen from the air and sunlight destroy the germs present in water.
Aeration is the process in which air under pressure is blown into filtered water, this also helps to kill the germs.

Permanent hardness of water is the hardness caused by the presence of soluble chloride and sulphate salts of calcium and magnesium dissolved in water. Unlike temporary hardness, permanent hardness cannot be removed by simple boiling and requires specific chemical or physical treatment methods. Permanent hardness can be removed by several methods. The first method is adding washing soda, which is sodium carbonate. When washing soda is added to hard water, it reacts with the calcium and magnesium chlorides and sulphates to form insoluble calcium carbonate and magnesium carbonate. These insoluble carbonates settle down and are removed by filtration, leaving behind soft water that is suitable for washing and other purposes. The second method is distillation, which is a physical process that can remove both temporary and permanent hardness from water. In distillation, water is heated to boiling point to produce steam, which is then condensed back into liquid form in a separate container. Since the dissolved salts and minerals do not vaporize along with water, they are left behind in the original container. The water obtained after distillation is called distilled water and is the purest form of water, containing no dissolved salts or minerals. Distilled water is suitable for use in laboratories, batteries, and other applications where pure water is required.

The process of breaking down of water molecules by the passage of electric current is known as electrolysis of water.
Electrolysis of Water:
A glass beaker is fixed with two carbon electrodes and it is filled with water up to one third of its volume.
The positive carbon electrode acts as anode and the negative carbon electrode acts as cathode.
Two test tubes are placed on the electrodes.
The electrodes are connected to a battery’ and current is passed until the test tubes are filled with a particular gas.
If the gas collected is tested using a burning splint we can notice that the gas in cathode side bums with a popping sound when the burning splint is brought near the mouth of the test tube.
This property is usually shown by hydrogen gas and so it is confirmed that the gas inside the test tube is hydrogen.
The burning splint placed near the anode side bums more brightly confirming that it is oxygen gas. This experiment shows that water is made up of hydrogen and oxygen.
The ratio of hydrogen and oxygen is 2:1. Hence, for every two volumes of hydrogen collected at the cathode, there is one volume of oxygen collected at the anode.

1. Domestic Sewage.
Untreated sewage contains impurities such as organic matter from food waste, toxic chemicals from household products and it may also contain disease-causing microbes.
2. Domestic waste and plastics.
Plastics block drains spreading vector borne diseases such as malaria and dengue. Waste in water bodies negatively impact aquatic life.
3. Agricultural activities
Fertilizers, pesticides and insecticides used in agriculture can dissolve in rainwater and flow into water bodies such as rivers and lakes.
This causes an excess of nutrients such as nitrates and phosphates as well as toxic chemicals into the water bodies and they can be harmful to aquatic life.
4. Industrial waste.
Many industries release toxic waste such as lead, mercury, cyanides, cadmium, etc.
If this waste is unregulated and is released into water bodies, it negatively impacts humans, plants, animals and aquatic life.
5. Oil spills.
Oil spills cause water pollution which is harmful to aquatic life.
6. Thermal pollution.
Water used for cooling purposes is discharged back to a river or to original water source at a raised temperature and sometimes with chemicals. This rise in temperature decreases the amount of oxygen dissolved in water which adversely affects the aquatic life.