Water Online Test 8th Science Lesson 18 Questions in English

Explanation

Three fourths of our planet earth is filled with water. Water exists in three states namely solid, liquid and gas.

Explanation

Water on the surface of the earth is found mainly in oceans (97.25%), polar ice caps and glaciers (2.05%) and the remaining is in lakes, rivers and aquifers – ground water.

Explanation

Even our body is made up of 65% of water but it is not apparent. Water is a chemically stable compound. Its chemical name is di hydrogen monoxide (H2O). It can be broken up into hydrogen (H2) and oxygen (O2) when an electrical current is passed through it.

Explanation

The process of breaking down of water molecules by the passage of electric current is known as electrolysis of water.

Explanation

Electrolysis of water can be easily demonstrated with the help of an experiment. In this experimental set up, 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.

Explanation

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 burns 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.

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The burning splint placed near the anode side burns more brightly confirming that it is oxygen gas.

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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.

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Water was first prepared in 1781 by an English scientist Henry Cavendish. He discovered hydrogen gas when active metals reacted with Sulphuric acid. The hydrogen gas released was highly inflammable and burnt to form a colorless product called water.

Explanation

Water is also produced by the reduction of metal oxide by hydrogen, burning of hydrogen in air and burning of hydrocarbons in air. Respiration of plants and animals also releases water.

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Henry Cavendish was a British philosopher, scientist, chemist, and physicist. Cavendish is noted for his discovery of hydrogen. He called it inflammable air. He mixed metals with strong acids and created hydrogen. He created carbon dioxide also by combining metals with strong bases.

Explanation

The apparatus used for the preparation of water in the laboratories. In this method, pure hydrogen gas is passed through anhydrous calcium chloride to absorb water vapor, if present. Dry hydrogen coming out of the opening is burnt with sufficient supply of air. The burnt hydrogen gas forms droplets of water, when it comes in contact with the cold flask. Distilled water without any dissolved matter is obtained by this method.

Explanation

Nature Pure water is a clear and transparent liquid. It is colorless, odorless and tasteless.

Explanation

The boiling point of water is 100°C at atmospheric pressure. At this temperature, water boils and changes to steam.

Explanation

The boiling point of water increases with increase in pressure. For example, when a pressure cooker is heated, a high pressure is built inside it. The high pressure increases the boiling point of water. Thus water remains a liquid at a higher temperature (> 100°C) in the cooker. This cooks the food faster.

Explanation

Pure water has the following physical properties.

• Pure water boils at 100° C at one atmospheric pressure.
• Pure water freezes at exactly 0°C at one atmospheric pressure.
• Pure water has a density of 1 gm/cm³

Explanation

Water freezes at 0°C and forms ice. Thus, the freezing point of water is 0°C. The freezing point of water decreases with increase in pressure.

Explanation

When the skaters move on ice, they exert pressure on it. This pressure lowers the freezing point. As a result, the ice melts underneath the skate and allows the skaters to glide across the ice with little effort. When the skaters move forward pressure is decreased and the water re-freezes to ice again.

Explanation

When ice cubes are put in a glass of water at room temperature, they float on the surface of the water. This is because ice is lighter than water. It means that the density of ice is lower than that of water.

Explanation

When the winter temperature is below 0°C, the water in the lake will start freezing. The frozen ice will float at the top and cover the lake. Since ice is a bad conductor of heat it does not allow heat to pass through it. So, the water below the ice remains in liquid form, where most of the aquatic life lives. This enables the aquatic animals and plants to survive even in extreme cold conditions.

Explanation

Density of water at different temperature

Explanation

For the same mass of ice and of water, the volume of ice is more than that of water. It is an unusual physical property of water. In the Himalayas the temperature can go down even below 0°C. The water in the water pipes will freeze at this temperature to ice. If the pipes are not strong they can crack, develop leaks or even burst. This is because freezing of water will cause an expansion in the volume.

Explanation

Latent heat of fusion of ice: If ice cubes are heated in a beaker in which a thermometer is placed, the thermometer does not register any rise in temperature till all the ice melts. The question arises where the heat energy goes if there is no rise in temperature. The heat energy is utilized in changing the state of ice from solid to liquid.

Explanation

The amount of heat energy required by ice to change into water is called latent heat of fusion of ice. Ice has the highest latent heat of fusion, i.e., 80 calories/g. or 336 J/g.

Explanation

The freshness of fish and meat can be maintained by placing them in contact with ice. With its larger latent heat, ice is able to absorb a large quantity of heat from the fish as it melts. Thus, food can be kept at a low temperature for an extended period of time.

Explanation

Latent heat of vaporization of water

When water attains the temperature of 100°C, it starts changing its state from liquid to gaseous state. However, the temperature of water does not rise above 100°C. It is because the supplied heat energy only changes the state of the boiling water. This heat energy is stored in steam and is commonly called latent heat of vaporization of steam. The steam has the highest latent heat of vaporization and its value is 540 calories/g or 2268 J/g.

Explanation

Specific heat capacity: The amount of heat that is needed to raise the temperature of a unit mass of a substance by 1°C is called specific heat capacity of that substance. The specific heat capacity of water is very high. One gram of water requires 1 calorie of heat to raise its temperature by 1°C. Due to its high specific heat capacity, water takes time to become hot as well as to cool down. Thus, water can absorb a lot of heat and retain it for a longer time.

Explanation

The property of specific heat capacity of water is used to cool engines. Water is circulated around car engine using the radiator pump and the heat is absorbed. Thus the engine is protected from getting too hot.

Explanation

Pure water is neutral and it shows no action towards litmus paper.

Explanation

Water is a very stable compound. It does not decompose into elements, when heated to ordinary temperatures. However, if it is heated to 2000°C, 0.02% of water decomposes to form hydrogen and oxygen gas.

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Water acts as a catalyst in a number of reactions. Perfectly dry hydrogen and chlorine gases do not react in the presence of sunlight. However in the presence of traces of water, the reaction takes place with explosion to produce hydrogen chloride.

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Water reacts with some metals. Metals such as sodium, potassium and calcium react vigorously with water at room temperature.

Explanation

Sodium reacts with water to form hydrogen gas and sodium hydroxide solution. Due to the heat evolved in this reaction the hydrogen (gas) catches fire and burns.

2Na + 2H₂O 2NaOH + H₂

Explanation

Magnesium is little more sluggish. It reacts with hot water and gives hydrogen and magnesium hydroxide solution.

2Mg + 2H₂O Mg(OH)₂ + H₂

Explanation

Many other metals react with water to form oxides and hydroxides. Iron is one such metal which forms iron oxide, called rust. Iron is used in many buildings, factories, bridges, ships and vehicles. The slow and gradual rusting of iron is called corrosion.

Explanation

Copper does not react with water at any temperature. That is why it is used for making pipes and boilers.

Explanation

Chlorine gas dissolves in water and produces hydrochloric acid.

2Cl₂ + 2H₂O Sunlight 4HCl + O₂

Explanation

A solvent is a substance which dissolves other molecules and compounds. For example, in a salt solution, water is the solvent and salt is the solute.

Explanation

Water has a unique property to dissolve more substances than any other liquids. It can dissolve solids such as salt and sugar, liquids such as honey and milk and gases such as oxygen and carbon dioxide in it. Water can dissolve more number of substances than any other solvent. Therefore, it is called as universal solvent.

Explanation

You can see a number of concentric rings of solid matter deposited on the watch glass. These are the dissolved solids left behind after the evaporation of water. Salts, minerals and impurities are the solids dissolved in water.

Explanation

Dissolved salts are important for the following reasons.

• They are essential for the growth and development of plants.
• They add taste to water.
• They supply the essential minerals needed for our bodies.
• Most of the chemical reactions important for our living take place in the cells of our body with the help of water.

Explanation

Tap water, river water and well water contain dissolved solids but rainwater and distilled water does not contain dissolved solids. Hence concentric rings are not formed in the rain water and distilled water after evaporation.

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Apart from solids and minerals, air is also dissolved in water. Air is present in dissolved state in all natural sources of water.

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The solubility of oxygen in water is higher than the solubility of nitrogen. Air dissolved in water contains approximately 35.6% oxygen along with nitrogen and carbon dioxide.

Explanation

Air being dissolved in water is important for the following reasons.

• Air dissolved in water is important for the living organisms to survive.
• Fish extracts the oxygen from the water and expels water through the gills. Fish can survive in water only through the dissolved oxygen present in water.
• Aquatic plants make use of dissolved carbon dioxide for photosynthesis.

Explanation

Carbon dioxide dissolved in water reacts with limestone to form calcium bicarbonate. Marine organisms such as snails, oysters, etc., extract calcium carbonate from calcium bicarbonate to build their shells.

Explanation

The salinity of water is more in the Dead Sea. It is actually a salt lake as it has a single source of water and is not connected to the ocean.  It is landlocked and this causes the water to evaporate. This has led to a steady increase in its degree of salinity. Now the salinity is so high such that the marine life cannot survive in it. This is why it is called the Dead Sea.

Explanation

Saline water contains salt commonly known as sodium chloride (NaCl). Such water is called saline water. It is not suitable for drinking and is said to be non-potable water.

Explanation

The water suitable for drinking is called potable water. Every liter of potable water contains 1- 2 grams of dissolved salts, mainly common salt.

Explanation

In addition to the common salt, there are small amounts of calcium (Ca), magnesium (Mg), potassium (K), copper (Cu) and zinc (Zn). The minerals in water give it a certain taste. In addition, these minerals are useful for our body’s metabolism. Potable water also contains dissolved air.

Explanation

The following are the characteristics of potable water.

• Potable water should be colorless and odorless.
• It should be transparent.
• It should be free from harmful micro-organisms such as bacteria, virus and protozoa.
• It should be free from suspended impurities.
• It should contain some minerals and salts, necessary for our body and some dissolved gases to add taste.

Explanation

Out of the total fresh water available on the earth only 1% is present in water bodies such as rivers and lakes and the rest is frozen in glaciers and polar-regions.

Explanation

Water from these water bodies is unfit for drinking, cooking, washing or bathing because it contains suspended and dissolved impurities. It also contains micro-organisms such as bacteria. If this water is consumed without purifying, it can cause water-borne diseases such as typhoid and cholera. Therefore, before water reaches our homes, it is treated and purified to make it potable. In conventional water treatment plants water is subjected to different processes for purification.

Explanation

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 process is called loading. The particles of potash alum combine with the suspended impurities and make them settle down at a faster rate.

Explanation

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.

Explanation

The filtered water is treated chemically to remove the remaining germs or bacteria. This process is called sterilization. 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.

Explanation

Ozonization is a process in which water is treated with ozone gas to kill the germs present in it.

Explanation

The sterilization 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.

Explanation

Aeration is the process in which air under pressure is blown into filtered water. This also helps to kill the germs.

Explanation

RO purifiers are the purifiers that can remove the dissolved impurities and germs. They also improve the taste of water. RO stands for the name of the technology, reverse osmosis, used in these purifiers. Some RO purifiers also have a UV (ultraviolet) unit that destroys the germs present in water.

Explanation

We use soaps and detergents to wash clothes. They form lather with water that quickens the process of removal of dirt from the clothes. Water contains a number of dissolved salts and minerals. When these salts are present in very small quantities in water it is called soft water. In this water, soaps or detergents form lather easily.

Explanation

Sometimes, minerals and salts are present in water in such a large quantity that soaps or detergents form a thick precipitate called scum instead of forming lather. This makes the removal of further dirt difficult. Such water is called hard water.

Explanation

Hardness of water is due to the presence of dissolved salts of calcium and magnesium. Hardness may be temporary or permanent.

Explanation

Temporary hardness is due to the presence of carbonate and bicarbonate salts of calcium and magnesium and permanent hardness results due to the presence of chloride and sulphate salts of calcium and magnesium.

Explanation

Disadvantages of Hard water

• It is not good for washing clothes. It forms scum with soap and detergents, which makes the soap ineffective and also spoils the clothes further.
• It damages the utensils and containers in which it is stored and forms a hard layer.
• It forms scales on the machine parts used in industries and decreases their efficiency.
• It results in stomach ailments if consumed for a long period.

Explanation

Boiling: Temporary hardness is easily removed from water by boiling. When heated, the calcium hydrogen carbonate decomposes producing insoluble calcium carbonate. The insoluble carbonates are then filtered and removed from water. This makes the hard water soft and fit for use.

Explanation

Adding washing soda: Washing soda is used to remove permanent hardness of water. Adding washing soda converts chlorides and sulphates into insoluble carbonates. These insoluble carbonates are removed by filtration.

Explanation

Ion – exchange: Another method used to remove the hardness of water is to pass it through a column of ion-exchange resins where calcium and magnesium ions get replaced by sodium ions. This converts hard water into soft water.

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Temporary and permanent hardness both can be removed by the method of distillation. The water obtained after distillation is called distilled water. It is the purest form of water.

Explanation

Distilled water and boiled water have no taste. The pleasant taste of drinking water is due to the presence of dissolved substances which include air, carbon dioxide and minerals.

Explanation

Fresh water resources are the sources of water that are useful to society for domestic, agricultural or industrial uses. These include surface and ground water. Examples of surface water include rivers, reservoirs, Eris and tanks.

Explanation

There are 17 major river basins in Tamil Nadu with 61 reservoirs and approximately 41,948 tanks. Eris and tanks are traditionally used in Tamil Nadu to collect rainfall during the monsoon which can be used throughout the year.

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Groundwater sources are called aquifers. Aquifers are layers below the ground made of coarse and gravel that contain spaces allowing rainwater collection. The use of groundwater is possible through open wells and bore wells.

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Shampoo, face wash, shower gel and toothpaste have small round pieces of plastic added to them. These are called micro beads. They are added for different reasons like scrub and clean your skin, polish your teeth etc. When we use products with micro beads, they go down our drain and pollute water bodies. Fish and other animals eat them by accident and get sick.

Explanation

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 in water bodies which is called Eutrophication and they can also be harmful to aquatic life.

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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.

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Oil spills cause water pollution which is harmful to aquatic life. The oil which remains floating on the water surface blocks sunshine, reduces the oxygen dissolved in water and suffocates marine organisms.

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Large amounts of water are used for cooling purposes in thermal and nuclear power plants and many industries. 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.

Explanation

Pollutants are generally classified as domestic pollutants, agricultural pollutants and industrial pollutants.

Explanation

Micro-plastics can be found in almost every freshwater source. They have been found from the freezing waters of the Arctic and Antarctic to the bottom of the deep-sea floor up to 5,000 meters deep. Micro plastics have been found in bottled water and tap water around the world.

Explanation

The largest source of water pollution in India is untreated sewage. On an average, a person uses 180 liters of water per day for washing clothes, cooking, bathing, etc.

Explanation

Some simple ideas to avoid water pollution are given below:

• Use detergents that are biodegradable and avoid those that contain toxic chemicals.
• Wear clothing that is made from natural fibres such as cotton and avoid wearing synthetic fibres such as nylon.
• Domestic waste water should be treated properly, and all harmful substances should be removed from it, so it can be reused for flushing toilets and gardening.
• Use bio-pesticides (natural pest control) instead of chemical pest control.
• Use compost made from cow dung, garden waste and kitchen waste as a fertilizer.
• Water released from industries should be treated before being discharged.

Explanation

Do not throw waste such as plastics into water bodies. Always separate your waste into recyclable, non-recyclable and biodegradable so that it does not cause pollution.