Water

The purest form of natural water is :

1. sea water
2. rain water
3. river water
4. spring water

Answer

rain water

Reason — Rainwater is the purest form of natural water. It is formed naturally by evaporation followed by condensation of water vapour in the atmosphere.
Seawater contains a large amount of dissolved salts, minerals and impurities. River water contains mud, microorganisms, dissolved salts, and pollutants. Spring water contains dissolved minerals and salts from rocks.

The ratio of hydrogen and oxygen by mass in a molecule of water is :

1. 1:16
2. 2:8
3. 1:8
4. 8:1

Answer

1:8

Reason — One atom of Hydrogen has mass of 1 amu, while one atom of Oxygen has mass of 16 amu. One molecule of water contains 2 atoms of Hydrogen and 1 atom of Oxygen, so their mass ratio:
1 x 2 : 16 x 1 = 1 : 8

Sea water contains a large amount of:

1. suspended impurities
2. dissolved common salt
3. oily substances
4. none of the above

Answer

dissolved common salt

Reason — Seawater contains a large amount of dissolved common salt.

Water returns on Earth's surface from the atmosphere by the process of:

1. combustion
2. respiration
3. photosynthesis
4. condensation

Answer

condensation

Reason — Condensation is the process by which water vapour in the atmosphere cools and changes back into liquid form, leading to the formation of clouds, dew, or even precipitation like rain, snow, etc. This is how water returns to Earth's surface from the atmosphere.
Combustion is burning of a substance in oxygen, producing heat and light. Respiration is the process of releasing energy from food in living organisms. Photosynthesis is the process by which plants make food using carbon dioxide, water, and sunlight.

Spring water is classified as:

1. surface water
2. above surface water
3. underground water
4. none of these

Answer

underground water

Reason — Spring water comes from underground water that has been naturally filtered and then rises to the surface through a spring.

Name the four main sources of water.

Answer

The four main sources of water are:

1. Ocean and sea water
2. River and lake water
3. Spring and well water
4. Rainwater

What is the importance of water cycle in nature?

Answer

The water cycle is important in nature because:

1. It helps in regulating the climate on the earth.
2. It makes water available in various forms on the earth.

Name the two gases from which water is formed. What is the chemical composition of these two gases in water by mass? Give the molecular formula of water.

Answer

Water is formed from hydrogen and oxygen gases. The chemical composition of hydrogen and oxygen gases is 1:8 ratio by mass in water. The molecular formula of water is H₂O.

Give reasons:

Water is used as a cooling agent.

Answer

Water has a high specific heat, i.e., it needs a large amount of heat to become hot. Thus, water can absorb a lot of heat without much rise in its temperature. Hence, it is used as an excellent cooling agent.

Give reasons:

Water pipes burst in severe winters.

Answer

During severe winters, when the temperature starts falling below 4°C, water in pipelines expands (due to property of anomalous expansion of water) and it exerts large pressure on the pipes causing them to burst.

Give reasons:

It is difficult to cook in hills as compared to plains.

Answer

The boiling point of water decreases with a decrease in pressure. Therefore, water boils at a lower temperature (i.e., less than 100°C) in hilly areas where the atmospheric pressure is lower as compared to the plains. Due to this, it takes a longer time to cook food making cooking difficult in hills.

Give reasons:

Ice floats on water.

Answer

The density of ice is less than the density of water, so ice floats on water.

Give reasons:

Sea water does not freeze at 0°C.

Answer

The presence of impurities in water lowers its freezing point. Sea water contains salt which lowers the freezing point of water. So sea water does not freeze at 0°C.

Give reasons:

Aquatic plants and animals survive in very cold regions.

Answer

Water shows anomalous expansion. As the temperature falls below 4°C, water at the surface becomes lighter and finally freezes at 0°C to form ice, which floats. This ice layer is a poor conductor of heat, so it acts like an insulating blanket and prevents the water below from freezing. Therefore, the lower layers remain liquid and aquatic plants and animals can survive in very cold regions.

Give reasons:

Water is used in room coolers and hot water bags.

Answer

Water needs a large amount of heat to become hot and takes a long time to lose heat, so it neither heats up nor cools down quickly. Hence, it can absorb a lot of heat from its surroundings without becoming very hot, making it an excellent cooling agent. Therefore, water is used in room coolers to absorb heat from air and in hot water bags because it stays warm for a longer time.

What is the effect on boiling point of water when:

(a) pressure is increased ?

(b) impurity is added ?

Answer

(a) When pressure is increased, the boiling point of water increases.

(b) When impurity is added, the boiling point of water increases.

Why is water very precious for all living beings? List four points.

Answer

Water is very precious for all living beings due to the following reasons:

1. Water is the source of life. It is the most essential substance for the existence of life.
2. It is required for everyday uses like drinking, cleaning, bathing, washing etc.
3. It provides a medium for all biochemical reactions inside our bodies to take place.
4. It regulates our body temperature through sweating and evaporation.

How does anomalous expansion of water help the aquatic organisms in cold climates?

Answer

In lakes, as the temperature drops, the surface water cools, sinks and mixes with the deeper water, this process continues until the temperature reaches 4°C. Beyond that point, the surface water expands, becomes lighter and it does not sink. So, further cooling takes place only at the surface while temperature of lower layers of water does not change. The water at the surface freezes to form a layer of ice that insulates the deeper water and allows aquatic organisms to survive in the water beneath the ice.

The gases dissolved in natural water are:

1. oxygen and carbon dioxide
2. hydrogen and oxygen
3. sulphur dioxide and hydrogen
4. chlorine and ammonia

Answer

oxygen and carbon dioxide

Reason — Many gases like oxygen, carbon dioxide and ammonia dissolve in water.

Colloids have the particle size ranging between:

1. 10^-9 to 10^-6 m
2. 10^-12 to 10^-10 m
3. 10^-7 to 10^-5 m
4. 10^-15 to 10^-12 m

Answer

10^-9 to 10^-6 m

Reason — A colloid is a homogeneous looking heterogeneous mixture in which particles of intermediate size are dispersed in a continuous medium. These particles remain suspended throughout the medium and do not settle down. Among the given options, the colloidal particle-size range is 10^-9 to 10^-6 m.

Which of the following salts is hydrated?

1. Common salt
2. Potassium nitrate
3. Blue vitriol
4. Ammonium chloride

Answer

Blue vitriol

Reason — Substances which contain water of crystallisation are called hydrated substances, e.g. Blue vitriol (CuSO₄.5H₂O).
Common salt is an anhydrous salt (does not contain water of crystallisation). Potassium nitrate is also an anhydrous salt. Ammonium chloride is an anhydrous salt.

Which of the following substances is hygroscopic in nature ?

1. silica gel
2. caustic soda
3. iron (III) chloride
4. calcium carbonate

Answer

silica gel

Reason — Certain substances absorb moisture (water vapour) from the atmosphere without dissolving in it. Such substances are called hygroscopic substances. E.g. silica gel.
Caustic soda (NaOH) is actually deliquescent, not just hygroscopic. Iron (III) chloride is also deliquescent, not simply hygroscopic. Calcium carbonate does not absorb moisture from air, so it is neither hygroscopic nor deliquescent.

The products formed when water is subjected to electrolysis are:

1. hydrogen and oxygen
2. hydrogen and sulphur dioxide
3. only hydrogen
4. only oxygen

Answer

hydrogen and oxygen

Reason — When water is subjected to electrolysis, it decomposes into hydrogen and oxygen gases at the cathode and anode respectively.

Classify the following into solutions, colloids and suspensions :

Sugar and water, chalk and water, milk, sand and water, salt and water, soap solution.

Answer

Solutions — Sugar and water, salt and water.

Colloids — milk, soap solution.

Suspensions — chalk and water, sand and water.

Name the gas produced when:

(a) steam is passed over hot coke.

(b) chlorine is dissolved in water and exposed to sunlight.

(c) a piece of calcium is added to water.

(d) when fossil fuel is burnt.

Answer

(a) Water gas [CO + H₂]

(b) Oxygen gas (O₂)

(c) Hydrogen gas (H₂)

(d) Carbon dioxide gas (CO₂)

Define the following terms :

(a) Solution

(b) Solute

(c) Solvent

Answer

(a) A solution is a homogeneous mixture of two or more substances whose composition can be varied.

(b) A solute is a substance that dissolves in other substances. It is in a smaller quantity in the solution.

(c) A solvent is a medium in which a solute dissolves. It is in a larger quantity in a solution.

What is meant by :

(a) Unsaturated

(b) Saturated and

(c) Supersaturated solutions ?

Answer

(a) A solution in which more of the solute can be dissolved at a given temperature is called an unsaturated solution.

(b) A solution that cannot dissolve any more of the solute at a given temperature is called a saturated solution.

(c) A solution that contains more solute than it can normally hold at a given temperature is called a supersaturated solution.

How do the solubility of a solid and a gas get affected by

(a) Increase in temperature ?

(b) Increase in pressure ?

Answer

(a) Solubility of a solid generally increases with an increase in temperature.
Solubility of a gas decreases with an increase in temperature.

(b) Pressure has practically no effect on the solubility of a solid in water.
In the case of gases, the amount of a gas dissolved in water increases with an increase in pressure and decreases with a decrease in pressure.

Define 'water of crystallisation'. Give two examples with formulae.

Answer

The fixed amount of water which is in a loose chemical combination with one molecule of a substance in its crystal form is called water of crystallization.

The two examples are:

Give two examples for each of the following :

(a) Hydrated substances

(b) Crystalline anhydrous substances

(c) Drying agents

(d) Deliquescent substances

(e) Efflorescent substances

(f) Colloids

(g) Solvents other than water.

Answer

(a) Blue vitriol (CuSO₄.5H₂O), Washing soda (Na₂CO₃.10H₂O)

(b) Common salt (NaCl), Potassium nitrate (KNO₃)

(c) Quicklime (CaO), Anhydrous calcium chloride (CaCl₂)

(d) Caustic soda (NaOH), Magnesium chloride (MgCl₂)

(e) Washing soda (Na₂CO₃.10H₂O), Glauber's salt (Na₂SO₄.10H₂O)

(f) Milk, blood

(g) Alcohol, Carbon tetrachloride

What do you observe when:

(a) Blue vitriol is heated?

(b) Washing soda is exposed to air?

(c) Blue litmus solution is added to water?

Answer

(a) When blue vitriol is heated, it loses its water of crystallisation. The blue crystalline solid of copper sulphate turns into white amorphous (powder) solid and drops of colourless liquid (which is water) condense on the upper cooler parts of the test tube.

(b) When washing soda is exposed to air the white crystalline substance changes into white powder.

(c) There is no change in colour of blue litmus solution when added to water as pure water is neutral.

What is metal activity series?

Answer

The reaction of metals with water under different conditions indicates the reactive nature of metals and enables us to arrange the metals on the basis of their reactivity which is called metal activity series.

Give balanced chemical equations for the reaction of water with:

(a) Sodium

(b) Iron

(c) Carbon dioxide

(d) Sodium oxide

Answer

(a) 2Na + 2H₂O ⟶ 2NaOH + H₂ (g)

(b) 3Fe + 4H₂O $\xrightarrow{\text{steam}}$ Fe₃O₄ + 4H₂ (g)

(c) CO₂ + H₂O ⟶ H₂CO₃

(d) Na₂O + H₂O ⟶ 2NaOH

Differentiate between:

(a) Solution and suspension

(b) Suspension and colloid

Answer

(a) Difference between Solution and suspension:

(b) Difference between Suspension and colloid:

Give reason :

Water is called a universal solvent.

Answer

Water is called a universal solvent because it can dissolve a large number of solids, liquids and gases.

Give reason :

Silica gel pouches are kept in unused water bottles.

Answer

Silica gel is a drying agent which absorbs moisture and keeps things dry. So, silica gel pouches are kept in unused water bottles.

Give reason :

Table salt becomes moist during rainy season.

Answer

Table salt contains impurities like magnesium chloride and calcium chloride which are deliquescent. They absorb water and become moist during rainy season.

Give reason :

On opening a bottle of a cold drink, a fizz sound is heard.

Answer

Cold drink bottles contain dissolved carbon dioxide under high pressure. On opening the bottle, the pressure decreases and the gas is released producing a fizz sound.

The salt which gives water vapour on heating is called :

1. Hydrated
2. Hygroscopic
3. Deliquescent
4. Anhydrous

Answer

Hydrated

Reason — Hydrated salts are salts that contain water molecules as part of their crystalline structure. When these salts are heated, they lose their water molecules, which evaporate as water vapour. For example, copper(II) sulphate pentahydrate (CuSO₄·5H₂O) gives off water vapour when heated.
Hygroscopic substances absorb moisture from air, but do not contain water of crystallisation. Deliquescent substances absorb moisture and dissolve in it, but they do not necessarily release water on heating. Anhydrous salts do not contain water of crystallisation, so they do not release water vapour.

Which of the following salts cause permanent hardness of water ?

1. Calcium sulphate
2. Calcium bicarbonate
3. Sodium chloride
4. Magnesium nitrate

Answer

Calcium sulphate

Reason — Calcium sulphate causes permanent hardness of water.
Calcium bicarbonate causes temporary hardness, which can be removed by boiling. Sodium chloride does not cause hardness of water. Magnesium nitrate is soluble, but not commonly responsible for water hardness in basic classification; hardness is mainly due to calcium and magnesium sulphates/chlorides.

The substance which can remove permanent hardness of water is :

1. sodium bicarbonate
2. sodium carbonate
3. sodium hydroxide
4. sodium chloride

Answer

sodium carbonate

Reason — Sodium carbonate can remove permanent hardness of water.
Sodium bicarbonate does not effectively remove permanent hardness of water. Sodium hydroxide is a strong base but not used for removing water hardness in this context. Sodium chloride does not affect hardness of water.

Temporary hardness of water can be removed by

1. filtering
2. boiling
3. loading
4. none of the above

Answer

boiling

Reason — Temporary hardness of water can be removed by simple physical method, i.e. by boiling.
Filtration removes insoluble impurities, not dissolved salts causing hardness. Loading is used in water treatment to speed up sedimentation, not to remove hardness.

Assertion (A): Water is a universal solvent.

Reason (R): We use water daily for drinking, washing, bathing, etc.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

Both A and R are true but R is not the correct explanation of A.

Explanation — A is true as water is often called a "universal solvent" because it can dissolve many different substances, such as salts, sugars, gases, and a variety of other compounds.
R is true as we use water daily for drinking, washing, bathing, etc. However, these are two separate facts.

Therefore, Both A and R are true but R is not the correct explanation of A is the correct option.

Assertion (A): Water exists in all the three states of matter.

Reason (R): On cooling, liquid water changes into ice, the solid state. On heating, it changes into vapour, the gaseous state.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

Both A and R are true and R is the correct explanation of A.

Explanation — A is true as water can exist as a solid (ice), liquid (water), and gas (vapour) under different conditions of temperature and pressure.
R is true as on cooling water becomes ice and on heating it becomes vapour. Water changes between solid, liquid, and gas states depending on temperature.

Therefore, Both A and R are true and R is the correct explanation of A is the correct option.

Assertion (A): Miscible liquids dissolve into each other.

Reason (R): Petrol and water are miscible liquids.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

A is true but R is false.

Explanation — Miscible liquids are those liquids which mix completely with each other to form a single, uniform layer. Hence, they can be said to "dissolve" in each other. So, Assertion (A) is true.
Petrol and water are immiscible; they do not mix and form two separate layers (petrol floats on water). So, Reason (R) is false.

Therefore, A is true but R is false is the correct option.

Assertion (A): A solution is a heterogeneous mixture of two or more substances with variable composition.

Reason (R): The component of a solution that dissolves is called as solute, while the other component in which the solute dissolves is called a solvent.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

A is false but R is true.

Explanation — A solution is a homogeneous mixture, i.e., its composition is uniform throughout. It is not a heterogeneous mixture. So, Assertion (A) is false.
In a solution, the substance which dissolves is called the solute, and the substance in which it dissolves is called the solvent. So, Reason (R) is true.

Therefore, Assertion (A) is false but Reason (R) is true.

Assertion (A): Pure water is tasteless but the water which we drink has taste.

Reason (R): Drinking water contains dissolved minerals and gases.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

Both A and R are true and R is the correct explanation of A.

Explanation — Pure water is tasteless, but the water we drink usually has a slight taste. So, Assertion (A) is true.
Drinking water contains dissolved minerals (salts like calcium and magnesium compounds) and dissolved gases, which impart taste to the water. So, Reason (R) is true.

Therefore, both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).

Assertion (A): Hard water does not form lather with soap easily.

Reason (R): Temporary hardness can be removed by boiling.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

Both A and R are true but R is not the correct explanation of A.

Explanation — Hard water contains dissolved calcium and magnesium salts. These ions react with soap to form an insoluble scum, therefore lather is not produced easily. So, Assertion (A) is true.
Temporary hardness (due to bicarbonates of Ca and Mg) can be removed by boiling. So, Reason (R) is true.
However, Reason (R) is not the correct explanation of Assertion (A) because it tells a method of removing temporary hardness, not the reason for poor lather formation.

Therefore, both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).

Fill in the blanks:

(a) Water has ..................... density and ................. volume at 4°C.

(b) Freezing mixture contains .................. and ................ .

(c) The solubility of a gas in water ................. with rise in temperature and ....................... with rise in pressure.

(d) ................... is the purest form of natural water.

(e) Use of excessive .................. by farmers causes water pollution.

(f) Boiling removes the ................... hardness of water.

(g) Water turns the colour of anhydrous copper sulphate ................... .

(h) The sticky substance formed when soap is added to hard water is ...................... .

Answer

(a) maximum, minimum

(b) salt, ice

(c) decreases, increases

(d) Rain water

(e) fertilizers

(f) temporary

(g) blue

(h) scum

Write true or false for the following statements and correct the false ones.

(a) Hard water forms lather easily with soap.

(b) Permanent hardness is due to bicarbonate salts of calcium and magnesium.

(c) Silica gel is a drying agent.

(d) The reaction of steam with magnesium produces hydrogen gas and magnesium hydroxide.

(e) An example of a colloid is ink.

Answer

(a) False
Correct Statement — Hard water does not form lather easily with soap.

(b) False
Correct Statement — Temporary hardness is due to bicarbonate salts of calcium and magnesium.

(c) True

(d) False
Correct Statement — The reaction of steam with magnesium produces hydrogen gas and magnesium oxide.

Mg + H₂O $\xrightarrow{\text{steam}}$ MgO + H₂

(e) True

Match the statements in column A with that in column B.

Answer

Give one word/words for the following statements:

(a) Water fit for human consumption.

(b) The harmful substances dissolved in water.

(c) The change of states of water from one form to another.

(d) The gaseous form of water found in air.

(e) A mixture of common salt and ice.

(f) A substance which does not contain water in its chemical structure.

(g) A property due to which a substance absorbs water without dissolving.

(h) Water molecules in loose chemical combination with other substances.

Answer

(a) Potable water

(b) Impurities

(c) Water cycle

(d) Water vapour

(e) Freezing mixture

(f) Anhydrous substance

(g) Hygroscopy

(h) Water of crystallisation

Define:

(a) Soft water

(b) Hard water

Answer

(a) Water which forms a lather with soap easily is called soft water.

(b) Water which does not forms a lather with soap easily rather forms a white sticky scum or precipitate is called hard water.

Name three water borne diseases.

Answer

Typhoid, cholera, hepatitis are the three water borne diseases.

Give reasons:

Alcohol is mixed with water and is used in car radiators.

Answer

Alcohol is mixed with water and is used in car radiators to prevent it from freezing in cold weather because the presence of impurities in water lowers the freezing point.

Give reasons:

Carbonated drinks are bottled under high pressure.

Answer

The amount of gas dissolved in water increases with an increase in pressure. That is why carbonated drinks containing carbon dioxide are bottled under high pressure as they contain a large amount of gas dissolved in them.

Name the compounds responsible for:

(i) temporary hardness of water

(ii) permanent hardness of water.

Answer

(i) Bicarbonates of calcium and magnesium

(ii) Sulphates and chlorides of calcium and magnesium

Suggest one method along with the reactions for the removal of

(i) temporary hardness of water

(ii) permanent hardness of water

Answer

(i) Boiling helps to remove the temporary hardness of water. When temporary hard water is boiled, the bicarbonates of calcium and magnesium break up to form their respective insoluble carbonates. These can be filtered out and the water becomes soft.

Ca(HCO₃)₂ $\xrightarrow{\text{boiled}}$ CaCO₃ + H₂O + CO₂ (g)

Mg(HCO₃)₂ $\xrightarrow{\text{boiled}}$ MgCO₃ + H₂O + CO₂ (g)

(ii) The permanent hardness of water is removed when it is treated with a small quantity of sodium carbonate. It reacts with soluble chlorides and sulphates of calcium and magnesium to form respective insoluble carbonates. These are filtered out and water becomes soft.

Na₂CO₃ + CaSO₄ ⟶ CaCO₃ ↓ + Na₂SO₄

Na₂CO₃ + MgCl₂ ⟶ MgCO₃ ↓ + 2NaCl

What are the main causes of water pollution? State three ways to control water pollution.

Answer

The main causes of water pollution are:

1. Chemical wastes from industrial and agricultural processes.
2. Thermal wastes from nuclear and thermal power plants.
3. Sewage and garbage disposed into water bodies.

It can be controlled by the following ways:

1. Domestic sewage should be treated before being discharged into water bodies.
2. Waste products of industries should be treated before being discharged into water bodies.
3. Use non-biodegradable materials such as detergents, pesticides and polythene responsibly to avoid pollution of water and the environment.

We all know that common salt (NaCl) adds taste and some nutrition to our food. One rainy day, Amaira wanted to add some salt to her meal. When she opened the jar and scooped a spoonful of salt out, she noticed that the salt had become moist and sticky.

(a) Why did the salt become moist ? Explain.

(b) Define deliquescence.

(c) How can we keep the salt dry in our homes during monsoon ?

Answer

(a) During the rainy season, the air contains a large amount of water vapour (high humidity). Common salt (NaCl), especially if it contains impurities like magnesium chloride, absorbs moisture from the air. This makes the salt moist and sticky.

(b) Deliquescence is the property of certain substances to absorb moisture from the air and dissolve in it, forming a solution.

(c) We can keep the salt dry in our homes during monsoon by :

1. Storing salt in an airtight container.
2. Keeping it in a dry place.

To explain the difference between hard water and soft water, a chemistry teacher took tap water in two beakers labelled A and B. A pinch of magnesium sulphate was dissolved in beaker A, while beaker B contained only tap water. Soap solution was then added to both the beakers and stirred. The students noticed a difference in the lather formed.

(a) What difference is observed by the students when soap solution is added to beakers A and B ?

(b) What conclusion can be drawn from their observations ?

(c) What substances cause (i) temporary hardness and (ii) permanent hardness ?

(d) Name a simple method that can be used to remove the temporary hardness of tap water in homes.

(e) Why is washing soda added to hard water ?

Answer

(a) When soap solution is added to beaker A, no or very little lather is formed; instead, scum appears.
When soap solution is added to beaker B, a good amount of lather is formed easily.

(b) Beaker A contains hard water due to magnesium sulphate.
Beaker B contains comparatively soft water, as it forms lather easily with soap.

(c) Temporary hardness is caused by calcium bicarbonate and magnesium bicarbonate.
Permanent hardness is caused by calcium sulphate, magnesium sulphate, calcium chloride, and magnesium chloride.

(d) A simple method to remove temporary hardness at home is boiling water.

(e) Washing soda is used with hard water because washing soda reacts with calcium and magnesium ions and forms insoluble carbonates, which get precipitated and removed. This helps in softening hard water and increasing lather formation.