ICSE Class 9 Chemistry Chapter 03 Water

Water

Syllabus

Water as a universal solvent:

Solutions as mixtures of solids in water; saturated solutions.

Qualitative effect of temperature on solubility (e.g. solutions of calcium sulphate, potassium nitrate, sodium chloride in water).

Hydrated and anhydrous substances

Hydrated substances

Water of crystallisation - meaning and examples

Anhydrous substances:

Meaning and examples only

Properties:

Efflorescence - Deliquescence - Hygroscopy

(Definition and examples of each of the above)

Drying and dehydrating agents

Meaning and examples only

Soft water and hard water.

Meaning (in terms of action of soap)

Advantages and disadvantages of soft water and hard water.

Types and causes of hardness.

Removal of hardness (i) by boiling (ii) by addition of washing soda.

3 A. Water

Formula: H2O

Chemical name: Dihydrogen oxide

Molecular mass: 2 x 1 + 16 x 1 = 18 amu

Water exists in all the three physical states: as solid (ice), as liquid (water) and as gas (water vapour).

Fig. 3.1 shows covalent bonding in a water molecule with two hydrogen atoms bonded to one oxygen atom.

A water cycle diagram is shown illustrating the processes of evaporation, condensation, precipitation, transpiration, and water flow through mountains and oceans.

3.1 Introduction

Water is the most important natural resource; more than 70% of Earth's area is occupied with water. However, hardly 2-5% of this water makes up the world's supply of fresh water, including the frozen water in polar ice caps and glaciers.

Water is a major constituent of all living things (plants, animals and human beings) and of the atmospheric environment in which we live. Nearly 70% of our body weight is water.

Water occurs in both free and combined states.

Ordinarily, water exists in the liquid state. When heated under normal pressure (760 mm Hg), water boils at 100-C and changes rapidly into its gaseous state (steam). When steam condenses back into water, heat is released. This heat is called latent heat of condensation, and it is equal to latent heat of vaporization.

It is the sudden release of latent heat of condensation that causes the violence associated with torrential rain.

3.2 Physical Properties Of Water

1. Nature: Pure water is a clear, transparent liquid. It is colourless, odourless and tasteless.

The taste in water is due to gases and solids dissolved in it, i.e. impurities present in it.

2. Boiling point: Under normal pressure, pure water boils at 100-C. The boiling point of water is affected by pressure because boiling point of a liquid is considered as the temperature when vapour pressure equals the atmospheric pressure. Thus, the greater the pressure, the higher the boiling point, and vice versa. This is the principle by which a pressure cooker works.

In the hills, water boils at temperature lower than 100-C with atmospheric pressure being low (70-C on top of Mount Everest) and food is not cooked properly there. Therefore, pressure cookers are useful in the hills.

The boiling point of water also increases due to the presence of dissolved impurities in it.

3. Freezing point of water or melting point of ice: Pure water freezes at 0-C under normal pressure (one atmosphere). The freezing point of water decreases with increase in pressure.

The freezing point of water also decreases due to the presence of dissolved impurities in it.

4. Density: At 4-C, water has its maximum density, 1g/cm3 or 1000 kg/m3, and minimum volume. Water expands on freezing, i.e. 92 volumes of water become 100 volumes of ice. Therefore, with relative density of ice being 0-92, it floats.

5. Anomalous expansion of water: Water has an unusual physical property. When cooled, it first contracts in volume, as do other liquids, but below 4-C, it starts expanding and continues to do so till the temperature reaches 0-C, the point at which it freezes into ice.

A graph shows water density (g/mL) on the y-axis ranging from 0.998 to 1.000, and temperature in degrees Celsius on the x-axis ranging from -5 to 20. The curve shows maximum density at 4-C.

The property of anomalous expansion of water enables life to exist in the colder regions of the world, because even when the water freezes on the surface, it remains a liquid below the ice layer (Fig. 3.4).

A cross-section diagram shows ice at the top with air below 0-C, then water layers at 0-C, 1-C, 2-C, 3-C, and 4-C below.

6. Latent heat of fusion of ice: The latent heat of fusion is the amount of heat required to change a substance from the solid state to its liquid state at its melting point without any change in temperature.

The amount of heat energy required by ice to change into water, is called latent heat of fusion of ice. Its specific value is 336 J/g or 80 cal/g. The same amount of heat is released when 1 g of water solidifies to form 1 g of ice at 0-C.

It is on account of high specific latent heat of solidification that lakes and rivers do not freeze suddenly.

Teacher's Note

Understanding water's anomalous expansion explains why ice skating is possible and why fish survive beneath frozen lakes during winter.

7. Latent heat of vaporization of water: When water is boiled, it changes to gaseous state.

The energy required to change water into its vapour at its boiling point without any change in temperature is called latent heat of vaporization of water, and its specific value is 2268 J/g or 540 cal/g. The same amount of heat is released when 1 g of steam condenses to form 1 g of water at 100-C.

It is on account of high specific latent heat of vaporization that steam causes far more serious burns than water at 100-C.

8. Specific heat capacity: It has been found that 1 g of water, when heated through 1-C, always absorbs 4.2 J (or 1 calorie) of heat energy. The fixed amount of heat absorbed by 1 g of water, when heated through 1-C, is called its specific heat capacity.

Water because of its high specific heat is commonly used as coolant in motor car radiators, desert coolers, etc.

Due to its high specific heat capacity, the presence of a large amount of water is able to modify the climate of the nearby land areas, making them warmer in winter and cooler in summer. Land and sea breeze are also set up because of this great moderating property of water.

Teacher's Note

Coastal cities like Mumbai and Chennai have moderate temperatures year-round because of the moderating effect of the Arabian Sea and Bay of Bengal on their climate.

3.3 Water As Universal Solvent

Water is a remarkable solvent. It dissolves many substances, forming aqueous solutions (water solutions). Water has a high dielectric constant. As a result, it reduces the electrostatic force of attraction between positive and negative ions and dissolves even inorganic compounds, which are usually electrovalent. Not only solids, but also gases and other liquids dissolve in water. Hence, it is difficult to find absolutely pure water in nature. For the same reason, water is also called universal solvent.

Substances that are apparently insoluble in water actually dissolve in it in traces. Even when we put water in a glass vessel, an extremely small amount of glass dissolves in it. It is for this reason that, when distilled water is kept in a sealed bottle for a long time, it leaves etchings on the inside surface of glass.

Experiment: To show that ordinary tap water contains dissolved solids.

Procedure: Put some tap water on a clean watch glass, and place it over a beaker containing water, as shown in Fig. 3.5. Boil the water in the beaker. When all the water has evaporated from the watch glass, remove it from the burner and let it cool.

A diagram shows a setup with a watch glass containing tap water placed over a silica glass tube, which is placed over a beaker of water. Heat is applied to the beaker. Another circular diagram shows the appearance of the watch glass after evaporation, displaying concentric rings.

Observation: On looking at the watch glass against light, a number of concentric rings of solid matter are seen. These are the dissolved solids left behind after evaporation of water.

Importance of dissolved salts in water

The dissolved solids in water are salts, minerals and impurities.

1. They are essential for growth and development of plants.

2. They add taste to water.

3. They supply the essential minerals needed by our bodies.

Note: Tap water, river water and well water contain dissolved solids, but rainwater and distilled water do not contain dissolved solids, and so concentric rings are not formed in their case.

Air Dissolved In Water

Air is present in dissolved state in all the natural sources of water. Of the two main components of air (nitrogen and oxygen), oxygen is more soluble in water than nitrogen.

Note: Since oxygen is more soluble in water compared to nitrogen, the air dissolved in water differs from ordinary air.

Experiment: To show that tap water contains dissolved gases (air).

Procedure: Fill a round-bottom flask completely with tap water, and arrange the apparatus as shown in Fig. 3.6. A graduated tube or burette filled completely with water is inverted over the end of the delivery tube in a trough of water.

A diagram shows a round-bottom flask filled with tap water connected via tubing to a graduated tube inverted in a trough of water. The apparatus is heated.

Observation: Heat the water in the flask. Gas bubbles are seen escaping from water. They are collected in the graduated tube by downward displacement of water. The water vapour present in the gases condenses on coming in contact with the cold water in the tube. Obviously, the remaining gases come from the air dissolved in water, and they escape on heating. Read the volume of the gas thus collected. The volume of the gas (air) collected in the graduated tube shows the presence of dissolved gases in tap water.

The solubility of a gas is often expressed by the volume of that gas dissolved in a certain volume of a solvent. For example, the solubility of H2 in water is expressed as 0-02 volume of hydrogen per unit volume of water, at 0-C or 506 volumes of hydrogen chloride per unit volume of water, at 0-C.

The composition of air dissolved in water is 33% oxygen (compared to 21% in ordinary air), 66% nitrogen and 1% carbon dioxide.

Note: The gases dissolved in water can be easily expelled by boiling the water (Fig. 3.6).

Note: Distilled water and boiled water have no taste. The pleasant taste of drinking water is due to the presence of dissolved matter, like air, carbon dioxide and minerals.

Importance Of Air Dissolved In Water

Air dissolved in water is biologically very important.

(1) Marine life, like fish, use the oxygen of the air dissolved in water for respiration, and thus aquatic life is sustained. 1 dm3 of water contains nearly 40 cm3 of dissolved oxygen.

(2) Carbon dioxide

(i) Aquatic plants make use of dissolved carbon dioxide for photosynthesis, i.e. to prepare their food.

The chemical equation shows: 6CO2 + 12H2O with chlorophyll and sunlight yields C6H12O6 + 6O2 + 6H2O (glucose)

(ii) Carbon dioxide dissolved in water reacts with limestone to form calcium bicarbonate.

CaCO3 + CO2 + H2O yields Ca(HCO3)2

limestone - bicarbonate

Marine organisms such as snails, oysters, etc. extract calcium carbonate from calcium bicarbonate to build their shells.

Teacher's Note

Fish in aquariums need air pumps because the dissolved oxygen gets depleted quickly in stagnant water, showing how vital dissolved gases are to aquatic life.

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