Water Chemistry PDF

# 8 Water ## Learning Outcomes - Describe that water dissolves many substances and is the universal solvent - Identify a solution, suspension, and colloid on the basis of properties - State the differences between saturated, unsaturated, and supersaturated solutions - Describe water of crystallizati...

# 8 Water ## Learning Outcomes - Describe that water dissolves many substances and is the universal solvent - Identify a solution, suspension, and colloid on the basis of properties - State the differences between saturated, unsaturated, and supersaturated solutions - Describe water of crystallization - Write equations for the reactions of of metals with cold water and steam - Describe hard and soft water - Describe the different methods of softening water ## UNIT 1 - DISSOLUTION OF SALTS IN WATER ### Dissolution of Salts in Water When a solid disappears in water, the solid is said to be dissolved and the product so obtained is called a solution. The process involving the disappearance of solid in water is called dissolution. - **Example:** When sugar is stirred in water, it disappears and hence dissolves and the product is called sugar solution, whereas the process involving the dissolving of sugar in water is called its dissolution. - **Solvent:** A liquid which dissolves other substances in it, is called solvent. For example in water, common salt or sugar dissolves, therefore, water is a solvent. - **Solute:** A substance which dissolves in a liquid is called solute. For example, common salt or sugar are solutes as they dissolve in water. - **Solution:** A homogeneous product formed by the dissolution of a solute into a solvent is called solution. For example, when common salt (solute) dissolves in water (solvent), then the product so obtained is called common salt solution. - **Saturated Solution:** A solution which cannot dissolve more of the solute at a given temperature is called saturated solution at that temperature. For example, if we take 100 cc of water and add 10 g of common salt and stir it well, the common salt will dissolve. However, if we go on adding more and more of common salt, a stage will come when common salt will stop dissolving in water, no matter how much you stir the solution. At this stage, the clear solution in contact with its own solute (common salt) is called saturated solution. ### Conditions for the formation of solution 1. There must be two substances, mutually soluble in one another. The substance in excess is generally called solvent, whereas the substance in small amount is generally called solute. 2. The temperature of the solvent should be slightly high as it helps in the quick formation of solution. 3. The solute should be stirred in the solvent as stirring increases the kinetic energy of the molecules of solute and hence the solution is formed rapidly. 4. The solute should be in the form of fine powder, as it helps in increasing the surface area and hence quick formation of solution. - **Unsaturated solution:** A solution which can dissolve more of the solute, at a given temperature, is called unsaturated solution at that temperature. You must keep in mind that a saturated solution at given temperature can be changed to unsaturated solution by (i) heating it (ii) adding more solvent to it. - **Supersaturated solution:** A solution which can hold more of solute at a given temperature, than saturated solution, is called supersaturated solution. ### Solubility of a Substance The maximum amount of solute which dissolves in 100 g of water at a given temperature so as to form a saturated solution is called solubility of the substance at that temperature. For example, if 36 g of common salt dissolves in 100 g of water at 20°C, to form a saturated solution then the solubility of common salt is 36 g/100 g of water at 20°C. It must be remembered that solubility is different for different substances as illustrated in the table: | Substance | Solubility in 100 g of water at 20°C | |--------------|------------------------------------| | Sugar | 204 g | | Sodium nitrate | 102 g | | Common salt | 36 g | | Ammonium chloride | 39 g | | Iron sulphate | 29 g | | Copper sulphate | 23 g | ## WATER AS UNIVERSAL SOLVENT Water can dissolve all kinds of solids, liquids or gases in it. However, certain substances dissolve rapidly, whereas certain substances dissolve very slowly. The substances which dissolve rapidly in water are called soluble substances. For example, common salt, sugar, etc., are soluble substances. The substances which dissolve very little or do not seem to dissolve are called insoluble substances. For example, glass, metals, stones, etc., are insoluble substances. It must be remembered that all substances dissolve in water over a period of time which may be a few minutes or a few centuries. It is because of this property that the water is called universal solvent. ### Importance of Dissolved Gases in Water (Dissolved gases in water provide taste to water. Oxygen dissolved in water helps in the respiration and survival of aquatic life. Oxygen present in the water also helps to kill some of the harmful germs and bacteria which may cause diseases. Similarly, carbon dioxide gas dissolves in water. It is a source of food for aquatic plants.) In summer, the fishes die in shallow ponds. It is because solubility of air decreases with the rise in temperature. Thus, the fishes do not get sufficient amount of oxygen and hence they die. When a soft drink bottle is opened it produces a lot of froth. It is because the solubility of carbon dioxide decreases when the crown is removed from the bottle. The removal of the crown reduces pressure on the soft drink. Now with the decrease in pressure the solubility of carbon dioxide gas decreases and hence, it bubbles out. ### Salinity of Seawater When the rivers discharge themselves in sea, they bring along dissolved salts. When heat of the Sun evaporates seawater, the salts are left behind. Water so evaporated changes into clouds and again causes rain over landmass. The rainwater again dissolves salts from land and brings them to the sea. Thus, the cycle continues and, hence, concentration of salt goes on increasing in seawater. Thus, seawater is more salty than river water. From the above statement it is not implied that the concentration of salt in seawater will continue increasing indefinitely. (It has been found that when concentration of salts reaches 35 g per litre of water, the concentration stops increasing. It is because the excess salts brought by the rivers crystallise and settle at sea floor. Thus, concentration of salts remains constant irrespective of the fact that rivers bring large amounts of salts from the landmass. ## SOLUTION, SUSPENSION AND COLLOID Solution, suspension and colloid differ from each other because of the difference into particle size. - **Solution:** A solution in which the particles of the solute are broken down to such a fine state that they cannot be seen even under a powerful microscope, is called a solution. In a solution, the particles of the solute are broken down to a diameter of the order of 10-7 cm or less. **Examples:** Solution of common salt, sugar, nitre, copper sulphate, etc., are true solutions. ### Characteristics of a Solution 1. A solution is always clear and transparent, i.e., light can easily pass through it without scattering. This path of light is not visible in the solution. 2. The particles of solute break down to almost molecular size and their diameter is of the order of 10-8 or less. 3. A solution can completely pass through filter paper as particle size of the solute is far smaller than the size of pores of filter paper. Thus, the particles of the solute cannot be separated by filtration. 4. A solution is homogeneous in nature. 5. In a solution, the particles of solute do not settle down, provided temperature is constant. Thus, the solution is stable in nature. 6. From a solution, the solute can easily be recovered by evaporation or crystallisation. - **Suspension:** A heterogeneous mixture of insoluble particles of solute, spread throughout a solvent, is called suspension. The particle size (diameter) in a suspension is more than 10-4 cm. The particles have a tendency to settle down at the base of solvent and can be filtered out, because their size is bigger than the size of the pores of filter paper. Following are some examples of common suspensions: 1. Muddy water, in which particles of sand and clay are suspended in water. 2. Slaked lime suspension used for white-washing has particles of slaked lime suspended in water. 3. Paints in which the particles of dyes are suspended in turpentine oil. ### Characteristics of Suspension 1. The size of particles is more than 10-5 cm in diameter and hence they can be seen with unaided eye. 2. The particles of suspension can be separated from solvent by the process of filtration. 3. The particles of suspension settle down, when the suspension is kept undisturbed. The process of settling of suspended particles under the action of gravity is called sedimentation. Thus, in a way the suspensions are unstable. 4. A suspension is heterogeneous in nature. 5. An excessive amount of scattering takes place in suspensions, because of bigger size of particles. Therefore, path of light is not visible. - **Colloidal Solution or Colloid:** A heterogeneous solution in which the particle size is in between 10-7 cm to 10-4 cm, such that the solute particles neither dissolve nor settle down in a solvent, is called colloidal solution. In a colloidal solution, relatively large suspended particles are called dispersed phase and the solvent in which the colloidal particles are suspended is called continuous phase or dispersing medium. Starch solution, egg albumin in water, human and animal blood, milk, soap solution, etc., are some examples of colloidal solution. ### Characteristics of Colloidal Solution 1. The size of a colloidal particles is in between 10-7 cm and 10-5 cm. 2. The particles of a colloidal solution are visible under powerful microscope. 3. The particles of a colloidal solution do not settle down with the passage of time. Therefore, colloidal solutions are quite stable. 4. The particles of a colloidal solution cannot be recovered by crystallisation or evaporation. However, they can be separated by the process of centrifugation. 5. The particles of a colloidal solution can easily pass through filter paper and hence, cannot be separated by filtration. 6. The particles of a colloidal solution scatter light, i.e., when strong beam of light is passed through the colloidal solution, the path of beam becomes visible. 7. Colloidal solutions are not transparent, but translucent in nature. 8. The colloidal solutions are heterogeneous in nature. ### How to Distinguish between a Solution and a Colloidal Solution 1. A solution is clear and transparent whereas a colloidal solution is translucent. 2. Pass a beam of light through a given solution in a dark room. The path of light does not become visible in case of a solution but, the path of light becomes visible in case of a colloidal solution. ### Differences between Solution and Colloidal Solution | S.No. | Solution | Colloidal Solution | |---|---|---| | 1 | The particle size is less than 10-8 cm. | The particles size is in between 10-7 cm to 10-5 cm. | | 2 | The particles are not visible under a powerful microscope. | The particles are visible under a microscope. | | 3 | The particles of a solution can be recovered by evaporation and crystallisation. | The particles of a colloidal solution cannot be recovered by evaporation and crystallisation. | | 4 | The particles of a solution do not scatter light. | The particles of a colloidal solution scatter light. | | 5 | Solutions are clear and transparent. | Colloidal solutions are translucent. | | 6 | Solutions are homogeneous. | Colloidal solutions are heterogeneous in nature. | ### Differences between Colloidal Solution and Suspension | S.No. | Colloidal Solution | Suspension | |---|---|---| | 1 | The size of the particles of solute is in between 10-7 cm to 10-5 cm. | The size of the particles of solute is more than 10-5 cm. | | 2 | The particles of solute do not settle down when a colloidal solution is allowed to stand. | The particles of suspension settle down when a suspension is allowed to stand. | | 3 | The particles of solute cannot be filtered out. | The particles of suspension can easily be filtered out. | | 4 | The particles of solute are not visible to the unaided eye. | The particles of suspension are visible to the unaided eye. | ## Exercise 8.1