Water: 21st Century Skills and Properties PDF
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Mother Teresa Mission Higher Secondary School
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This document discusses water's importance, sources, physical and chemical properties. It describes the characteristics of solutions, including solubility and different types like saturated and unsaturated solutions. It also examines the reactivity of metals with water.
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# Water ## 21st Century Skills * Hands-on Learning * Understanding * Experiential Learning * Remembering * Analysing * Critical Thinking ## Importance of Water We use water for various purposes, i.e., drinking, bathing, cleaning, washing, etc. We also need water for digesting food and removing t...
# Water ## 21st Century Skills * Hands-on Learning * Understanding * Experiential Learning * Remembering * Analysing * Critical Thinking ## Importance of Water We use water for various purposes, i.e., drinking, bathing, cleaning, washing, etc. We also need water for digesting food and removing the waste materials from our body. Plants and animals also require water. So, water is the chief life-giver. About three-fourths of the earth's surface is covered with water. Water can exist in all the three states of matter, i.e., solid, liquid and gas. We need to drink 7-8 glasses of clean water everyday to remain healthy. Water which is fit for drinking is called potable water. ## Sources of Water The various sources of water on the earth are rainwater, surface water, snow, river water, sea water and groundwater. ### Rainwater Rainwater is the purest form of naturally occurring water. But, the polluted gases like sulphur dioxide, nitrogen dioxide, etc., present in air cause acid rain. ### Surface Water On the earth's surface, water exists as snow, river water and sea water. Have you ever tasted sea water? It is saline. The river water flowing into seas brings lots of salts with it and these salts are dissolved in sea water to make it saline. Even though we have abundant supply of water on our earth, we cannot use this water directly for drinking and agricultural purposes. But, still seas and rivers play an important role in supplying water to our homes. Is sea water potable? No, sea water is not potable as it contains a lot of dissolved salts in it which make it saline. Even distilled water is not potable as it does not contain essential salts required by our body. ## Groundwater Groundwater is present beneath the earth's surface. Rainwater is the main source of groundwater The level of water under the ground is known as the water table ## Physical Properties of Water Pure water shows the following properties: * It is a good solvent. * It does not conduct electricity. * It is tasteless, transparent, colourless and odourless. * Pure water boils at 100°C at normal atmospheric pressure to form steam and freezes at 0°C at normal atmospheric pressure to form solid ice. * It can exist as solid, liquid or gas. * It has a density equal to 1 g cm³. ## Water is a good solvent Water does not dissolve everything in it, but it dissolves many substances in it. It is, therefore, called a universal solvent. Some substances like glass, metals and sand are insoluble in water. Some other substances like common salt, sugar, etc., dissolve in water and are called soluble substances. ## Solution Solution is a homogeneous mixture which is formed by dissolving one or more substances. For example, when we dissolve sugar in water, sugar solution is obtained. ### Characteristics of a Solution 1. A solution is always clear and transparent (Note: it is not always colourless, e.g., copper sulphate solution in water is blue in colour). 2. A solution is a homogeneous mixture of solute and solvent 3. A solution cannot be separated into solute and solvent by filtration 4. A solute and solvent can be in any physical state, e.g., | Solution | Example | |----------|---------| | Solid in liquid | Salt solution | | Liquid in liquid | Milk in water | | Gas in liquid | Oxygen in water | In a solution, the component which is less in amount is termed as a **solute** and the component which is more in amount is termed as the **solvent**. For example, one teaspoon of sugar dissolves in a glass full of water. | Solute | Solvent | |--------|---------| | Sugar | Water | But if, in a bowl full of sugar, few teaspoons of water are added, then: | Solute | Solvent | |--------|---------| | Water | Sugar | In air, nitrogen gas has the maximum percentage. So, it is termed as the solvent and all the other gases are called solutes. ### Classification of Solutions The solutions are classified: * Based on amount of the solute in the solvent * Based on the size of the solute particles ### Based on amount of the solute in the solvent Based on amount of the solute in the solvent, solutions are classified into three parts as explained below: - **Unsaturated solution:** A solution in which more solute can be dissolved at a given temperature is called an unsaturated solution. For example, first, we take about 20 ml of water in a beaker and add a small amount of salt to it. Mix it thoroughly to dissolve. Then, add some more amount of salt and mix it thoroughly. We will find that salt gets dissolved. This solution is called an unsaturated solution. - **Saturated solution:** A solution which cannot dissolve more amount of the solute at a given temperature is called a saturated solution. For example, first, we take 20 ml of water in a beaker. Add a small amount of salt to it and mix it thoroughly. Salt will dissolve. If we go on adding more salt, we will observe that a stage will come when we will not be able to dissolve any more salt. We will see some solid salt remaining undissolved. - **Solubility:** The maximum amount of the solute which can get dissolved in 100g of water at a given temperature to obtain a saturated solution is called solubility of a substance. For example: | Salt | Temperature | Solubility | Conclusion | |-----|--------------|-----------|------------| | Ammonium nitrate | 20°C | 150 g | At 20°C, 150 g of ammonium nitrate in 100 g of water can be dissolved to form a saturated solution. | | Ammonium nitrate | 40°C | 297 g | On increasing the temperature, the solubility of ammonium nitrate is increased. | | Ammonium sulphate | 20°C | 74.4 g | The solubility of ammonium sulphate at 20°C is less than that of ammonium nitrate. | - **Supersaturated solution:** A saturated solution in which more solute is made to get dissolved by increasing the temperature is called a supersaturated solution. For example, a saturated solution of sugar is prepared. This solution is heated at higher temperature. As a result, more sugar can be dissolved to produce supersaturated solution. But as soon as the solution is removed from direct heat and begins to get cooled, the extra sugar dissolved can be seen settling at the bottom of the container. ### Based on the size of the solute particles Based on the size of the solute particles, the solution is classified into three parts as explained below: - **True solution:** A homogeneous solution in which the size of the solute particles is very small which cannot be seen even under a microscope is called a true solution (commonly referred to as solution). The diameter of particle size of solute is less than or equal to 10-9 cm. For example, sugar solution, lemon juice, glucose solution, etc. - **Suspension:** A heterogeneous solution in which the solute particles are large and insoluble in the solvent is called a suspension. The diameter of particle size of the solute is less than 10-5 cm. For example, muddy water, chalk dust and water, digene medicine, paints, etc. - **Colloidal solution:** A heterogeneous solution in which the solute particles are neither dissolve nor settle down is called a colloidal solution (referred to as colloid). The solute particles can be seen. The diameter of the particle size of solute is between 10-9 cm to 10-5 cm. For example, milk, starch in water, egg albumin in water, etc. **Table 8.1** shows the comparison of properties of true solution, suspension and colloidal solution. | Property | True solution | Colloidal solution | Suspension | | ---------|----------------|--------------------|--------------| | Particle size | < 10-9 m | > 10-9 m to 10-5 m | > 10-5 m | | Nature | Homogeneous | Heterogeneous | Heterogeneous | | Appearance | Clear and transparent | Translucent | Cloudy | | Filtration | Solute particles can pass through the filter paper | Solute particles can pass through the filter paper | Solute particles can be separated from the solvent by filtration. | | Sedimentation (keeping the solution undisturbed) | No change, solute particles do not settle down. | No change, solute particles do not settle down. | Solute particles settle down and get separated from the solvent. | Effect of heating | Solvent will evaporate and solute can be obtained separately. | Solvent will not get evaporated and the solute particles cannot be obtained separately. | Solvent will get evaporated and solute can be obtained separately. | ## Water of Crystallisation Some of the compounds in their structure have few loosely bound water molecules. Few important points related to water of crystallisation are as follows: * There is no chemical reaction between the compounds and their water molecules. * Only few compounds have water molecules present in their structure. * The number of water molecules per molecule of the compound is fixed. * These water molecules easily leave the compound on heating because they are loosely bound. * Presence of these water molecules is represented as: (Formula of the compound) (Number of water molecules) (Formula of the compound) For example, CuSO<sub>4</sub>.5H<sub>2</sub>O. Thus, the fixed number of loosely bound water molecules present per molecule of some compounds (salts) is called **water of crystallisation**. **Hydrated salts:** The compounds having fixed water molecules loosely bound per molecule of the salt are called hydrated salts. Due to the presence of water of crystallisation, some of these salts acquire a colour but not always. For example: | Hydrated salts | Chemical formulae | Colours | |-----------------|---------------------|---------| | Magnesium sulphate | MgSO<sub>4</sub>.7H<sub>2</sub>O | White | | Zinc sulphate | ZnSO<sub>4</sub>.7H<sub>2</sub>O | White | | Copper sulphate | CuSO<sub>4</sub>.5H<sub>2</sub>O | Blue | | Iron sulphate | FeSO<sub>4</sub>.7H<sub>2</sub>O | Green | | Sodium carbonate | Na<sub>2</sub>CO<sub>3</sub>.10H<sub>2</sub>O | White | Thus, the residue left when a hydrated substance loses its water of crystallisation is called **anhydrous substance**. **Anhydrous salt:** Since water of crystallisation is loosely bound to the salt, it can easily be removed on heating. Some important points related to conversion of hydrated salts into anhydrated salts are as follows: * Hydrated salts may or may not loose all the water molecules bound to them when heated. For example: > CuSO<sub>4</sub>.5H<sub>2</sub>O(s) Heat, CuSO<sub>4</sub>(s) + 5H<sub>2</sub>O(g) <br> > Hydrated salts Anhydrated salts > CoCl<sub>2</sub>.6H<sub>2</sub>O Heat, CoCl<sub>2</sub>. 2H<sub>2</sub>O + 4H<sub>2</sub>O <br> > Hydrated salts Anhydrated salts * On heating the hydrated salt, the colour of it may change.. ## Chemical Properties of Water Chemical properties of water are as follows: ### Action of Water with Metals Metals displace hydrogen from water, i.e., it is a displacement reaction. When water reacts with metals, the rate of a reaction depends on the reactivity of the metals. The arrangement of metals in the decreasing order of reactivity is called **reactivity series**. Metals at the top of the series react with water very easily. But, as we move down to the bottom, the reactivity decreases. | Reactivity series | |-------------------| | Lithium | Highly reactive with water. | | Potassium | | | Barium | | | Strontium | They displace hydrogen very easily to produce metal. | | Calcium | | | Sodium | | | Magnesium | | | Aluminium | Less reactive with water at high temperature.| | Zinc | | | Chromium | | | Iron | | | Nickel | Less reactive with water. But, they displace hydrogen to produce metal. | | Tin | | | Lead | | | Hydrogen (nonmetal) | | | Copper | | | Mercury | They do not displace hydrogen from water under any conditions. | | Silver | | | Gold | | | Platinum | | Metals like potassium, sodium and calcium react with cold water to produce the corresponding metal hydroxides and hydrogen gas. 2Na(s) + 2H<sub>2</sub>O(l) → 2NaOH(l) + H<sub>2</sub>(g) <br> sodium cold water sodium hydrogen hydroxide 2K + 2H<sub>2</sub>O → 2KOH + H<sub>2</sub>(g) <br> potassium cold water potassium hydrogen hydroxide Ca + 2H<sub>2</sub>O → Ca(OH)<sub>2</sub> + H<sub>2</sub>(g) <br> calcium cold water calcium hydrogen hydroxide Metals like magnesium, zinc and iron are less reactive. Hence, they react with water in the form of steam. Mg(s) + H<sub>2</sub>O(g) → MgO(g) + H<sub>2</sub>(g) <br> magnesium steam magnesium oxide hydrogen 2Al(g) + 3H<sub>2</sub>O(g) → Al<sub>2</sub>O<sub>3</sub>(g) + 3H<sub>2</sub>(g) <br> aluminium steam aluminium oxide hydrogen Zn(s) + H<sub>2</sub>O(g) → ZnO(g) + H<sub>2</sub>(g) <br> zinc steam zinc oxide hydrogen Fe(s) + H<sub>2</sub>O(g) → FeO(g) + H<sub>2</sub>(g) <br> iron steam iron oxide hydrogen Metals like tin, lead, copper, etc., do not react with water at all. ### Action of Water with Nonmetal When nonmetals like carbon and chlorine react with water, they form different products as follows: C + H<sub>2</sub>O Heat, CO + H<sub>2</sub> <br> carbon steam carbon monoxide hydrogen Cl<sub>2</sub> + H<sub>2</sub>O → 2HCl + O<sub>2</sub> <br> chlorine steam hydrochloric acid oxygen ### Action of Water with Metallic Oxides Water dissolves metallic oxides to form metallic hydroxides, i.e., alkalis. MgO + H<sub>2</sub>O → Mg(OH)<sub>2</sub> <br> magnesium oxide water magnesium hydroxide CaO + H<sub>2</sub>O → Ca(OH)<sub>2</sub> <br> calcium oxide water calcium hydroxide ### Action of Water through Electrolysis When an electricity is passed through water, it decomposes into hydrogen and oxygen gases. 2H<sub>2</sub>O Electricity, 2H<sub>2</sub> + O<sub>2</sub> ## Soft and Hard Water The water which lathers readily with a soap is called **soft water**. Some examples of soft water are distilled water and tap water. The water which does not lather with a soap easily is called **hard water**. Some examples of hard water are sea water, well water, etc. ## Hardness of Water The hardness of water is due to the presence of certain dissolved salts of calcium and magnesium in water. The salts of calcium and magnesium when dissolved in water cause hardness of water. These salts are as follows: * Calcium sulphate (CaSO<sub>4</sub>) * Magnesium sulphate (MgSO<sub>4</sub>) * Calcium chloride (CaCl<sub>2</sub>) * Calcium bicarbonate [Ca(HCO<sub>3</sub>)<sub>2</sub>] * Magnesium bicarbonate [Mg(HCO<sub>3</sub>)<sub>2</sub>] * Magnesium chloride (MgCl<sub>2</sub>) The differences between hard water and soft water are listed in **Table 8.2**. | Hard water | Soft water | |------------|-------------| | 1. Hard water does not lather readily with a soap. | 1. Soft water lathers readily with a soap. | | 2. It contains dissolved chlorides, sulphates or bicarbonates of calcium and magnesium in it. | 2. It does not contain chlorides, sulphates or bicarbonates of calcium and magnesium in it. | | 3. River water and sea water are examples of hard water. | 3. Distilled water is an example of soft water. | Water is a very good solvent. When natural water flows in rivers, seas and also under the ground like in wells, it dissolves the salts of calcium and magnesium present in rocks and soil. These dissolved salts are responsible for the hardness of well, sea and river water. ### Types of hardness of water The hardness of water is of the following types: * **Temporary hardness of water** * **Permanent hardness of water** **Temporary hardness of water:** The hardness of water which is caused due to the presence of dissolved calcium and magnesium bicarbonates in it is called temporary hardness of water. Temporary hardness can be removed by simply boiling the hard water. **Permanent hardness of water:** The hardness of water which is caused due to the presence of dissolved calcium and magnesium chlorides and sulphates in it. Permanent hardness of water cannot be removed by boiling. ### Removal of Hardness of Water The hardness of water can be removed by following methods: **Removal of temporary hardness by boiling:** Temporary hardness caused due to dissolved calcium bicarbonate and magnesium bicarbonate can be removed by boiling. **What happens to carbon dioxide gas?** Carbon dioxide gas escapes leaving behind soft water. This happens because on heating for some time, soluble calcium bicarbonate and magnesium bicarbonate salts present in water decompose to form calcium carbonate and magnesium carbonate respectively. These carbonates remain insoluble in water and are then separated by filtration. Following reactions take place: Ca(HCO<sub>3</sub>)<sub>2</sub> Heat CaCO<sub>3</sub> + CO<sub>2</sub> + H<sub>2</sub>O <br> calcium bicarbonate Boil calcium carbonate (insoluble) carbon dioxide water Mg(HCO<sub>3</sub>)<sub>2</sub> Heat MgCO<sub>3</sub> + CO<sub>2</sub> + H<sub>2</sub>O <br> magnesium bicarbonate Boil magnesium carbonate (insoluble) carbon dioxide water **Removal of temporary and permanent hardness by adding washing soda:** Washing soda (sodium carbonate, Na<sub>2</sub>CO<sub>3</sub>) can be used to remove both types of hardness, i.e., temporary as well as permanent hardness. Washing soda reacts with calcium and magnesium salts present in hard water to form insoluble carbonates of calcium and magnesium which are then removed by filtration. **Removal of temporary hardness caused due to dissolved calcium and magnesium bicarbonates:** Calcium bicarbonate present in hard water reacts with washing soda to form calcium carbonate and sodium bicarbonate (baking soda). Ca(HCO<sub>3</sub>)<sub>2</sub> + Na<sub>2</sub>CO<sub>3</sub> → CaCO<sub>3</sub> + 2NaHCO<sub>3</sub> <br> calcium bicarbonate sodium carbonate calcium carbonate sodium bicarbonate Similarly, magnesium bicarbonate present in hard water reacts with washing soda to form magnesium carbonate and sodium bicarbonate. Mg(HCO<sub>3</sub>)<sub>2 </sub> + Na<sub>2</sub>CO<sub>3</sub> → MgCO<sub>3</sub> + 2NaHCO<sub>3</sub> <br> magnesium bicarbonate sodium carbonate magnesium carbonate sodium bicarbonate **Removal of permanent hardness caused due to calcium and magnesium chlorides and sulphates:** Calcium sulphate present in hard water reacts with washing soda to form calcium carbonate and sodium sulphate. CaSO<sub>4</sub> + Na<sub>2</sub>CO<sub>3</sub> → CaCO<sub>3</sub> + Na<sub>2</sub>SO<sub>4</sub> <br> calcium sulphate sodium carbonate calcium carbonate sodium sulphate Magnesium sulphate present in hard water reacts with washing soda to form magnesium carbonate and sodium sulphate. MgSO<sub>4</sub> + Na<sub>2</sub>CO<sub>3</sub> → MgCO<sub>3</sub> + Na<sub>2</sub>SO<sub>4</sub> <br> magnesium sulphate sodium carbonate magnesium carbonate sodium sulphate Calcium chloride present in hard water reacts with washing soda to form calcium carbonate and sodium chloride. CaCl<sub>2</sub> + Na<sub>2</sub>CO<sub>3</sub> → CaCO<sub>3</sub> + 2NaCl <br> calcium chloride sodium carbonate calcium carbonate sodium chloride Magnesium chloride present in hard water reacts with sodium carbonate to form magnesium carbonate and sodium chloride. MgCl<sub>2</sub> + Na<sub>2</sub>CO<sub>3</sub> → MgCO<sub>3</sub> + 2NaCl <br> magnesium chloride sodium carbonate magnesium carbonate sodium chloride Sodium salts remain in water after filtering the carbonates of calcium and magnesium and do not make the water hard. **Table 8.3** shows the differences between temporary and permanent hardness. | Temporary hardness | Permanent hardness | |-----------------------|-----------------------| | 1. Temporary hardness is caused due to the presence of dissolved bicarbonates of calcium and magnesium in water, i.e., calcium bicarbonate [Ca(HCO<sub>3</sub>)<sub>2</sub>] and magnesium bicarbonate [Mg(HCO<sub>3</sub>)<sub>2</sub>]. | 1. Permanent hardness is caused due to the presence of dissolved chlorides and sulphates of calcium and magnesium in water, i.e., calcium chloride (CaCl<sub>2</sub>), magnesium chloride (MgCl<sub>2</sub>), calcium sulphate (CaSO<sub>4</sub>) and magnesium sulphate (MgSO<sub>4</sub>). | | 2. It can be removed both by boiling and by adding washing soda | 2. It cannot be removed by boiling but can be removed by adding washing soda (Na<sub>2</sub>CO<sub>3</sub>). | ### Advantages of using hard water The advantages of hard water are as follows: * The minerals like calcium and magnesium present in hard water are good for the formation of healthy teeth and bones. * Dissolved calcium and magnesium salts impart a pleasant taste to water. ### Disadvantages of using hard water The disadvantages of hard water are as follows: 1. Hard water is unfit for drinking purpose as it upsets stomach. 2. It is unfit for washing clothes or bathing as it forms sticky scum rather than lather which gets settled on the clothes. 3. If hard water is used in boilers, it causes hard deposit on the wall of the boiler. **Is hard water fit for washing clothes?** When clothes are washed in hard water, the calcium and magnesium salts react with soaps which result in the formation of a dirty white curdy precipitate called scum which makes washing difficult. This results in the wastage of a lot of water. Also, the washed clothes appear dull due to the formation of scum. Since water is very important for the survival of life on the earth, it is our duty to ensure responsible citizens that: * We should not waste water. * We should create awareness among people to save water. * We should try to reduce water pollution. **Why do we need water?** The population of the world is increasing day-by-day and so, there is a demand of water. So, we must use water wisely and save it properly. ## Wrapping it up * Rainwater is the purest form of naturally occurring water. * The level of water under the ground is called water table. * Water that is fit for drinking is called potable water. * A homogeneous solution of soluble particles of solute with size 10<sup>-9</sup> cm is called a true solution. * A heterogeneous solution of insoluble particles of solute with particle size more than 10<sup>-5</sup> cm is called suspension. * A heterogeneous solution in which particle size is 10<sup>-7</sup> cm to 10<sup>-5</sup> cm is called a colloidal solution. * A solution having a large amount of solute is called a concentrated solution and a solution having a small amount of solute is called a dilute solution. * The fixed number of water molecules loosely bound to one molecule of a substance is called water of crystallisation. * A substance having fixed number of loosely bound water molecules present is called hydrated substance. * A substance which absorbs moisture does not change the physical state is called hygroscopic substance. * Water which produces lather with soap solution easily is called soft water. * Water which does not produce lather with soap solution easily is called hard water.
