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Grade 8 Chapter 13

Module Objectives

• Biological Significance of Water• Action of Metals and Non-Metals on Water• Chemical Properties of Water• Classification of Substances • Hardness of Water• Methods of removal of hardness of Water

Importance of Water

About 65% of our body is made up of water

Water plays an important role in various vital processes that are carried out by our body

All cellular processes take place with water as a medium.

Water is used to treat common health problems in Ayurvedic “Hydrotherapy”.

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Biological significance of some physical properties of Water: transparent, dipolar, universal solvent, adhesion, cohesive forces.

1. Water, being transparent and colourless transmits sunlight, enabling aquatic plants to have photosynthesis, and also enabling us to see, as our eyes are coated with water.

2. Water molecules are dipolar, meaning they have a positively and a negatively charged region. These charges attract other charged substances that are dissolved in it, and the water molecules form a layer around each charged ion, keeping the substance in solution.

3. Water is known as a ‘universal solvent. This is because it dissolves many more substances than most common solvents.

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4. Water is ‘wet’ because it sticks to things. Its molecules form hydrogen bonds with other polar substances. This is called adhesion.

5. The attraction between molecules of similar substances is called cohesion. In this way water molecules stick together and move along very narrow spaces, such as through the very narrow xylem of tall plants and trees, against gravity. This process is called capillarity.

Cohesion, is easily observed if you carefully overfill a glass with water. The water molecules hold together on the outside almost like a ‘net’ above the rim, until gravity overtakes the hydrogen bonding and water spills down the side of the glass. This type of cohesion is surface tension. This surface tension of water allows water spiders to literally walk on water.

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1.Property 2. Definition 3. Biological importanceUniversal Solvent

A liquid substance which can dissolve a wide range of molecules

Allows aquatic plants to photosynthesise.

Adhesion The tendency of molecules to be attracted to other molecules of a different type

Molecules are pulled inward causing it to move upwards through the xylem tissue.

Cohesion The tendency of molecules of a substance to attract one another

A strong force of attraction between the water molecules and the walls of the xylem allow transpiration to take place. This is what causes water to be wet.

Transparent The ability for visible light to penetrate a substance fully

Allows water to act as a transport medium for polar molecules causing them to dissociate. It also removes metabolic wastes such as urea and ammonia.

Activity 1: Column 3 has got mixed up. Align them with the relevant property and definition. :

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Property Definition Biological importance

Universal Solvent

A liquid substance which can dissolve a wide range of molecules

Allows water to act as a transport medium for polar molecules causing them to dissociate. It also removes metabolic wastes such as urea and ammonia.

Adhesion The tendency of molecules to be attracted to other molecules of a different type

A strong force of attraction between the water molecules and the walls of the xylem allow transpiration to take place. This is what causes water to be wet.

Cohesion The tendency of molecules of a substance to attract one another

Molecules are pulled inward causing it to move upwards through the xylem tissue.

Transparent The ability for visible light to penetrate a substance fully

Allows aquatic plants to photosynthesise.

Purification of WaterWhat is potable Water?Water that be used for drinking.

Name some traditional methods of obtaining potable water. Boiling , making sunlight fall on water, adding Tulsi Leaves, camphor, dropping a red hot iron ball into water , storing it in copper pots etc.

Name some modern ways of getting potable water.Methods of removal of color, odour, suspended matter and microbes using ceramic filter candles in water filtersDifferent methods of removal of hardnessWater treatments that uses coagulation, sedimentation, filtration, decoloration, deodorisation and sterilisation with chlorine or ozone.

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Reactivity SeriesAll metals react with water and acids to different extents. A more reactive metal will react with weaker or colder solutions and will displace less reactive metals from their compounds.

Metals can be placed in a Reactivity series, ranking them from the most to the least reactive.

If metals are left exposed to air, the more reactive metals form a dull oxide coating ( lithium, sodium and potassium), whereas the less reactive metals need heating to form an oxide.

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Activity 2: Work in your groups of 4. Some information about Metals is given below.

Read it and then arrange them according to their reaction with water, cold or hot, steam or dilute acids, placing the most reactive metal on top of the series.

The metals are:Ag, Cu, Fe, Al, Na, Au,Ca, Li, K, Mg, Zn, H

Although hydrogen is a non-metal it is included in the reactivity series of metals due to the fact that it behaves like a metal in most chemical reactions i.e., the hydrogen ion has a positive charge [H+] like other metals.

A Tip:Approximating position of metals:

1. Zinc has no reaction with cold water. When the metal is heated in steam zinc, oxide and hydrogen are formed.

2. Quite reactive with dilute hydrochloric acid forming the colourless soluble salt zinc chloride and hydrogen gas.

3. K, Na, Li and Ca are too reactive to add safely to acids – calcium can be used if the acids is very dilute.

4. Aluminium has no reaction with water or steam due to a protective aluminium oxide layer of Al2O3. Slow reaction with dilute hydrochloric acid to form a colourless soluble salt aluminium chloride and hydrogen gas.

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6. Ca reacts very gently with cold water, but gradually theFlask gets warm and calcium hydroxide which is milky is formed.

7.With Mg there is hardly any reaction with cold water, but gets coated with insoluble Magnesium Hydroxide and this prevents any further reaction. However with boiling water Mg reacts strongly and burns with a bright white light to produce white MgO. It reacts vigorously with cold dilute acids – the mixture gets very hot and a solution of magnesium sulfate or magnesium chloride is produced.

8. K reacts explosively with cold water forming the alkali potassium hydroxide and hydrogen gas (which is ignited)

9. Na darts around the surface of water with a hissing sound. Heat and some sparks are produced.

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9. Cu -No reaction with cold water or when heated in steam. No reaction with dilute hydrochloric acid or dilute sulphuric acid.

10. Zn burns in steam to form a white powder ZnO which is yellow only when hot.

11. Fe reacts slowly with steam to form a reddish brown oxide. This reaction is reversible so Fe needs to be heated continuously. No reaction with cold water (rusting is a joint reaction with oxygen). Slow reaction with dilute hydrochloric acid forming the soluble pale green salt Iron (II) chloride and hydrogen gas.

12. H- None of the metals below hydrogen can react with acids or water to form hydrogen gas.

13. Ag, Au- No reaction with cold water or when heated in steam. No reaction with dilute hydrochloric acid

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Displacement Reaction: If a reactive element comes into contact with the compound of a less reactive element a chemical reaction may take place. The less reactive element is removed from the compound and replaced by the more reactive element.

Displacement Reaction: If a reactive element comes into contact with the compound of a less reactive element a chemical reaction may take place. The less reactive element is removed from the compound and replaced by the more reactive element.

Check your list with this one

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1. Zinc oxide + calcium 2. Magnesium + iron oxide 3. Zinc + tin oxide 4. Magnesium sulphate + Zinc 5. Calcium + copper oxide 6. Magnesium + iron sulphate 7. Tin oxide + Copper 8. Gold + copper oxide

REACTIONREACTIONREACTIONNO REACTIONREACTIONREACTIONNO REACTIONNO REACTION

Activity 3: With your partner and knowledge of the Reactivity Series of Metals, predict whether the following chemical reaction will take place or not.

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Some conclusions on the Reactivity Series of Metals

Metals above hydrogen in the reactivity series react with water (or steam) to produce hydrogen

If the metal reacts with cold water then a metal hydroxide and hydrogen are formed

If a metal reacts with steam , then a metal oxide and hydrogen are formed.

As you go down the reactivity series, the reactions become less & less vigorous (strong)

metal + cold water = metal hydroxide + hydrogen

Metal + steam = metal oxide + hydrogen

Metals below hydrogen in the reactivity series do not react with water or steam

Activity 4: In the same groups, write word and symbol equations for the reactions of the metals in the series.

Example: Potassium reacts with cold water to produce

Potassium Hydroxide and Hydrogen gas. 2K + 2H2O (cold) 2KOH (aq) + H2 (g)

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Check your equations here......

1.2Na(s) + 2H2O(cold ) 2NaOH(aq) + H2(g)

2.Ca(s) + 2H2O(cold) Ca(OH)2(aq/s) + H2(g)

3.Mg(s) + 2H2O(warm water) Mg(OH)2(aq/s) +H2(g)

4.Aluminium has no reaction with water or steam due to a protective aluminium oxide layer of Al2O3.

5.Zn(s) + H2O(steam) ZnO(s) + H2(g)

6.3Fe(s) + 4H2O(steam) Fe3O4(s) +4H2(g)

7.No reaction of the metals below Hydrogen with water in any condition.

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Activity 4: Here is a Mnemonic to remember the activity series. Can you come up with a better one? Share it with the class.

Now you will be able to answer these questions.

1. If you dip a piece of blue or red litmus paper in pure water, it remains unchanged. What inference do you draw from this activity?

2. Can we store water safely in any metallic container?

3. Metals like potassium and sodium are kept under kerosene. Why?

4. Why must metals like Na and K always be handled with a pair of tongs?

5. Why are water pipes made of copper or aluminium?

Action of steam on Non-MetalsWhen steam is passed over redhot coke, mixture of carbon monoxide and hydrogen is formed.

When steam is passed over red hot silicon, which is a non metal, silicon dioxide and hydrogen gas are formed.

Find out: Mathura oil refinery is situated very close to Taj Mahal. The supreme court issued an order to shift it to a far away place. Why?

Acid RainActivity 5: Watch Video 1 and come up with a definition for acid rain. (In pairs).

What is it and how is it measured?

Normal rainfall has a pH of 5.6. Rain in the atmosphere reacts with gases like carbon dioxide, sulphur dioxide and various nitrogen oxides to form weak acids, thus lowering the pH. Many human activities contribute to acid rain formation. Its acidity can be measured with pH paper.

What is a pH Scale? ƒThe pH scale ranges between 0 and 14, with 7 being neutral. A substance with a pH less than 7 is acidic, and greater than 7 is basic or alkaline. Eachunit change in pH is a tenfold increase or decrease in the strength of the acid or base being measured. This acid rain falls on plants, animals, water, soil and building material and causes much harm.The figure on the next slide will better illustrate this.

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pH Scale

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What 2 important conclusions can you draw from this chart?

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Activity 6: In groups of 4, think of some actions we can take to reduce the pollutants that cause acid rain from going into the air. Share the ideas with the class.

Deliquescent Substances

What is Deliquescence?

•Certain water-soluble substances, when exposed to the atmosphere at ordinary temperature, absorb moisture from the air to become moist and ultimately dissolve in the absorbed water. These are deliquescent substances.

•Many Substances absorb moisture - magnesium chloride, solid sodium hydroxide, solid potassium hydroxide, anhydrious calcium chloride, calcium nitrate and Ferricc Chloride.

•Jaggery becomes moist when exposed to Air for some time and so is deliquescent.

Try the above activity by using caustic Soda or anhydrous calcium chloride or with some chemical fertilizers. Write your observations

Find out: Deliquescence is more common in coastal area. Why?

Know this: Compounds that can remove water from the surrounding air are called desiccants or dying agents. Eg. Concentrated sulphuric acid and calcium chloride monohydrate.

We often find small packs of solid desiccants placed inside electronic equipment or in medicine tablet bottles.

Efflorescent Substances

• Certain hydrates lose water when exposed to air of moderate humidity

• Example: Sodium sulphate( Na2SO4·10H2O)

Washing Soda (Na2CO3·10H2O)

• Loss of water from hydrated compounds to the surroundings is called Efflorescence.

• Looking at the formulae of the compounds can you tell why they are efflorescent?

What is Hard Water?• Hard water does not produce much lather with soap, so soap is

wasted.• Water from regions having limestone and other mineral rocks

which dissolve in it is hard water• Calcium, magnesium bicarbonates, sulphates and chlorides cause

hardness in water• Calcium carbonate does not cause hardness because it is insoluble

in water• If used in boilers, it forms insoluble deposits of salts of calcium and

magnesium• They produce scales on the inner walls of boilers which reduces the

heating efficiency• There are chances of explosion of boilers

What is Soft water?

• Soft water contains small amounts of dissolved salts • Soft water produces a rich lather with little soap• Rain water and spring water are examples of soft water

Methods of removal of Hardness of Water

Hard water that contains Calcium and Magesium bicarbonate can be made soft by boiling

In this reaction, Calcium bicarbonate decomposes to insoluble calcium carbonate and carbon dioxide

Hardness can be removed by the addition of washing soda to water. In this reaction , the soluble salts of calcium and magnesium are converted into insoluble carbonates which can be filtered out.

The soluble sodium salts do not cause hardness.

Washing soda

Materials required: Beaker, glass rod, washing soda , hard

water

Take some hard water in a beaker and ad a small quanity

of washing soda to it. Stir it with a glass rod. Allow the

insoluble carbonates to settle down or precipitate . Filter or

decant the liquid.

Materials required: Beaker, glass rod, washing soda , hard

water

Take some hard water in a beaker and ad a small quanity

of washing soda to it. Stir it with a glass rod. Allow the

insoluble carbonates to settle down or precipitate . Filter or

decant the liquid.

Activity – Soft and Hard water

Write the equation of softening of water showing magnesium salts and sodium carbonate

Experiment to show the removal of hardness by washing soda

You have Soft water

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75% of the Earth’s surface is covered by water97.5% of that water is the oceansOnly 1% is available for drinking1.5 billion people lack ready access to drinking waterDesalination may be the answer.

De-salinationWhat is it? It is removal of salt from salty waterWhy do we need to do it?

Small quantities are not harmful, but it is counterproductive (it just makes you more thirsty!). Eventually, it can be dangerous, ultimately producing fatal seizures, heart arrhythmias and kidney failure.

Desalination De-saliniation occurs by the natural process of the Water Cycle.

Some artificial (1)process like boiling of sea water followed by condensation and by (2) using membrane filters is used to convert saline water (sea water)to potable water which is suitable for human consumption.

This process produces table salt as a by-product.

Teacher demonstrates

Materials required: Hard water, 1 g of soap, 200ml warm distilled water, 0.5 of Epsom salt and washing soda

1.Make a soap solution by dissolving about 1g of soap in 100ml of warm distilled water. Allow the solution to cool 2. Prepare a solution of really hard water by dissolving about 0.5g of Epsom salt in 100ml of distilled water 3. In a test tube, take a small quantity of hard water and add the soap solution to it drop by drop. Stir it well. What do you observe? You may observe very little lather being formed. 4. In another test tube, take the same quantity of the hard water and add a pinch of crushed washing soda and shake vigourously. Allow the precipitate to settle. To this , add the soap solution drop by drop and stir it well. What do you observe? You will find that a large amount of lather is formed.

Q: Why is all Salty water not hard water?**********************

1.Make a soap solution by dissolving about 1g of soap in 100ml of warm distilled water. Allow the solution to cool 2. Prepare a solution of really hard water by dissolving about 0.5g of Epsom salt in 100ml of distilled water 3. In a test tube, take a small quantity of hard water and add the soap solution to it drop by drop. Stir it well. What do you observe? You may observe very little lather being formed. 4. In another test tube, take the same quantity of the hard water and add a pinch of crushed washing soda and shake vigourously. Allow the precipitate to settle. To this , add the soap solution drop by drop and stir it well. What do you observe? You will find that a large amount of lather is formed.

Q: Why is all Salty water not hard water?**********************

Behaviour of Soap in different water