Water

1. Water Science – 9th Grade

2. Module Objectives Molecular Structure of Water. Polar nature of Water Water as a universal solvent. Electrolytes Electrolysis Faraday’s law of electrolysis Uses Electroplating and uses State of colloids Properties of colloids Comparison between the properties of true solution, colloids and suspension Uses of colloids. • .

3. Introduction • We have already learnt: • Availability of water in earth • Main source of water • Properties of water • Chemical composition of water • Electrolysis of water • Conservation of water Water is the essential for existence of life All the bio-chemical reaction take place in water medium Nearly 65-70% of the total body weight of living beings consists of water.

4. Molecular Structure of Water • Keep in mind : Sharing of electrons between Hydrogen and Oxygen atoms fulfills the requirement of attaining stable state. Look At this Video: http://www.youtube.com/watch?v=KiZJOTt3Dl0 H H O O H H

5. Polar Nature of Water • Keep in mind. • Dipole: • Bond having both positive and negative end in a single molecule is called dipole • Polarity: • Dipole phenomenon is called Polarity Shared electrons tend to be much closer towards oxygen atom. This decides the charge Attraction O H - + Polarity

6. Water as an universal solvent • Solvent: • A solvent is a liquid or gas that dissolves a solid, liquid, or gaseous solute, resulting in a solution. Water is a widely used solvent. Hence it is an universal solvent (eg. Water, carbon disulphide, ether and alcohol) • Solute: • A Solute is a liquid or gas that gets dissolved in a solvent (eg. Sugar, salt • ionic nature • Components that dissolve in water and dissociate into ions are called ionic nature • Dissociation of compounds • Positive and negative ions surrounded by water molecules are known as dissociation of compounds • Electronegativity • Capacity of an atom to attract shared electrons is called Electronegativity

7. Electrolytes Substance that conducts electricity either in liquid form or in solution form Non-electrolyte is a substance which does not conduct electricity in molten or solution state. Most organic compounds are non-electrolyte

8. Activity - Electrolyte • Arrange the apparatus as shown in the figure. • Dissolve a little quantity of common salt in water • Suspend two carbon rods in the vessel containing salt solution • Connect the carbon electrodes to dry cell of 6-9 volts and a bulb with the help of wire • Observe the bulb glowing when current is flowing through the circuit • Perform the same with sugar solution • Record your observation Watch the experiment: http://www.youtube.com/watch?v=1XWnovm6JLs

9. Electrolysis We have already learnt: H Electric Current Acidified Water O Passes through Decomposes to Process of Electrolysis: • Electrolyte decomposes to ions Ions moves towards electrodes Once ions reach the respective electrodes, they give up the charges to electrodes and become molecules or atom groups - + - + Anode (+ve) Cathode (-ve)

10. Faraday’s first law of electrolysis The Mass of the substance liberated at each electrode is proportional to the quantity of electric charge passed through the electrolyte According to this law: mass α charge m α Q m = ZQ, or Z=m/Q Electro chemical Equivalent = mass of substance liberated / amount of charges passed through But, Q=It Therefore, Z=m/It Example: 1 coulomb of charge flows through a silver nitrate solution Mass of silver liberated at cathode is 0.0108 gram Find ECE (Electro chemical Equivalent ) of silver? Use this formula. Solution: Electro chemical Equivalent of silver= m/Q m/Q=0.00118 gram/coulomb

11. Faraday’s Second law of electrolysis If the same quantity of electric charge is passed through different electrolytes the mass of substance liberated at different electrodes are proportional to their respective chemical equivalents. Chemical equivalent of any element is the ratio between its atomic mass and valency Example: Atomic mass of copper = 63.55amu Valency of copper = 2 Chemical equivalent of copper = mass/valency Find the Chemical equivalent of copper? Answer: 63.55/2 = 31.77 Mass of copper atom is 64 Chemical equivalent of copper atom is 32 Video explaining the 1st and 2nd faraday’s law in Hindi: http://www.youtube.com/watch?v=2OdlVM2sCWU

12. Do it yourself – Faraday’s 2nd law 1. Prepare a list of chemical equivalent of the following elements. 2. Take copper sulphate solution • Suspend two carbon rods • Connect the carbon electrodes to dry cell of 6-9 volts and a bulb with the help of wire • Observe the bulb glowing when current is flowing through the circuit. • Answer: The bulb glows as copper sulphate is an electrolyte

13. Experiment to verify the second law • Measure the masses of cathodes of silver voltameter and copper voltameter. • Connect the silver voltameter, copper voltameter dry cells and a switch, in a series. • Pass the electric current for a period of time • Measure the masses of cathode again. • The difference in the mass of cathode in the silver voltameter is the silver deposited and in the cathode of the copper voltameter is the mass of copper deposited.

14. Applications of Electrolysis 2. Extraction of metals: Metal ore are subjected to electrolysis to obtain the metal. 1. Refining of metals: Impure metals are purified by the process of electrolysis. 3. Electro typing: Blocks of printing matter are prepared by electrolysis 4. Electro plating: To give a metal coating to another metal electrolysis is used

15. Electroplating • Electroplating is an important industrial application of electrolysis The article to be plated is cleaned thoroughly The article is suspended in the aqueous salt solution of the metal to be coated The article is connected to the negative terminal of an electric source The metal to be coated is connected to the positive terminal Direct current is passed for a predetermined time The metal ions move from anode to cathode and deposit on cathode The thickness of deposit depends on the strength of the current and duration of passing current

16. Colloids Colloids is a mixture in which the size of the particles is between the particles that are found in a true solution and a suspension. The colloidal particles are called disperse phase and they are dispersed in a dispersion medium Examples: Mayonnaise, hand cream, whipped cream, shaving cream, jelly

17. Activities for Colloids • Activity 1 • Prepare a sugar solution in a beaker. Stir well and keep it aside • Try to recognize the sugar particles separately in water • Verify that the sugar particles cannot be seen separately • Add sand to another beaker • Verify that the sand particles can be seen separately • Activity 2 • Take finely powdered starch and dissolve it in boiling water • The mixture obtained is not homogenous • Yet it hardly shows heterogeneous character • The particles of starch do not settle down but remain in suspension • To form such a system, the size of the particles should be intermediate between the true solution and suspension

18. Types of Colloids By changing the disperse phase and medium, we can get different types of colloids

19. Properties of Colloids 1. Tyndall Effect: The scattering of light by the colloidal particles is known as Tyndall Effect The colloidal particles are sufficiently large to scatter light is the reason for this phenomenon. http://www.youtube.com/watch?v=E2ULbn7Uxsk

20. Activity for Tyndall effect • Take a sugar solution, water and a muddy suspension in 3 different beakers. • Keep the beakers in a straight line. • Make a small hole at the centre of a circular black paper and paste it to the glass of the torch • Pass the bean of bright light through the beaker. • Add few drops of milk to the beaker containing water slowly till the path of light is seen. • It can be clearly seen in the beaker with diluted milk and water. • It can be faintly seen in the muddy suspension • Light path is not seen in sugar solution

21. Do it yourself.. • Dissolve some Hypo in water (Sodium thiosulphate) • Keep the solution in a glass beaker and keep it in a dark place. • Pass bright light through the solution • Observe the tyndall effect • Take a mirror and reflect the beam of sunlight • Tap the duster near the beam of light • Observe the movement of chalk particles • Why • What happens when the room is dust free?

22. Properties of Colloids 2. Brownian Movement The zig-zag movement of the colloidal particles in the medium, can be seen through a ultra microscope. This movement is called Brownian movement. This movement is due to the collision of the molecules of dispersion medium with colloidal particles. http://www.youtube.com/watch?v=hy-clLi8gHg

23. Properties of Colloids 3. Coagulation of colloids Colloidal particles are charged They have random motion in a medium They do not settle down When these charges are neutralized, then these particles settle down http://www.youtube.com/watch?v=yNbnqWSbw2U

24. Activity for Coagulation of colloids • Collect water in a beaker • Add a little quantity of potash alum • Observe that the particles settle down in the beaker. • Why?

25. Comparisonbetween the Properties of true solution Colloids and suspension http://www.youtube.com/watch?v=sQUcKiwxtOE

26. Comparisonbetween the Properties of true solution Colloids and suspension.. cntd..

27. Uses of Colloids Human body absorbs some medicines containing metal components in the form of colloids. So colloids are used in medicines Sulphur colloids are used as pesticides Clay particles present in the drinking water are separated by adding potash alum. Silver bromide colloids is used in photographic plates Colloids are used in preparation of ink, boot-polish and creams Blood is a colloid. Potash alum is used to coagulate the blood forming a clot http://www.youtube.com/watch?v=9QVTHDM90io Tannim is used to give hardness to leather