Water

Science – 9th Grade

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.

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 lifeAll the bio-chemical reaction take place in water mediumNearly 65-70% of the total body weight of living beings consists of water.

Molecular Structure of Water• Keep in mind :

Hydrogen OxygenAtomic Number 1 8

Electronic Configuration 1S1 1S2, 2S2 2p4

How many electrons needed to attain a stable state

1 electrons 2 electrons

H

H

O

Look At this Video: http://www.youtube.com/watch?v=KiZJOTt3Dl0

H

HO

Sharing of electrons between Hydrogen and Oxygen atoms fulfills the requirement of attaining stable state.

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

Hydrogen Oxygen

Tendency to attract electrons

Weaker tendency Stronger tendency

Charge Positive charge Negative charge

Shared electrons tend to be much closer towards oxygen atom. This decides the charge

H + O-Attraction

Polarity

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

Hydrogen oxygen

Electronegativity 2.1 3.5

ElectrolytesSubstance that conducts electricity either in liquid form or in solution form

Strong Electrolyte Weak Electrolyte

Capacity to dissociate into ions when dissolved in water

Completely dissociated Partially dissociated

Examples Sodium chloride, Sodium Hydroxide, Potassium Nitrate

Acetic acid, Ammonium Hydroxide

Real time example Salt Hard water

Non-electrolyte is a substance which does not conduct electricity in molten or solution state. Most organic compounds are non-electrolyte

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

Electrolysis

Positive IonsReceive electrons from cathode and become atoms

Negative ionsgives the charges to anode and become atoms

++--

Cathode (-ve)

Anode (+ve)

Process of Electrolysis:

Ions moves towards electrodes

Once ions reach the respective electrodes, they give up the charges to electrodes and become molecules or atom groups

Electric Current Acidified WaterH

ODecomposes toPasses through

We have already learnt:

Electrolyte decomposes to ions

Faraday’s first law of electrolysisThe 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 chargem α Qm = ZQ, or Z=m/QElectro chemical Equivalent =

mass of substance liberated /amount of charges passed through

But, Q=ItTherefore, Z=m/It

Example:1 coulomb of charge flows through a silver nitrate solutionMass of silver liberated at cathode is 0.0108 gramFind ECE (Electro chemical Equivalent ) of silver?

Use this formula.

Solution:Electro chemical Equivalent of silver= m/Qm/Q=0.00118 gram/coulomb

Faraday’s Second law of electrolysisIf 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.

Video explaining the 1st and 2nd faraday’s law in Hindi: http://www.youtube.com/watch?v=2OdlVM2sCWU

Chemical equivalent of any element is the ratio between its atomic mass and valency

Example: Atomic mass of copper = 63.55amuValency of copper = 2Chemical equivalent of copper = mass/valencyFind the Chemical equivalent of copper?

Answer: 63.55/2 = 31.77

Mass of copper atom is 64Chemical equivalent of copper atom is 32

Do it yourself – Faraday’s 2nd law1. 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

Element Atomic mass Valency Chemical equivalentSilverOxygenHydrogenNickelMagnesiumSodiumZincAluminium

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.

Applications of Electrolysis1. Refining of metals:Impure metals are purified by the process of electrolysis.

2. Extraction of metals:Metal ore are subjected to electrolysis to obtain the metal.

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

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

Colloids

Homogenous mixture Heterogenous mixtureAre composed of two or more components that are uniformly distributed throughout the system

Are composed of two or more components that are not uniformly distributed though out the system

Inseparable by mechanical means Separable by mechanical means

Ex:Salt dissolved in water, Soapy water, Sugar dissolved in water

Ex:Sand & water, Oil & water, Orange juice with pulp in it

The Homogenous mixture of two or more substances is called true solution

The Heterogenous mixture of two or more substances is called suspension

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

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

Types of ColloidsBy changing the disperse phase and medium, we can get different types of colloids

Disperse Phase Medium Example

Solid Solid Coloured glass

Solid Liquid Starch, blood

Solid Gas Smoke in air

Liquid Solid Butter, cheese

Liquid Liquid Milk, face cream

Liquid Gas Mist in air, clouds

Gas Solid Air bubbles in silicate

Gas Liquid Water vapour in air

Properties of Colloids

1. Tyndall Effect:

The colloidal particles are sufficiently large to scatter light is the reason for this phenomenon.

The scattering of light by the colloidal particles is known as Tyndall Effect

http://www.youtube.com/watch?v=E2ULbn7Uxsk

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

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?

Properties of Colloids

2. Brownian Movement

http://www.youtube.com/watch?v=hy-clLi8gHg

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.

Properties of Colloids

3. Coagulation of colloids

Colloidal particles are charged

They have random motion in a medium

They do not settle downWhen these charges are neutralized, then these particles settle down

http://www.youtube.com/watch?v=yNbnqWSbw2U

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?

Comparison between the Properties of true solution Colloids and suspension

True solution Colloids Suspension

Homogenous mixture Homogenous mixture Heterogeneous mixture

It is not possible to see the solute and solvent particle through the microscope

Dispersed particles can be seen through the microscope

The suspended particles can be seen with naked eye

Size of the Particles are less than 10nm

Size of the Particles are between 10nm to 1um

Size of the particles are more than 1um

http://www.youtube.com/watch?v=sQUcKiwxtOE

Comparison between the Properties of true solution Colloids and suspension.. cntd..

True solution Colloids Suspension

Particles do not settle down, when Coagulationtakes place

Particles do not settle down

Particles settle down,

It is not possible to separate particles with a filter paper

It is not possible to separate particles with a filter paper

It is possible to separate the suspended particles with a filter paper

Solute particles have constant kinetic motion

Colloidal particles show Brownian movements

Suspended particles show negligible Brownian movements

Tyndall effect not observed Tyndall effect is observed Little Tyndall effect is observed

Uses of ColloidsHuman 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

Tannim is used to give hardness to leather

http://www.youtube.com/watch?v=9QVTHDM90io