Class 7 Science How things change and react with one another Exam Notes

Class 7 Science How things change and react with one another Exam Notes. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

Covers the following topics:

1.Physicals and chemical changes

2.Change

3.Rusting and its prevention

4.Crystals and Crystallisation

Physical And Chemical Changes

There are two possible definitions for physical and chemical change :

*A physical change is reversible, a chemical change is not always. For example, the freezing of water would be a physical change because it can be reversed, whereas the burning of wood is a chemical change and the burning cannot be withdrawn once having burnt.

*A physical change is a change in which no new substance is formed, a chemical change results in the formation of one or more new substances. Again, consider the previous examples: freezing water into ice just results in water molecules, which are stuck together it is still H₂ Whereas burning wood results ash, carbon dioxide etc, all new substances, which were not, there when you started burning.

A physical change in a substance does not change what the substance is. In a chemical change where there is a chemical reaction, a new substance is formed and energy is either given off or absorbed. For example, if a piece of paper is cut up into small pieces, it still is paper. This would be a physical change in the shape and size of the paper. If the same piece of paper is burned, it is broken up into different substances that are not paper. Physical changes can be reversed; chemical changes cannot be reversed with the substance changed back without extraordinary means, if at all. For example, a cup of water can be frozen when cooled and then can be returned to a liquid form when heated. If one decided to mix sugar into water to make sugar water, this would be a physical change as the water could be left out to evaporate and the sugar crystals would remain. However, if one made a recipe for a cake with flour, after, sugar and other ingredients and baked them together, it would take extraordinary means to separate the various ingredients out to their original form. When heat is given off in a chemical change or reaction, it is called an exothermic reaction. When heat is absorbed in a chemical change or reaction, it is called an endothermic reaction.The speed at which chemical reactions take place depend on the temperature pressure and how concentrated the substances involved in the chemical reaction are. Sometimes substances called catalysts are used to speed up or help along a chemical reaction.

There are two types of changes:

Changes

Physical Change

In a physical change, there is only a change of state. The new substance has the same properties as the old one. No new substance(s) are produced. Ice-water-steam (They are all still water !) For example: ice melting to water or water boiling. In all of these changes, you can get the original materials back!

physical change may also involve changing the shape of the substance. Paper cut into pieces is still paper, ploughing a field but the field still remains as soil, cutting wood into pieces is still wood, and moulding a sculpture is still cement or marble Physical changes are reversible changes.

The following are the examples of physical change, which is also a temporary change:-

(a) Stretching rubber band

(b) Chewing of food

(c) Putting an electric switch on and off

(d) Breaking a stick

(e) Tearing clothes

(f) Ploughing a farm

(g) Sawing wood etc.

(h) Melting of ice-cream etc.

(i) Dissolution of resin in alcohol

(j) Formation of dew, hail, snow, frost, mist and fog

(k) Evaporation of water from sea, river lake etc

◊Chemical symbols of some familiar chemical elements

→Chemical change

In a chemical change, one or more new substances are created. The new substance is different from the original. It has properties that are different from those of the starting materials. In addition, you can get the original matter back conveniently.

Consider an ordinary box of matches. A single match in a box can remain unchanged forever. However, if someone were to take the match and then light it a flame light up and the burns out. What remains will have changed forever. The match can never be lighted again. The match has undergone a chemical reaction. Chemical changes are irreversible changes.

The following are the examples of chemical changes:

a. Rusting of iron

b. Burning of fuel such as, gas, petrol etc.

c. Oxidising food for energy

d. Lighting a match

e. Decay of food, dead plant and animal bodies

f. Cooking of food

g. Butter turning sour

h. Photosynthesis in plants

i. Fermentation of fruit juice etc.

j. Respiration in living being

k. Digestion

In the chemical change, gas is evolved; change of state, precipitate formation and the energy transfer takes place. Consider the following examples of chemical changes:

a. Zinc reacts with hydrochloric acid to form Zinc chloride and hydrogen gas is evolved.
Zn + 2HCl → ZnCl₂ + H₂

b. In the process of photosynthesis, solar energy is transferred or change to chemical energy

CO₂ + 12 H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂

c. Two gaseous substance H2 and O2 react together to form wter, which is a liquid. This satisfies that the change of state takes place during a chemical change.
2H₂(gas) + O₂(gas) → 2H₂O (liquid)

d. Sindoor (vermillion) is chemically lead nitrate. When it is heated, the compound liberates oxygen and forms a yellowish substance called lead oxide.
2Pb₃O₄ → 6PbO + O₂

→RUSTING AND ITS PREVENTION

Rusting is a significant difficulty because it wears off the structure of bridges, iron railings, machine parts, agricultural equipments such as khurpi, spade, sickle etc. Rusting is a type of corrosion. Eating away of metals due to oxygen, carbon dioxide, water vapour, hydrogen sulphide (a gas with pungent smell) etc.
present in the atmosphere is called corrosion. Eating away of metals due to oxygen alone is called rusting. The powdery substance that is deposited on iron (metals) is called rust. Prevention of rusting has always been a challenge and involves a huge expenditure.

Fe (iron) + O₂ (Oxygen from atmosphere + H2O (from moisture water) = Fe₂O₃ (Ferric Oxide or rust) Rusting is a process of slow oxidation. It is a chemical change which is responsible for destroying the iron articles such a khurpi, kodal etc. slowly.
By greasing and oiling of metal accessories, tools and machines.

◊ The process of coating a metal with other suitable material to stop its contact with air, water or moisture is called galvanization. Iron sheets are dipped in molten zinc to coat the surface of iron objects or sheets with a thin layer of zinc. A thin layer of zinc on iron protects the surface or iron from rusting or corrosion. Bicycle wheels and wheels of automobiles are coated with nickel and chromium, which makes them lustrous, protects from rusting and becomes durable.

◊ Painting on a metal forms a layer on the metal surface to detach it from the contact of air, water or moisture. This prevents a metal from getting corroded or rusted.

→CRYSTALS AND CRYSTALLISATION
Solids with regular geometric shapes such as square, a triangle, a quadrilateral or a pentagon etc. formed from regular arrangements of particles are called crystals. The edges of crystals are straight and the surfaces flat. Substances that form a crystal are called crystalline. Solids without a regular shape are called amorphous substances. The process of obtaining crystals from its saturated solution at a higher temperature is called crystallisation. It is a physical change as no new substance is formed. A crystal has a similar set of chemical property as that of mother liquid. The main aim to crystallize a substance is to obtain a pure substance from their naturally occurring impure substance.

◊ Common Salt and the sea water
Common salt is obtained from seawater by the process of crystallization. Seawater is richest source of salt. Seawater is rich of aquatic lives. Their excreta and dead of aquatic plants and animals keep on decaying and salts from their remains keep on adding salt to the seawater. Various rivers and rain also add salt to seawater. Water is the best solvent known to us. Deposition of salt in seawater is much larger than the capacity it can dissolve. Seawater is saturated solution of salt. Excess of salt sediments to the bottom of sea. A saturated solution of seawater contains around 35 gm of dissolved salt per litre of water. This why seawater is saline

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