Physical and Chemical Changes

Notes of chapter: Physical and Chemical Changes are presented below. Indepth notes along with worksheets and NCERT Solutions for Class 7.

(1) Physical Properties-

Physical properties are size, shape, colour and state of the substance.

Eg:- Physical properties of Iron sheet

(i) Size – 5 × 4 feet

(ii) Shape – Rectangle

(iii) Colour – Black

(iv) State – Solid

(2) Physical change-

A change in which a substance undergoes a change in its physical properties is called a physical change.

Characteristics of physical change-

(i) It is generally a reversible change.

(ii) No new substance is formed in physical change.

Example 1

Cut a piece of paper in four pieces. Cut each square piece in four pieces. Lay these pieces on a table so that the pieces acquire the shape of the original piece of paper. There is no change in the property of paper but only in size.

Showing Diagrammatic Presentation of Physical Change from NCERT Chapter Physical and Chemical Changes

Piece of paper

 

Showing Diagrammatic Presentation of Physical Changes NCERT Chapter Physical and Chemical Changes

Four parts of sheet

 

Showing physical changes

Small parts of sheet

Example 2

Collect the chalk dust and add some water in it. Roll the paste and give shape of chalk. Keep it to dry. You will get chalk. It shows change in size of the chalk.

Example 3

Take some ice in the glass and melt small portion of it. Now, place the glass with mixture of ice and water in a freezing mixture (ice plus common salt). You will find ice again. It shows change in the state of the ice, i.e., from solid to liquid.

Example 4

Boil some water and collect its steam on the inverted pan. You will find water droplets on the inner surface of the pan. It shows change in gas state to liquid state.

Example 5

Hold a hack saw blade in a tong and keep it on the gas stove. It turns to red. Now cool the tip of blade. You will get its original colour. It shows change in colour.

Red tip after heating from NCERT Chapter Physical and Chemical Changes

Metal Regain Black Colour after Cooling from NCERt Chapter Physical and Chemical Changes

(3) Crystallisation –

Crystallisation is a process in which large crystals of pure substance can get from their solutions.

It is an example of physical change.

Example 1

Getting salt crystals from  sea water.

Example 2

Separation of fitkari(alum) from impure solution.

Example 3

Add a few drops of dilute sulphuric acid in a cupful of water. Heat the water. Add copper sulphate when it stars boiling. Stir it continuously. Add copper sulphate powder continuously in the water till it stops dissolving. Filter the solution. Allow it to cool. After some time you can see the crystal of copper sulphate.

Crystalisation of copper sulphate

(4) Chemical change-

A change in which one or more new substances are formed is called chemical change. It is also known as chemical reaction.

Eg:- Medicines, plastic, detergent etc. are produced by chemical change.

In addition to new products other changes can occurs in a chemical change which are given below:

(i) Heat, light or any other radiation may be given off or absorbed.

Eg:-Burning of any substance

(ii) Sound may be produced.

Eg:- Explosion of fireworks that produces light, heat, sound and gases.

(iii) A change in smell may take place or a new smell may be given off.

Eg:-Rotten food items produce smell.

(iv) A colour change may take place.

Eg:- colour of apple, potato and brinjal slice changed to brown if it is not consumed immediately.

(v) A gas may be formed.

Eg:- Explosion of fireworks, candle flame produces gas.

Example 1

Get a small piece of a thin strip of magnesium. After cleaning its tip with sand paper brings it near a candle flame. It burns with a white light. After burning completely it leaves new product ash.

Magnesium ribbon burning from NCERT Chapter Physical and Chemical Changes

Chemical Reaction:

Magnesium(Mg) + Oxygen(o2) → Magnesium Oxide (MgO)

Now, collect ash and mix with water .Stir it well. A new substance magnesium hydroxide[Mg(OH)2] is produced. It is a base.

Chemical reaction is given below:

Magnesium oxide (MgO) + Water(H2O) → Magnesium hydroxide[Mg(OH)2]

Effect of Magnesium hydroxide on litmus paper

Example 2

Take a teaspoonful of copper sulphate or neela thotha in half a cup of water in a beaker. Add few drops of dilute sulphuric acid in it. You will get blue coloured solution. Put a nail it and wait for half an hour. The colour of solution will change to green.

The nail gets a brown coating when dipped in copper sulphate. Copper is deposited on the nail and gives brown colour. In this reaction we get two new substances, first is Iron sulphate (green Colour) and second is copper.

Change in colour of the copper sulphate solution due to reaction with iron.

Chemical reaction is given below:

Copper sulphate solution (blue) + Iron (nail) → Iron sulphate solution (green) + Copper (brown deposit)

Example 3

Take a teaspoonful of vinegar in a test tube. Add a pinch of baking soda to it a gas will come out with hissing sound. Pass this gas in fresh lime water. Lime water will turn milky because of formation of calcium carbonate. The turning of lime water in to milky is a standard test of carbon dioxide.

Chemical reaction in the test tube is as follows:

Vinegar (Acetic acid) + Baking soda (Sodium hydrogen carbonate) → Carbon dioxide + other substances

The reaction between carbon dioxide and lime water is given below:

Carbon dioxide (Co2) + lime water [Ca(OH)2] → Calcium Carbonate(CaCO3) + Water (H2O)

Carbon dioxide, Water and calcium carbonate are new products that are produced in chemical change.

(5)Rusting –

Rusting is a process in which the iron products get a brownish substance on them in the presence of oxygen and water. This brownish substance, Iron oxide is called rust.

Chemical reaction is given below:

Iron (Fe) + Oxygen (O2) + Water (H2O) → rust(iron oxide Fe2O3)

If the moisture in air is high rusting becomes faster.

(6) To prevent rusting iron items they should be painted or greased regularly. It will prevent iron from coming in contact with oxygen and water. Another way to prevent rusting on Iron is to deposit a layer of a metal like chromium or zinc.

(7) Galvanisation –

The process of depositing a layer of zinc on iron is called galvanisation.

Eg:-

(i)The iron pipes that is used in our homes to carry water are galvanized to prevent rusting.

(ii) Ships get rust on the lower part of the ship that remains under salty water of sea. Even above water ship gets rust because of the water drops. So a fraction of its part has to change every year in spite of being painted.

 

Helping Topics

Effect of Magnesium Hydroxide on Litmus Paper

Physical and Chemical Changes of a Burning Candle

Physical and Chemical Changes of Making Paper 

Physical and Chemical Changes of Burning of Wood and Cutting it into Small Pieces

Identification of Physical and Chemical Changes in Different Processes 

NCERT Solutions Class 7

Worksheet Class 7

 

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