Changes in Nature
We studied in the previous class that the changes can be of two types, reversible and irreversible changes , and we studied some of the examples too, but broadly they may be classified into Physical and Chemical changes. Why only Physical and Chemical Changes? Because in Science we need to be systematic. Folding a paper and burning a paper, both are changes, but folding can be undone. The folded paper is a paper and it can be got back into its original state, while burning of paper gives us ash, which is totally a different thing. Hence the folding of paper is physical change, while the burning of the same paper is a chemical change. Many such examples can be thought of and experienced in real life.
Physical Changes
- Folding of paper.
- Inflation and deflation of balloon.
- Boiling of water.
- Melting of ice.
- Drawing a picture with pencil on a paper.
- Beating metal sheet to get a very thin metal sheet.
- Condensation.
- Rinsing a cloth piece with water.
- Freezing of ice.
- Cutting or tearing a cloth piece.
- Typing a rope in a knot.
- Trimming hairs and nails.
- Dyeing of hair.
- Cracking an egg.
- Slicing a bread piece.
All the above written are examples of Physical changes. here one point must be noted is that the Physical changes can be reversible or irreversible.
Now we can easily define Physical changes.
Physical Changes are those changes which tend to change the shape, size, color and configuration of the object. It is the type of change in which we do not get a new product after the change has occurred.
Chemical Changes
The chemical changes are the new changes that bou will learn about in this class. What happens when we burn a paper? Do we get the paper back. The new product formed is ash and smell. Are these same as the paper? Many such examples can be quoted for the Chemical changes.
- Burning of paper.
- Bursting of cracker.
- Rusting of iron.
- Burning of coal.
- Digestion of Food.
- Conversion of milk into curd.
- Photosynthesis.
All the above written examples are examples of Chemical changes. We come across many chemical changes in real life. But the most number of chemical changes can be visualized in a chemistry lab or a chemical factory. The mixing of different types of solutions and gases are done there. These are done under special conditions and precautions. Now we can define a chemical change.
Chemical changes are those changes i which the reactants completely change their nature and we get products which are different from the initial substances. These changes are irreversible in nature. One thing must be remembered that in chemical changes new substances are formed.
Key words:
- Chemical Change: A change in which new products are formed which are different from the products which we used initially. Eg: Converting milk to Curd.
- Chemical reaction: A chemical change is also called a chemical reaction. The formation of new products from reactants is shown through Chemical reaction.
- Crystallization: the process of formation of pure crystals of some substances from their solutions is called crystallization. It is an example of Physical Change.
- Galvanization: The process of depositing a layer of zinc on surface of iron so as to save it from rusting is called Galvanization. It is an example of physical change , because it does not change the nature of iron, but it shields the iron surface only.
- Physical Change: The change which involves the change of properties like shape, size, color and state of the substance is called Physical change. It may be a reversible or irreversible change. In such change no new substance is formed.
- Rusting: A change in which moisture in air converts iron into iron oxide is called rusting. It is an example of a chemical change as iron gets damaged and cannot be regained.
NCERT Exercise Questions:
Q.1.Classify the changes involved in the following processes as physical or chemical changes:
(a) Photosynthesis- Chemical change
(b) Dissolving sugar in water- Physical Change
(c) Burning of coal- Chemical Change
(d) Melting of wax- Physical Change
(e) Beating aluminum to make aluminum foil- Physical change
(f) Digestion of food- Chemical change
Q 2. State whether the following statements are true or false:
(a) Cutting a log of wood into pieces is a chemical change. False
(b) Formation of manure from leaves is a physical change. False
(c)Iron pipes coated with zinc do not get rusted easily. True
(d)Iron and rust are the same substances. False
(e)Condensation of steam is not a chemical change. True
Q. 3. Fill in the Blanks:
(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of Calcium Carbonate.
(b) The chemical name of baking soda is Sodium Hydrogen Carbonate.
(c) Two methods by which rusting of iron can be prevented are Painting and Galvanization.
(d) Changes in which only Physical properties of a substance change are called physical changes.
(e) Changes in which new substances are formed are called Chemical changes.
Q 4. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.
Answer- The reaction between baking soda and lemon juice can be given as below:
Lemon juice + Baking soda ——–——-> C02 (bubbles) + Other substances
(Citric acid) (Sodium hydrogen carbonate) (Carbon dioxide)
It is a chemical change
Q.5. When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both the chemical and physical changes take place.
Answer- When a candle burns, both physical and chemical changes occur:
(i) Physical change: melting of wax, vapourisation of melted wax.
(ii) Chemical change: Burning of vapours of wax to give carbon dioxide, heat and light.
LPG is another example in which physical change occurs when LPG comes out of cylinder and is converted from liquid to gaseous state and a chemical change occurs when gas burns in air.
Q.6. How would you show that setting of a curd is a chemical change?
Answer- We can say that setting of curd is a chemical change because we can not get the original substance, i.e., milk back and a new substance is formed with different taste, smell and other chemical properties
Q 7. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.
Answer- Burning of wood is a chemical change because in burning new substances are formed as
Wood + Oxygen —-———–> Charcoal + Carbon dioxide + Heat + Light
But cutting it into small pieces is physical change because no new substance is formed. We can only reduce the size of wood.
Q 8. Describe how crystals of copper sulphate are prepared.
Answer- Take a cupful of water in a beaker and add a few drops of dilute sulphuric acid. Heat the water. When it starts boiling, add copper sulphate powder slowly. Continue to add copper sulphate powder till no more powder can be dissolved. .During this process continuously stir the solution. Filter the solution. Leave it for cooling. Look it after some time, you can see the crystals of copper sulphate
Q 9. Explain how painting of an iron gate prevents it from rusting?
Answer- It is known that for rusting the presence of oxygen and moisture is essential. Painting prevents the iron gate from coming in contact with oxygen and moisture.
Q 10.Explain why rusting of iron objects is faster in coastal areas than in deserts.
Answer- Content of moisture in the air in coastal areas is higher than in the air in deserts. So, the process of rusting is faster in coastal areas.
Q 11.- The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder it exists as a liquid. When it comes out from the cylinder it becomes a gas (Change- A) then it bums (Change-B).
Answer- (ii) Process-B is a chemical change.
Q. 12.- Anaerobic bacteria digest animal waste and produce biogas (Change-A). The biogas is then burnt as fuel (Change-B).
(iii) Both processes A and B are chemical changes.