1. Physical Changes -
ü In physical change, a substance undergoes changes only in its physical properties like change in its shape, size, colour and state, and no new substance is formed.
ü Characteristics of physical changes include following - these changes are temporary changes which can be easily reversed to form the original state. And no new substance or substances are formed. A temporary change in colour may or may not take place. Very little energy (heat, etc) is either absorbed or evolved during the change.
2. Chemical Changes-
ü Chemical changes are also termed as chemical reactions.
ü In a chemical reaction two or more similar or two or more different species interact to form product. A chemical change occurs when two substances react chemically to form a new substance having different properties.
ü Characteristics of chemical changes include following - these changes are permanent changes which can not be easily reversed or can not be reversed at all to form the original state. And new substance or substances are formed. A permanent change in colour may take place. Energy (heat, etc) is either absorbed or evolved during the change.
ü A change with which we are quite familiar is the rusting of iron. Almost every iron object kept in the open gets rusted slowly. It acquires a coating of a brownish substance called rust and the process is called rusting.
ü We can usually see iron gates of parks or farmlands, iron benches kept in lawns and gardens, almost every article of iron, kept in the open gets rusted.
ü The agricultural tools such as spades and shovels, also get rusted when exposed to the atmosphere for some time.
ü In the kitchen, a wet iron pan (tawa) often gets rusted if left in that state for some time. Rust is not iron. It is different from iron on which it gets deposited.
3. Chemical Changes Around Us-
ü In the process of digestion, the various food materials break down to form new substances which can be absorbed by the body, so the process of digestion is a chemical change.
ü An explosion of a firework (or crackers) is also a chemical change which produces heat, light, sound and unpleasant gases that pollute the atmosphere.
ü When food gets spoiled, it produces a foul smell. This shows that new substances have been formed in the spoiled food which has a foul smell. So, the spoilage of food is a chemical change.
ü When an acid reacts with a base, then a neutralisation reaction takes place in which two new substances, salt and water, are formed. So, neutralisation is a chemical change.
ü During photosynthesis, the plants intake carbon dioxide and water in the presence of chlorophyll and sunlight to form two new substances, glucose (food) and oxygen. So, photosynthesis is a chemical change.
ü If we cut an apple into slices and kept in the open for some time, we will find that the cut surface of apple acquires a brown colour. This change in colour is due to the formation of the new substance by the action of oxygen (or air). So, this change in colour is a chemical change.
ü Similarly, the cut surface of potato or brinjal turns black on keeping in air for some time due to the chemical change.
4. Rusting of Iron-
ü When an iron object is left exposed to moist air, it chemically reacts with oxygen and water in the air to form a red-brown flaky substance called rust.
ü The process of rusting can be represented by the following equation-
Iron (Fe) + Oxygen (O2) (From air) + Water (H2O) → Rust (Iron oxide, Fe2O3)
ü Rusting occurs in the presence of both oxygen and water.
ü The more humid the air, the faster the rusting occurs.
ü The rust slowly eats away or corrodes the iron, leading to considerable loss.
ü Since iron is used in making bridges, ships, * cars, truck bodies and many other articles, the monetary loss due to the rusting is huge.
ü Prevention of Rusting - Rusting can be prevented by not allowing the iron to come in contact with moisture and air. The simplest method is to coat the iron with oil, grease or paint. These coats should be applied regularly to prevent rusting.
ü A more efficient method is to coat the iron with another metal such as zinc or chromium. The process of depositing a layer of zinc on iron is called galvanization. The iron pipes we use in our homes to carry water are galvanized to prevent rusting.
NOTE- Rusting of ships is a major problem in the shipping industry as the body of a ship is always in contact with water and the air around it is also very humid.
The salt in water speeds up the process of rusting. This leads to huge monetary loss to the shipping industry. Rusting of iron can be prevented by allowing it to make stainless steel.
Stainless steel is made by mixing iron with carbon and metals like chromium, nickel and manganese. It does not rust.
5. Crystallization-
ü Large crystals of pure substances can be obtained from their solutions by the process of crystallization.
ü It is an example of a physical change. The process of cooling a hot concentrated solution of a substance to obtain crystals is called crystallization.
ü The process of crystallization is used to obtain crystals of a pure solid substance from the impure solid substance.
ü Impure copper sulphate powder can be purified by the process of crystallisation to obtain large crystals of pure copper sulphate.
NCERT PRACTICE QUESTIONS
1. Classify the changes involved in the following processes as physical or chemical changes-
(a) Photosynthesis (b) Dissolving sugar in water (c) Burning of coal (d) Melting of wax
(e) Beating aluminium to make aluminium foil (f ) Digestion of food
Ans.- a) Chemical change b) Physical Change c) Chemical change d) Physical Change e) Physical Change f) Chemical change
2. State whether the following statements are true or false. In case a statement is false, write the corrected statement in your notebook.
(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)
3.Fill in the blanks in the following statements-
(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of _________.
(b) The chemical name of baking soda is _________.
(c) Two methods by which rusting of iron can be prevented are _________ and _________.
(d) Changes in which only _________ properties of a substance change are called physical changes.
(e) Changes in which new substances are formed are called _________ changes.
Ans.- (a) Calcium carbonate. (b) Sodium hydrogen carbonate. (c) galvanization and painting (d) physical
(e) chemical
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.
Ans.- This is a chemical change, reaction can be given as-
Lemon juice + Baking soda → Carbon dioxide + other substances
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.
Ans.- (a) Physical change is melting of wax and chemical change is the burning of gas with the evolution of CO2
(b) Physical change is the breakdown of larger food molecules to simpler ones. And chemical change is the digestion of food by the action of HCl and enzymes.
6. How would you show that setting of curd is a chemical change?
Ans.- The setting of curd is a chemical change as curd cannot be turned back to milk. The properties of milk and curd are different so we can say new product is formed.
7. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.
Ans.- Cutting of wood is a physical change as it does not change the nature of the wood. On the other hand, burning of wood is a chemical change as wood is converted to charcoal with the liberation of CO2 gas.
8. Describe how crystals of copper sulphate are prepared.
Ans.-
ü Take a cupful of water in a beaker.
ü Add a few drops of dilute sulfuric acid to this.
ü Heat the water and when it starts boiling, add copper sulphate powder while still stirring.
ü Add the copper sulphate powder till the solution becomes saturated. Filter into a china dish and allow it to cool.
ü The solution should be kept undisturbed. Slowly, the crystals of copper sulphate separate out.
9. Explain how painting of an iron gate prevents it from rusting.
Ans. - Rusting of iron requires contact with water (moisture) and oxygen. By painting an iron gate, we prevent the contact between iron, atmospheric Oxygen and water (moisture), this helps in preventing rusting of iron.
10. Explain why rusting of iron objects is faster in coastal areas than in deserts.
Ans. - Rusting of iron requires contact with water (moisture) and oxygen. In coastal areas, humidity is more in comparison to that in deserts; hence, rusting of iron objects is faster in coastal areas than in deserts.
11. The gas we use in the kitchen is called liquefied petroleum gas (LPG). In the cylinder it exist as a liquid. When it comes out from the cylinder it becomes a gas (Change – A) then it burns (Change – B). The following statements pertain to these changes. Choose the correct one.
(i) Process – A is a chemical change.
(ii) Process – B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change
Ans. - (ii) Process – B is a chemical change.
12. Anaerobic bacteria digest animal waste and produce biogas (Change – A). The biogas is then burnt as fuel (Change – B). The following statements pertain to these changes. Choose the correct one.
(i) Process – A is a chemical change.
(ii) Process – B is a chemical chang
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change
Ans. - (iii) Both processes A and B are chemical changes.
Junior Science Topic - Physical And Chemical Changes. Chapter notes, video lectures, power point presentation and NCERT exercise solution. (Class 7, CBSE syllabus, chapter 06)
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