PHYSICAL AND CHEMICAL CHANGES Class 7th Science Lesson 6

 NOTES PREPARED BY 

ASHAQ HUSSAIN BHAT 

TEACHER SCHOOL 

EDUCATION DEPARTMENT 

JAMMU AND KASHMIR

6:- PHYSICAL AND CHEMICAL CHANGES

points to Remember

• Change is the nature of life.

• Many changes occur in nature daily.

• Changes may be classified as Physical change and Chemical change.

• There is always a cause for a change.

• cannot be controlled.

• Some changes can be controlled while some other changes

• Physical change is a temporary change and can be reversed.

• In a physical change, no new substance is formed.

• A chemical change is a permanent change and cannot be easily reversed.

• In a chemical change, new substances with entirely different properties are formed.

• Changes can be grouped together by finding similarities  between them.

• Properties such as shape, size, colour and state are called physical properties

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

• Ribbon of magnesium burns with a brilliant white Iight

• When carbon dioxide is passed through lime wate., lime water turns milky.

•  Chemical changes may accompany with sound, light, heat, smell, gas, colour change.

• . Burning is a chemical change and is always accompanied by production of heat.

•  Ozone acts as a protective layer in the atmosphere.)

• Rusting needs the presence of both oxygen and water.

•  Iron is protected by depositing a layer of zinc on it and this process of depositing zinc on other metal is called galvanization

• Stainless steel is a mixture of carbon, chromium, nickel and manganese.

• Iron can also be painted to avoid rusting.

• Crystallization is the process of obtaining large particles of definite geometrical shape of a compound from its saturated solution.

IMPORTANT DEFINITIONS

Physical change. The change in which physical properties of substance change and no new substance with new properties are formed is called physical change, e.g. powdering a lump of sugar.

Chemical change. The change due to which the substance changes completely to form new substances having new properties is called chemical change, e.g. burning of paper.

Rusting. The process of acquiring brownish film by iron when kept in moist air is called rusting.

Galvanization. The process of depositing a layer of zinc on iron to prevent it from rusting is called galvanization.

Crystallization. The process of obtaining large crystals of a substance from its solution is called crystallization.

TEXT BOOK EXERCISES

Q. 1. Classify the changes involved in the following process as physical or chemical change :

(a) Photosynthesis
(6) 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) Photosynthesis-Chemical change.

(6) Dissolving sugar in water-Physical change

(c) Burning of coal-Chemical change

(d) Melting of wax-Physical change

(e) Beating aluminium to make aluminium foil—Physical change

(f) Digestion of food-Chemical change.

Q. 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. (True/false)
(b) Formation of manure from leaves is a physical change. (True/false)
(c) Iron pieces coated with zinc do not get rusted easily. (True/false)
(d) Iron and rust are the same substances. (True/false)
(e) Condensation of steam is not a chemical change. (True/false)

Ans.(a) False--cutting a log of wood into pieces is a physical change.

(6) False--formation of manure from leaves is a chemical change.

(c) True--Iron pieces coated with zinc do not get rusted easily.

(d) False-Iron and rust are not same. Rust is iron oxide.

(e) True-Condensation of steam is not a chemical change.

Q. 3. Fill in the blanks in the following statements 

(a) When carbondioxide is passed through lime water, it turns milky due to 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.

Ans. (a) Calcium carbonate (b) Sodium bi-carbonate

(c) Painting, galvanization (d) physical.

Q. 4. When baking soda is mix with lemon juice, bubbles are formed with the evolution of gas. What type of a change is it? Explain.

Ans. When baking soda (a base) is mixed with lemon juice (a weak acid) carbondioxide is evolved which forms bubbles. This change is an example of chemical change.

Q. 5. When a candle burns, both physical and Give another example of familiar process in which both chemical changes take place. Identify these changes: chemical and physical changes take place.

Ans. When a candle burns, wax melte. It is a

change and some wax vapours along with wick burn in Oxygen to give carbon dioxide, which is chemical change. Cooking gas (LPG) when burnt first gets changed into its gaseous state (Physical change) and then it burns in oxygen to give carbondioxide (Chemical change). So burning of LPC is an example in which both chemical and physical changes take place

Q. 6. How would you show that setting of a curd is a chemical change ?

Ans. When curd is set, the new substance formed is entirely different from milk. The change is a permanent change and cannust be reversed to its constituents. So, setting of curd is a chemical

change.

Q. 7. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.

Ans. Burning of wood is a chemical change as wood on burning produces gases and ash.

Cutting of wood is a physical change as wood pieces obtained have same physical properties.

Q. 8. Describe how crystals of copper sulphate are prepared.

Ans. Preparation of copper sulphate crystals. Boil a cup of water in a beaker. Add few drops of dilute sulphuric acid. Dissolve copper sulphate powder slowly and stirring it continuously. Continue adding copper sulphate till no more as 

salt can be dissolved. Filter the solution and keep it aside for some time when large crystals of copper sulphate will be formed.

Q. 9. Explain how painting of an iron gate prevents it from rusting.

Ans. Prevention of iron gate from rusting by

painting. Rusting occurs in the presence of both water and air. If a coat of paint is applied to iron gate, then its contact with air is broken. So rusting is prevented,

Q. 10. Explain why rusting of iron objects is faster in coastal areas than in deserts.

Ans. Faster rusting in coastal areas. It is due to the presence of humid air, which increases the rate of rusting. But on the other hand in deserts, air is dry, which inhibits the rate of rusting.

Q. 11. The gas we use in kitchen is called liquefied 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 burns (Change B). The following statements pertain to these changes. To Choose the correct one:
(1) 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 is a chemical change.

Ans. (ii) Process-B is a chemical change.

Q. 12. Anaerobic bacteria digests animal waste and produces 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 change.
(iii) Both processes A and B are chemical changes.

Ans. (iii) Both processes A and B are chemical changes.

Choose the correct one:

Q. 13. Rust is :

(a) Carbon dioxide

(6) Iron

(c) Oxygen

(d) Iron oxide.

Ans. (d) Iron oxide.

Q. 14. Which of the following is not a physical change?

(a) Rusting of iron

(b) Melting of ice

(c) Freezing of water

(d) Dissolving sugar in water.

Ans. (a) Rusting of iron.

LONG ANSWER QUESTIONS

Q. 1. Distinguish between physical and chemical

changes.

Ans. Distinction between Physical and Chemical

changes :

S.No. Physical change           Chemical change

1. It is temporary change.      1. It is a permanent

                                                         change.

2. No new substance is            2. New substances with

formed.                                            new properties are                                                                                         formed.

3.It can be reversed by             3. It cannot be reversed bysimple physical                        simple physical   methods

methods.                               

methods.

4. No change in the composi-    4. The composition        of thetion of the substance                new    substance changes   takes place.                                      Greatly

5. No energy changes are           5. Usually energy changes

involved.                                          are involved.

Q. 2. What is crystallization ? How will you prepare crystals of alum ?

Ans. Crystallization. It is the process of separating a pure substance in the form of crystals having well defined geometrical shape from its hot saturated solution by cooling. This process is very commonly used to purify solid substances.

 To prepare crystals of alum 

Take some distilled water in a beaker. Go on adding alum powder slowly simultaneously with continuous stirring of the solution with a glass rod. Heat the beaker gently and try to dissolve more alum powder till it stops dissolving. Filter the hot solution in a glass beaker to cool. Observe the solution. Well defined crystals of alum will be obtained after sometime.

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