CHAPTER : PHYSICAL AND CHEMICAL CHANGES
1. what are slow and fast changes?
Ans- The changes that take longer time to complete are called slow changes.
Eg-Rusting of iron, change of seasons.
The changes that take place in very short interval of time are called fast changes.
Eg:- Burning of a cracker , lighting of an electric bulb.
2. what are natural and man made changes ?
Ans- Some of the changes take place in nature by themselves, which are called natural changes.
Eg: change of day and night, earthquakes
Any change that occurs due to efforts of human beings is called man made changes .
Eg : Cooking of food, Formation of Steel from iron.
3.What is periodic and non periodic change?
Ans- Changes that are repeated at regular intervals of time are called periodic changes.
Eg: change of day and night, change of seasons ,etc.
Changes that are not repeated at regular intervals are called non periodic changes.
Eg: earthquakes ,landslides.
4.What are reversible and Irreversible changes ?
Ans- When change the substance can be reversed by changing the conditions it is said to be reversible changes.
If a substance cannot be brought back to its original state after it change it is said to be an Irreversible change.
5. Explain physical change with examples.
Ans- It is the temporary change in which no new substances formed and the chemical composition of the original substance remains the same. Its physical properties like colour, state, shape may change.
Eg: formation of dew ,melting of ice, sublimation of iodine and camphor.
6. What are the characteristics of physical change?
Ans- The characteristics of physical change are as follows:-
(a) No new substance is formed.
(b) There is no change in the chemical composition of the original substance.
(c) The change is temporary and it can be reversed by reversing the conditions
(d) There is no change in the masses of the substance involved in the physical change.
(e) There is usually no loss or gain of energy as a result of physical change.
7.Explain chemical change with examples.
Ans- A chemical change is a permanent change in which new substances are formed whose chemical composition and physical and chemical properties are different from those of the original substance.
Eg: Burning of paper ,Burning of wood.
8. What are the characteristics of a chemical change.
Ans-
(a) New substances are formed.
(b) The change is permanent and Irreversible.
(c) There is usually a change in the mass of the original substance.
(d) Exchange of energy takes place.
(e) The composition of the original substance changes completely.
9. Classify the following as a physical or a chemical change:-
(a) Drying of wet clothes- Physical change
(b) Manufacture of Salt from seawater- Physical change
(c) Butter getting rancid- Chemical change
(d) Boiling of water -Physical change
(e) Burning of paper- Chemical change
(f) Melting of wax- Physical change
(g) Burning of coal -Chemical change
(h) Formation of Clouds -Physical change
(i) Making of a sugar solution -Physical change
(j) Glowing of an electric bulb-Physical change.
10.Fill in the blanks:-
(a) The process of a liquid changing into a solid is called freezing.
(b) A change which alters the composition of a substance is known as a chemical change.
(c) There is no change in the composition of a substance during a physical change.
(d) The reaction in which energy is evolved is called exothermic reaction.
11. Give reason:-
(a) Freezing of water to Ice and evaporation of water are physical changes.
Ans- Freezing of water and evaporation of water are physical changes because if the freezed water or ice cubes are taken out of refrigerator it melts again into water, this indicates that the properties of water and ice are the same (the chemical compositions are same )and in the same way the vapour can be condensed to water again so these changes are called as physical changes which can be reversed.
(b) Burning of a candle is both a physical and chemical change.
Ans- When a candle is lighted some of the solid wax first melts and turns into liquid then turns into vapour to produce a flame .New substances CO2 and H2O vapour are formed along with the evolution of light and heat energy this shows a chemical change.
When some of the molten wax drops to the floor it again solidfiles which shows a physical change. Thus the melting of a candle wax is a physical change and the burning of wax to produce CO2 and H2O is a chemical change.
12.Give four differences between physical and chemical changes.
Ans- PHYSICAL CHANGE
- No new substance is formed.
- Change is temporary.
- The change can be reversed by simple physical methods.
- Heat or light energy may or may not be released or absorbed.
CHEMICAL CHANGE
- Substance with entirely different chemical composition and properties is formed.
- Change is permanent.
- Change cannot be reversed by simple physical methods.
- Heat or light energy or both are given out or absorbed.
13. Write the product and balanced equation of these reactions:-
(a) Fe + S --------->
(b) Mg + O2 --------->
(c) Pb + S --------->
(d) Fe + Cl2 -------->
Ans-
(a) Fe + S --------> FeS
(b) 2Mg + O2 ---------->2MgO
(c) Pb + S ---------> PbS
(d) 2Fe + 3Cl2 ----------> 2FeCl3
14. What is a reagent ?
Ans- A substance used to start a chemical reaction is called a reagent.
It is also one of the reactants .
Common reagents used in the laboratory are hydrochloric acid sulphuric acid and sodium hydroxide.
15. What are qualitative and quantitative properties ?
Ans- Descriptive properties of a substance which cannot be given a mathematical value are called qualitative properties.
Eg-smell, taste, colour.
The properties which can be measured and given a specific mathematical value are called quantitative properties.
Eg- melting point , boiling point mass , solubility and density.
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These notes are very helpful and easy to understand...
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