Term 2 Science syllabus

CHAPTER 6
PHYSICAL AND CHEMICAL CHANGES

EXTRA QUE/ ANS

Que 1: Define rusting of iron with reaction.

Ans: If we leave a piece of iron in the open for some time, it acquires a film of
brownish substance. This substance is called rust and this process is called
rusting.

Reaction:- 4Fe + 3O2 + 2H2O ---→ 2Fe2O3 . 2H2O
(Water) Rust(Iron oxide)
(Iron) (Oxygen)

For rusting, the presence of both oxygen and water (or water vapour) is
essential.

Que2: What are the different methods of preventing rusting of iron?

Ans: Different methods of preventing rusting of iron are listed below:-

1. By painting

2. By greasing

3. By galvanisation

4. By alloying

Que3 : Define galvanization. Give one use of it.

Ans: The process of depositing a layer of zinc on iron is called galvanization.

USES: The iron pipes we use in our homes to carry water are galvanized to
prevent rusting.

Que4. Write down the chemical reaction between vinegar and baking

soda.

Ans Vinegar + Baking Soda ----→ CO2 + Other Substances

(Acetic Acid) (Sodium Bicarbonate) (Carbon dioxide)

Que5. What happens when iron nail is dropped in copper sulphate
solution? What type of change is it?

Ans: When iron nail is dropped in copper sulphate solution, Chemical change
takes place. The colour of the solution changes from blue to green due to the
formation of iron sulphate and there is a brown deposit of copper on iron nail.

Reaction : Fe + CuSo4 ------→ FeSO4 + Cu
(Iron)
(Copper sulphate) (Iron sulphate) (Copper)

(Blue) (Green)

Que6. Explain how following are chemical change.

Ans: 1. Burning of a substance:- Burning of a substance is a chemical change
because during burning, new substance like carbon dioxide , ash etc. are
formed with the production of heat.

2. Explosion of fire cracker :- Explosion of a firework is a chemical change
because explosion produces heat , light , sound and unpleasant gases that
pollute the atmosphere which are new substance.

3. Spoiling of food :- It is a chemical change because when food gets spoiled, it
produces a foul smell with the formation of new substances.

4. A slice of an apple acquires brown color:- It is a chemical change because a
slice of an apple acquires brown color due to the formation of new substances.

5. Neutralisation :- It is a chemical change because in neutralization, an acid
reacts with base to form new substances like salt and water with the
production of heat.

Que7 . Define Physical and Chemical change.

Ans Physical change :- A change in which a substance undergoes a change in its
physical properties like shape , size , state etc is called a physical change. In
such a change no new substance is formed.

Example:- Melting of ice , Melting of wax , Tearing of paper , Boiling of
wateretc.

Chemical change :- A change in which one or more new substances are formed
is called a chemical change. A chemical change is called irreversible change.

Example:- spoiling of food , burning of wood , formation of curd etc.

Que8. Differentiate between chemical and physical changes.

Ans:

Physical changes Chemical changes

1. No new substance is formed. 1. New substance is formed.

2. It is a temporary change. 2. It is a permanent change.

3. It is generally reversible. 3. It is generally irreversible.

4. Very little energy is obtained or 4. A lot of energy is absorbed or given
given out in this change. out.

Que9. Give standard test of the evolution of carbon dioxide with the
help of reaction.

Ans: When carbon dioxide is passed through lime water, calcium carbonate is
formed, which makes lime water milky. The turning of lime water into milky is
a standard test of carbon dioxide.

Reaction : Carbon dioxide + lime water ---→ Calcium carbonate + water

(CO2) [Ca(OH)2] (CaCO3) (H2O)

Que10. What is rust? Give example.

Ans: If we leave a piece of iron in the open for some time, it acquires a film of
brownish substance. This substance is called rust.

For Example: 1. Iron gates of parks or farm lands, iron benches kept in lawns
and gardens , almost every article of iron , kept in open gets rusted.

2. Shovels and spades get rusted when exposed to the atmosphere for some
time.

3. In the kitchen , a wet iron pan (tawa) often gets rusted if left in that state for
some time.

Que11: Give an activity to show that burning of magnesium ribbon is
a chemical change.

Ans: Activity:

1. Take a small piece of a thin strip or ribbon of magnesium.

2. Clean its tip with sand paper.

3. Bring the tip near a candle flame.

4. It burns with a brilliant white light.

5. When it is completely burnt it leaves behind a powdery ash.

Reaction:- 2Mg + O2 ----→ 2MgO

(Magnesium) (Oxygen) (White powdery ash)

The magnesium oxide formed is a new substance. So, burning of magnesium
ribbon is a chemical change.

Que12: In addition to new substance what may accompany a chemical
change?

Ans: In addition to new products, the following may accompany a chemical
change.
1. Heat, Light or any other radiation (ultraviolet for example) may be given off
or absorbed.
2. Sound may be produced.
3. A change in smell may take place or a new smell may be given off.
4. A color change may takes place.
5. A gas may be formed.

Que13: Define crystallization. What type of change is it?

Ans: Crystallization :- The process of obtaining pure crystals of a new
substance from the solution is called crystallization. It is a physical change.

Que14. What is stainless steel? How it is made?

Ans: Stainless steel is an alloy of iron. Stainless steel is made by mixing iron
with carbon and metals like chromium, nickel and manganese. It does not
rust.

Que15. Can we call a breaking down of O3 (ozone) is a chemical
change.

Ans: Yes, breaking down of ozone is a chemical change because Ozone allows
ultraviolet radiations and breakdown to oxygen. Oxygen is different from
ozone.

Ultraviolet

Reaction:- 2O3 --------------------------------→ 3O2
Radiation