class 7 - Science - Ch6 - Physical and Chemical Changes

NCERT Solutions and other Q & A

Melting of Ice Cream (Physical Change)
(Source: openclipart.org)

Q1: What do you mean by physical properties of matter?

Answer: A physical property can be measured and observed without changing the composition or identity of a substance. E.g. Odour, colour, volume, state (gas, liquid, or solid), density, melting point, and boiling point are physical properties of a substance.

Q2: What do you mean by physical changes?

Answer: Physical changes are changes in the physical properties of substances. No new substances are formed in these changes. These changes may be reversible. E.g. melting of ice.


Q3: Define chemical properties of  matter.
Answer: Chemical property of a substance is its ability to form new substance.

Q4: What do you mean by chemical change? Give an example.
Answer: A change in which one or more new substances are formed is called a chemical change. A chemical change is also called a chemical reaction. In general, chemical changes are irreversible. The chemical properties of the substance formed are different from the original substances.

E.g. Burning of wood is a chemical change. It gives off heat and gases and leaving a residue of ashes. In this process, the wood is changed to several new substances.


Q5: 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

Answer:

Physical Changes	Chemical Changes
Dissolving sugar in water Melting of wax Beating aluminium to make aluminium foil	Photosynthesis                                    Burning of coal Digestion of food
(No new substances are formed)	(new substances are formed)

Q6: State whether the following statements are true or false. In case a statement is false, write the corrected statement in your notebook.
Answer:

(a) Cutting a log of wood into pieces is a chemical change. (True/False) 
Correction: Cutting of wood is a physical change because identity and composition of wood does not change.

(b) Formation of manure from leaves is a physical change. (True/False)
Correction: Formation of manure from leaves is a chemical change because manure formed has a different composition from leaves.

(c) Iron pipes coated with zinc do not get rusted easily. (True/False)

(d) Iron and rust are the same substances. (True/False)
Correction: Iron and rust are two different substances. Rust is Iron Oxide.

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


Q7. Fill in the blanks in the following statements:
Answer:

(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of _Calcium Carbonate (CaCO₃)_.

(b) The chemical name of baking soda is _Sodium Bicarbonate (NaHCO₃)_.

(c) Two methods by which rusting of iron can be prevented are _painting or greasing__ and _galvanizing with Chromium or Zinc_.

(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.

Q8: 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:
When baking soda (sodium bicarbonate) is mixed with lemon juice (citric acid), a chemical change occurs. In this recation, new substances like Carbon di-oxide is formed and heat is evolved. This change is irreversible.

Sodium BiCarbonate  +  Citric Acid  →  Sodium Citrate  + Carbon DiOxide + Water + Heat

Q8: 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:
Physical Changes in Burning Candle: On heating, candle wax melts is a physical change. Since it again turns into solid wax on cooling. The change is reversible.
Chemical Changes in Burning Candle: The wax near to flame burns and gives new substances like Carbon Dioxide, Carbon soot, water vapours, heat and light.



Cooking of food, hard-boiling of egg are examples where both physical and chemical changes occurs. In both cases the physical appearance of the substances change and new substances are formed.

Q9: How would you show that setting of curd is a chemical change?
Answer:  Setting of curd is a chemical change since a new substance is formed which differs in taste from the original substance i.e. milk. Also the change is irreversible (you cannot convert curd into milk). The composition of curd (lactic acid) is different from milk (fats and proteins).

Q10: 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 wood on burning reduces to new substances like ash(carbon), carbon dioxide gas, water vapours, heat and light. The change is irreversible.

While cutting of wood in smaller pieces is a physical change because the original composition of wood does not change. No new substances are formed.

Q11: Describe how crystals of copper sulphate are prepared.
Answer:

1. Take a cupful of water in a beaker and add a few drops of dilute sulphuric acid.
2. Heat the water. When it starts boiling add copper sulphate powder slowly while stirring continuously.
3. Continue adding copper sulphate powder till no more powder can be dissolved.
4. Filter the solution. Allow it to cool.
5. Do not disturb the solution when it is cooling.
6. You can see you see the crystals of copper sulphate after few hours.

(Watch the you tube video showing preparation of copper sulphate crystals.)


Q12: Explain how painting of an iron gate prevents it from rusting.
Answer: For rusting, the presence of both oxygen and water (or water vapour) is essential. The coat of the paint prevents direct contact of iron with air and oxygen and thus prevents it from rusting.


Q13: Explain why rusting of iron objects is faster in coastal areas than in deserts.
Answer: For rusting, the presence of both oxygen and water (or water vapour) is essential.

Iron (Fe) + Oxygen (O2, from the air) + water (H2O) → rust (iron oxide Fe2O3)

In coastal areas the content of moisture in air is high (highly humid) as compared to desert regions (low humid), rusting becomes faster.

Q14: The gas we use in the kitchen is called liquified 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.
Answer:  (ii) Process – B is a chemical change.
Explanation:
(i) Process – A is a physical change. The LPG in cylinder is in liquid form because of high pressure. When it comes from cylinder, it turns into gas. It is a physical change.
(ii) Process – B is a chemical change. Burning of gas is a chemical change.


Q15: 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 change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.

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

Study Notes and Q & A

Q16: What are the characteristics of Chemical changes?

Answer:

1. Chemical change, in general is a permanent change.
2. This change is generally irreversible.
3. New substances are formed .
4. Energy in the form of heat or light is either absorbed or released.
5. The chemical composition of the new substance differs from that of the initial substance.