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NCERT Class 7 Science Chapter 6 Physical and Chemical Exercises
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Physical and Chemical Exercises
Chapter: 6
NCERT TEXT BOOK EXERCISES
Q. 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 aluminum to make aluminum foil
(f) Digestion of food.
Ans: Physical Change: (b), (d), (e)
Chemical Change: (a), (c), (f)
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.
Ans: False.
(b) Formation of manure from leaves is a physical change.
Ans: False.
(c) Iron pipes coated with zinc do not get rusted easily.
Ans: True.
(d) Iron and rust are the same substances.
Ans: True.
(e) Condensation of steam is not a chemical change.
Ans: True.
Q. 3. Fill in the blanks in the following statements:
(a) When carbon dioxide is passed through limewater, it turns milky due to the formation of _________.
Ans: Calcium carbonate.
(b) The chemical name of baking soda is _________.
Ans: Sodium hydrogen carbonate.
(c) Two methods by which rusting of iron can be prevented are _________ and _________.
Ans: Coating, galvanization.
(d) Changes in which only _________ properties of a substance change are called physical changes.
Ans: Physical.
(e) Changes in which new substances are formed are called _________ changes.
Ans: Chemical.
Q. 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: The reaction between baking soda and lemon juice can be given as below:
Lemon juice + Baking soda →
(Citric acid) (Sodium hydrogen carbonate)
CO2, (bubbles) + Other
Carbon dioxide Substances
It is a chemical change.
Q.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: Physical change: Melting of wax. Chemical change :Burning of candle.
LPG is another example in which physical change occurs. When LPG comes out of cylinder and is converted from liquid to gaseous state and a chemical change occurs when gas burns in air.
Q. 6. How would you show that setting of curd is a chemical change?
Ans: The conversion of milk into curd, i.e., setting of curd is a permanent as well as irreversible and lead to the production of a new substance. The new substance, curd is formed has different composition and properties from the milk. Hence, 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 because, in addition to new products burning is always accompanied by production of heat.
Cutting of wood into small pieces is a physical change because, pieces of wood underwent changes in size and no new substance is formed.
Q. 8. Describe how crystals of copper sulfate are prepared?
Ans: Preparation of crystals of copper sulfate: Take about 100 mL of water in a beaker and add a few drops of dilute sulphuric acid to it. Heat the water over a burner till it boils. Add copper sulphate slowly to the hot water with constant stirring. Continue to add copper sulphate till no more copper sulphate can
Fig. 6.8. Making crystals of copper sulfate.
be dissolved. This will give us saturated solution of copper sulphate. Filter the hot saturated solution of copper sulphate to remove insoluble impurities. Allow the hot and concentrated solution of copper sulfate to cool slowly. Do not disturb the solution when it is cooling. After some time we will see large copper sulfate crystals at the bottom of the beaker.
Separate the copper sulphate crystals from solution by filtration and dry. The soluble impurities present in copper sulphate do not crystallise and hence remain behind in the solution.
Q. 9. Explain how painting of an iron gate prevents it from rusting.
Ans: The process of rusting can be represented by the following equation:
Iron (Fe) + Oxygen (O2, from the air) + Water (H2O) → Rust (iron oxide, Fe2O3) (H₂O)
For rusting the presence of both oxygen and water (or water vapour) is essential.
In fact, if the content of moisture in air is high, which means if it is more humid, rusting becomes faster. So, to prevent iron gate from coming in contact with oxygen, or water, or both one simple way is to apply a coat of paint or grease. In fact, these coats should be applied regularly to prevent rusting.
Q. 10. Explain why rusting of iron objects is faster in coastal areas than in deserts.
Ans: The water of coastal areas contain many salts. The salt water makes the process of rust formation faster. Thus, rusting of iron objects is faster in coastal areas than deserts.
Q. 11. The gas we use in the 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. Choose the correct one.
(i) Process ― A is a chemical change.
(ii) Process ― B is a chemical change.
(ii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.
Ans: (iii) Both process A and B are chemical changes.
Q. 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 change.
(ii) Both process A and B are chemical changes.
(iv) None of these processes is a chemical change.
Ans: (iii) Both process A and B are chemical changes.
ADDITIONAL IMPORTANT QUESTIONS
Very Short Answer Questions
Q. 1. Which property of water changes during its evaporation?
Ans: Physical state.
Q. 2. Why is a physical change consi- dered temporary?
Ans: A physical change is considered temporary because it can be reversed on removing the cause of change.
Q. 3. Magnesium ribbon is burnt in air. Name the white residue left after the reaction.
Ans: Magnesium oxide.
Q.4. What are the necessary conditions for rusting of iron to take place?
Ans: (i) Presence of air (or oxygen).
(ii) Presence of water (or moisture).
Q. 5. Name the process by which common salt is obtained from the sea water.
Ans: Evaporation.
Q. 6. Give two examples of physical changes.
Ans: (i) Melting of ice.
(ii) Boiling of water.
Q. 7. What kind of change is rusting?
Ans: Chemical change.
Short Answer Questions
Q. 8. What is the colour of copper sulphate solution?
Ans: Blue color.
Q. 9. What kind of change takes place when iron reacts with copper sulphate?
Ans: Chemical change.
Q. 10. Name the gas released when baking soda is mixed with vinegar.
Ans: Carbon dioxide.
Short Questions Answer
Q. 1. Formation of clouds is a physical change. Explain.
Ans: Formation of clouds is a physical change. Clouds are formed by condensation of water vapours present in the atmosphere. When rainwater goes back on the earth no new product is formed. Therefore, it is a physical change..
Q. 2. Giving reason, explain which of the following is a physical change:
(i) Electric current is passed through water and hydrogen and oxygen gas are formed.
(ii) Electric current is passed through a bulb and it glows.
Ans: (ii) Electric current is passed through. a bulb and it glows-is a physical change because when current is stopped, bulb does not glow and no new substance is formed.
Q.3. Does the energy change takes place both in physical and chemical changes?
Ans: Yes, all types of changes involve energy. During a change, energy is either evolved or absorbed. Burning of candle is a chemical change and energy is evolved. Evaporation of liquid is a physical change and energy is absorbed.
Q.4. When a grey coloured object made of metal A is left exposed to damp air for a considerable time, it gets covered with a red brown flaky coating by the process called B which eats up the whole object gradually. It is said that the presence of C and D is necessary for this process to take place. If this object is galvanized by metal E, then the process B does not occur.
(a) Name the metal A of which the object is made.
Ans: Iron.
(b) Name the process B.
Ans: Rusting
(c) What are:
(i) C and
(ii) D?
Ans: (i) Oxygen (or air)
(ii) Water (or water vapor).
(d) Name the metal E.
Ans: Zinc.
Q. 5. When an ant stings a person, the solution of substance A is rubbed on the stung area of the skin to get relief from pain. The substance A is also used in kitchen for cooking purposes. Another substance B is sour in taste and contains an organic acid C. It is used in making pickles. When a solution A is mixed with B, then a change D takes place to produce bubbles of gas E.
(a) What are substances:
(i) A and
(ii) B?
Ans: (i) Baking soda.
(ii) Vinegar.
(b) Name the acid C.
Ans: Acetic acid.
(c) What type of change is D?
Ans: Chemical change.
(d) Name the gas E.
Ans: Carbon dioxide.
Q. 6. Why is the tearing of paper a physical change?
Ans: Tearing of paper is a physical change because there is no change in the properties of paper and no new substance is formed. No energy is either absorbed or evolved.
Q. 7. Why is vaporisation of water a physical change?
Ans: When water is heated to boiling, it gets converted into vapor. The vapor so formed when cooled gives back liquid water. The condensed water has all the properties of the original water. So, the vaporisation of water is a physical change.
Short Answer Questions
Q. 1. When magnesium is burned in air, a powdery ash X is formed, X on dissolving in water forms Y.
(a) What are
(i) X, and
(ii) Y?
Ans: (a) (i) Magnesium oxide.
(ii) Magnesium hydroxide.
(b) What is the action of Y on litmus paper?
Ans: Y turns red litmus.
(c) What is the common name of the indigestion relieving medicine which contains Y?
Ans: Milk of magnesia. paper blue.
Q. 2. What happens when iron nails are dipped in copper sulphate solution?
Ans: When iron nails are dipped in copper sulphate solution, then after some time a brown layer of copper gets deposited on iron nail. This change is due to chemical reaction between copper sulphate and iron. The colour of copper sulphate solution also changes from blue to green colour due to formation of iron sulphate.
Iron nail + Copper sulphate solution →
(blue)
Iron sulfate + Copper
(green) (brown)
Q.3. What happens when an iron blade of a knife is dipped in copper sulphate solution? What kind of chemical reaction takes place?
Ans: When an iron blade of a knife is dipped in copper sulfate solution iron blade is coated with reddish deposit of copper. Thus, we can say that iron (more reactive element), displaces copper (less reactive element), from its aqueous copper sulphate solution. Chemical displace-ment reaction takes place. It is a chemical change.
Q.4. Why is the conversion of milk into curd a chemical change?
Ans: During the conversion of milk into curd many reactions involving proteins, organic acids and fat takes place.
The taste and other properties of curd are different from those of milk. It is not possible to obtain milk from the curd. Therefore, the con- version of milk into curd is a chemical change.
Q. 5. Identify the type of change and state whether energy is evolved or absorbed. in each one of the following: Burning of a candle, lighting of a bulb, preparation of food by green plants.
Ans: (i) Burning of a Candle: Chemical change, energy evolved.
(ii) Lighting of a Bulb: Physical change, energy evolved.
(iii) Preparation of Food by Green Plants: Chemical change, energy absorbed.
Long Answer Questions
Q. 1. Classify the following in a chemical or physical change:
(i) Cutting of trees.
Ans: Cutting of trees: Physical change.
(ii) Melting of butter in a pan.
Ans: Melting of butter in a pan: Physical change.
(iii) Water boils to form steam.
Ans: Water boils to form steam: Physical change.
(iv) Electric current is passed through water and it is broken down into hydrogen and oxygen.
Ans: Electric current is passed through water and it is broken down into hydrogen and oxygen: Chemical change.
Q. 2. (a) What is rusting? What is rust?
Ans: The formation of brown powdery material on the surface of iron in the presence of moist air is called rusting of iron.
Iron when exposed to moist air gets covered with a layer of brown powdery material called rust.
(b) Write the chemical reaction that occurs during rusting.
Ans: Rust is mainly hydrated iron(III) oxide.
(b) Iron + Oxygen + Water →
(from air) (from air)
Hydrated iron (III) oxide
(rust)
(c) What are the conditions required for rusting to take place?
Ans: Conditions required for rusting:
(i) Presence of air (or oxygen).
(ii) Presence of water (or moisture).
(d) Describe galvanisation of iron.
Ans: Coating of an iron object with a thin layer of zinc is called galvanisation. Galvanisation of iron prevents its rusting.
Q. 3. Write five differences between chemical and physical changes.
Ans:
| Chemical Change | Physical Change |
| 1. A new substance is formed. | 1. No new substance is formed. |
| 2. It is a permanent change. | 2. It is a temporary change. |
| 3. The composition of new changes. substance | 3. No change in the composition of change takes place. |
| 4. It is irreversible. | 4. It is reversible. |
| 5. Heat/light evolved or absorbed during change. | 5. No heat/light evolved or absorbed or may be evolved or absorbed. |
HOTS QUESTIONS
Q. 1. On heating, calcium carbonate gets converted into calcium oxide and carbon dioxide.
Is this a physical or a chemical change?
Ans: Chemical change.
Q.2. Classify each of the following as a physical or a chemical change. Give reasons.
(a) Drying of a shirt in the sun.
(b) Rising of hot air over a radiator.
(c) Burning of kerosene in a lantern.
(d) Change in the color of black tea on adding lemon juice to it.
(e) Churning of milk cream to get butter.
Ans: Physical changes: (a), (b), (e)
Chemical changes: (c), (d),
Q.3. When electricity is passed through water, then a change V Occurs which leads to the formation of two gases W and X. On the other hand, when water is heated strongly, then a change Y takes place leading to the formation of gas Z.
(a) What are gases:
(i) W
(ii) X and
(iii) Z?
Ans: (i) Hydrogen.
(i) Oxygen.
(iii) Steam.
(b) What type of change is:
(i) V and
(ii) Y?
Ans: (i) Chemical change.
(ii) Physical change.
SKILL BASED QUESTIONS
Q. 1. Observe the following figure and answer the questions that follow:
Fig. 6.9.
(i) What does this figure show?
Ans: This figure shows the action of acetic acid over baking soda.
(ii) Name the gas released.
Ans: The gas is carbon dioxide.
(iii) Write the effect of the gas on the lime water.
Ans: The lime water becomes milky.
(iv) Write the reaction.
Ans: (iv) Lime water + Carbon dioxide → Calcium carbonate (milky) + Water
Q. 2. Observe the diagram given below and suggest a suitable title for the diagram. Can we prevent the process depicted in the figure 6.10?
Fig. 6.10, Rusting of iron.
Ans: (i) Rusting of iron.
(ii) Rusting of iron can be prevented
(a) by applying paint or grease on the surface of iron.
(b) by depositing a layer of metal like chromium or zinc on iron.
FORMATIVE ASSESSMENT
1. Fill in the following blanks with suitable words:
(i) Changes in which new substances are
formed are called __________ changes.
Ans: Chemical.
(ii) Melting of wax is a ________ change but burning of wax is a _______ change.
Ans: Physical, chemical.
(iii) Souring of milk is a ________ change.
Ans: Chemical.
(iv) The chemical name of baking soda is _________.
Ans: Sodium hydrogen carbonate.
(v) When carbon dioxide is passed through lime water, it turns milky due to the formation of __________.
Ans: Calcium carbonate.
(vi) The two methods by which rusting of iron can be prevented are _______ and _________.
Ans: Painting, gal- vanisation.
(vii) The process of depositing a thin layer of zinc on iron objects is called ________.
Ans: Galvanisation.
(viii) The presence of _________ in sea water makes the process of rust formation on ships faster.
Ans: Salt.
2. State whether the following statements are true or false:
(i) Cutting a log of wood into pieces is a chemical change.
Ans: False.
(ii) Formation of manure from leaves is a physical change.
Ans: False.
(iii) Condensation of steam is not a chemical change.
Ans: True.
(iv) Iron and rust are the same substance.
Ans: False.
(v) Iron pipes coated with zinc do not get rusted easily.
Ans: True.
(vi) During burning of fuels, heat energy is liberated.
Ans: True.
(vii) Souring of milk is a physical change.
Ans: False.
(viii) Evaporation of water is a chemical change.
Ans: False.
3. Matching Type Questions:
| Column – A | Column – B |
| (i) Rust | (a) physical change |
| (ii) The condensation of steam | (b) hydrated(III) oxide |
| (iii) The burning of paper | (c) crystallization |
| (iv) Coating of an iron object with a thin layer of zinc | (d) galvanisation |
| (v) The process of formation of crystals | (e) chemical change |
Ans:
| Column – A | Column – B |
| (i) Rust | (b) hydrated(III) oxide |
| (ii) The condensation of steam | (a) physical change |
| (iii) The burning of paper | (e) chemical change |
| (iv) Coating of an iron object with a thin layer of zinc | (d) galvanisation |
| (v) The process of formation of crystals | (c) crystallization |
4. Multiple Choice Questions:
(i) The layer of rust is actually deposit of:
(a) iron (III) oxide
(b) mud
(c) water
(d) oxygen
Ans: (a) Iron(III) oxide.
(ii) Which of the following is not a chemical change?
(a) burning of coal
(b) digestion of food
(c) melting of ice
(d) rusting of iron
Ans: (c) Melting of ice.
(iii) Which of the following is not a physical change?
(a) Melting of wax
(b) Tearing a piece of paper
(c) Dissolution of salt in water
(d) Digestion of food
Ans: (d) Digestion of food.
(iv) Which of the following is a chemical change?
(a) Breaking of a brick
(b) Evaporation of a liquid
(c) Burning of a candle
(d) Moving of a wheel.
Ans: (c) Burning of a candle.
(v) Which of the following is a physical change?
(a) Glowing of an electric bulb
(b) Formation of curd from milk
(c) Cooking of food
(d) Growth of a plant
Ans: (a) Glowing of an electric bulb.
(vi) In which type of change a new substance is formed?
(a) physical change
(b) chemical change
(c) both in physical and chemical change
(d) none of the above
Ans: (b) Chemical change.
5. Write one word for the following:
(i) A change that can be reversed by reversing the conditions.
Ans: Reversible change.
(ii) A change in which only the physical properties of any substance get changed.
Ans: Physical change.
(iii) The process in which a metal is displaced by a more reactive metal.
Ans: Metal displacement reaction.
(iv) A change which is irreversible, permanent and leads to the formation of one or more new substances.
Ans: Chemical change.
(v) Prevention of rusting of iron by coating with zinc.
Ans: Galvanisation.
(vi) A flaky brown deposit formed over the surface of iron in the moist air.
Ans: Rust.
(vii) The process of obtaining crystals from its hot saturated solution.
Ans: Crystallisation.
(viii) The process of giving a thin coating of tin on metals.
Ans: Tinning.
