6. Physical and chemical changes
| 1. | Choose the correct option. (R/U) | ||
| a) | The process by which sugar is obtained from a sugar solution is known as ______________. | ||
| (i) | neutralisation | (ii) | alloying |
| (iii) | crystallisation | (iv) | anodising |
| ans: | Crystallisation |
| b) | A change is considered as a chemical change when it is accompanied by: | ||
| (i) | absorption or release of heat | (ii) | evolution of gas |
| (iii) | formation of a new substance | (iv) | all of the above |
| ans: | all of the above | ||
| c) | The process used to prevent rusting of iron by coating iron articles with zinc is called: | ||
| (i) | electroplating | (ii) | galvanisation |
| (iii) | sedimentation | (iv) | none of the above |
| ans: | galvanisation | ||
| 2. | Classify the following changes into physical and chemical changes. (U) | ||
| a) | Beating of aluminium metal to form aluminium foil. | ||
| ans: | Physical change | ||
| b) | Melting of wax | ||
| ans: | Physical change | ||
| c) | Burning of coal | ||
| ans: | Chemical change |
| 3. | Name the processes involved in each of the following physical changes: (R) |
| a) | Conversion of water to ice |
| ans: | Freezing |
| b) | Conversion of water vapour to water |
| ans: | Condensation |
| c) | Formation of salt from sea water |
| ans: | Evaporation |
| 4. | Assertion and Reason type questions: |
| In the following questions, Assertion and Reason have been put forward. Read the statements carefully and choose the correct alternative from the following: (A) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion. (B) Both Assertion and Reason are true but the Reason is not a correct explanation of the Assertion. (C) Assertion is true but the Reason is false. (D) Both Assertion and Reason are false. (An) | |
| a) | Assertion: Magnesium ribbon when burnt in air forms a white powder of magnesium oxide. |
| Reason: Magnesium ribbon reacts with carbon dioxide present in the air. | |
| ans: | (C) Assertion is true but the Reason is false. |
| b) | Assertion: The blue colour of copper sulphate changes to green on adding a piece of iron. |
| Reason: Iron is placed above copper in the reactivity series. | |
| ans: | (A) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion |
| 5. | Answer the following in one sentence each. (R/U) |
| a) | Name the metals which are mixed (alloyed) with iron to make stainless steel. |
| ans: | Metals like chromium and nickel are mixed (alloyed) with iron to make stainless steel. |
| b) | What are the harmful effects of rusting? |
| ans: | Rusting destroys iron articles. It causes damage to iron structures like bridges, building, rails, etc. that may result is accidents or disasters. This may cause great economic loss. |
| c) | What happens when sodium chloride reacts with silver nitrate? |
| ans: | The reaction between sodium chloride and silver nitrate produces a white precipitate of silver chloride. |
| 6. | Give reasons for the following. (Ap) |
| a) | Painting of an iron gate prevents it from rusting. |
| ans: | Painting of an iron gate prevents the direct exposure of iron to moisture and air and thus prevents rusting. |
| b) | Tearing of paper into pieces is a physical change. |
| ans: | Tearing of paper into pieces is a physical change because when the paper is torn, only the shape and size of the paper is changed, no new substance is formed |
| 7. | Observe the picture and answer the questions given below. |
| a) | Tom’s mother brought some fruits from the market and left them in the fruit basket. No one ate the fruits for 3-4 days. What has happened to the fruits? |
| ans: | The fruits have changed their colour and become mushy./The fruits got spoilt/rotten. |
| b) | Which type of change have the fruits undergone? (Physical change or Chemical change) |
| ans: | The fruits have undergone a chemical change. |
| c) | What could be the reason behind the change in the fruits? |
| ans: | The chemicals present in the fruits react with the oxygen present in the air and change their colour and get rotten/ Microorganisms act on the fruit and cause decay. |
| 8. | Identify the words from the given clues. (R) | |||||||||||||
| a) | A reaction in which an acid and a base react to form salt and water. | |||||||||||||
| ans: | N | E | U | T | R | A | L | I | S | A | T | I | O | N |
| b) | The original substances that take part in a chemical reaction and changes into a new substance/s. | |||||||||||||
| ans: | R | E | A | C | T | A | N | T | S | |||||
| c) | The gas released when vinegar reacts with baking soda. | |||||||||||||
| ans: | C | A | R | B | O | N | D | I | O | X | I | D | E |
| 9. | Observe the experimental set-up shown below and answer the questions that follow. (Ap) |
| a) | In which test tube the iron nail will rust? |
| ans: | Test tube C |
| b) | What are the essential conditions required for rusting? |
| ans: | Rusting of iron requires contact with water (moisture) and oxygen. |
| c) | What is the function of anhydrous calcium chloride? |
| ans: | Anhydrous calcium chloride absorbs the moisture present in the test tube. |
| 10. | Higher Order Thinking Skills (HOTS) (An) |
| Identify the type of reaction given below. | |
| Copper(II) oxide, a black solid, and colourless dilute sulphuric acid react to produce copper(II) sulphate, giving a characteristic blue colour to the solution. Blue copper(II) sulphate pentahydrate crystals are obtained from this solution. | |
| ans: | Neutralisation reaction |
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