Class 7 Science
Chapter 6 – Physical and Chemical Changes
Very Important Questions Answers - Set 4
Q.1. If an equation has '+ heat' written on its right-hand side, what type of reaction is it?
Ans: '+ heat' sign signifies that the reaction is accompanied with the evolution of heat. The reaction is exothermic.
Q.2. Identify what kind of reaction each of the following is:
(a) X + Y → XY
(b) XY → X + Y
(c) AB + XY → AY + XB
(d) AB + X → AX + B
(e) AB + CD → AD + BC
(f) Acid + base → salt + water
Ans:
(a) Combination reaction
(b) Decomposition reaction
(c) Double displacement reaction
(d) Single displacement reaction
(e) Double displacement reaction
(f) Neutralisation reaction
Q.3. What method will you use to get a pure liquid if it has dissolved impurities present in it?
Ans: We will use crystallisation method to obtain a pure liquid if it has dissolved impurities in it.
Q.4. Why are water pipes galvanized?
Ans: Water pipes are made up of iron. Rusting of iron occurs when iron interacts with air and water. Therefore, to prevent rusting of iron, water pipes are galvanized, i.e., iron pipes are coated with metals such as zinc or chromium to prevent iron from interacting with air.
Q.5. When water is frozen, it forms ice that has different properties. Why then do we say that freezing is physical change?
Ans: In a physical change, only the physical properties of a substance change and no new substances are formed.
Freezing of water is a physical change because only the physical state of water is changed; i.e., water is converted into ice. Hence, freezing is a physical change.
Q.6. Give one example each of a chemical reaction showing the following characteristics.
(a) evolution of a gas
(b) change of colour
(c) formation of precipitate
(d) change of state from liquid to gas
(e) change of state from gas to liquid
Ans:
(a) Zn + H2SO4 → ZnSO4 + H2↑
Zinc Sulphuric acid Zinc sulphate Hydrogen
(b) CuCO3 → CuO + CO2↑
Copper carbonate Copper oxide Carbon dioxide
(green) (black)
(c) CuSO4 + H2S → CuS↓ + H2SO4
Copper Hydrogen sulphide Copper sulphide Sulphuric acid
(d) 2H2O (l) → 2H2 (g) + O2 (g)
Water Hydrogen Oxygen
(e) 2H2 (g) + O2 (g) → 2H2O (l)
Hydrogen Oxygen Water
Q.7. Distinguish between endothermic reactions and exothermic reactions.
Ans:
|
Endothermic Reaction |
Exothermic Reaction |
|
These reactions are accompanied with absorption of heat. |
These reactions are accompanied with evolution of heat. |
|
Melting of solids, fusion, evaporation and vaporisation are examples of endothermic reaction. |
Combustion of petrol and oxidation of ammonia are examples of exothermic reaction. |
Q.8. What is a combination reaction? Give an example.
Ans: In a combination reaction, a compound is formed from its elements or from simpler substances. For example, magnesium reacts with oxygen on heating to form magnesium oxide.
2Mg + O2 → 2MgO
Q.9. When is a substance said to be reduced in a chemical reaction?
Ans: In a chemical reaction, a substance is said to be reduced if hydrogen is added to it or oxygen is removed from it.
Q.10. What is crystallization?
Ans: Crystallisation is a process in which pure solid is separated from a solution in the form of its crystals.
For example, common salt obtained from sea water contains soluble impurities such as magnesium chloride. To purify common salt, crystallisation process is applied.
Q.11. What is a supersaturated solution?
Ans: A solution in which more solute is dissolved by increasing the temperature of a saturated solution is called supersaturated solution. When a supersaturated solution is cooled to room temperature, it crystallises easily.
Q.22. Discuss the methods by which rusting of iron can be prevented.
Ans: Rusting of iron can be prevented by following methods:
(a) Painting: A coat of paint applied on the surface of an iron object prevents oxygen and moisture present in the atmosphere from interacting with the iron object.
(b) Oil or grease: Applying oil or grease on the surface of iron tools helps to keep out the oxygen and moisture of the atmosphere.
(c) Galvanization: Galvanization is a process of coating iron objects with a thin layer of metal such as zinc or chromium. A thin coating of zinc or chromium protects the iron from rusting.
Q.13. Which gas is given off when dilute hydrochloric acid is added to calcium carbonate?
How will you test the gas? Write the chemical equations for both reactions.
Ans: Carbon dioxide is given off when dilute hydrochloric acid is added to calcium carbonate. The equation involved is:
CaCO3 + HCl → CaCl2 + H2O + CO2↑
Carbon dioxide can be tested by passing it over lime water. Lime water turns milky. The equation involved is:
Ca(OH)2 + CO2 → CaCO3 + H2O
Lime Carbon dioxide Calcium carbonate Water
The calcium carbonate formed in the reaction is white in colour and not much soluble in water. It gives milky appearance and is used as a test for the presence of carbon dioxide.
Q.14. Explain the steps involved in preparing the base magnesium hydroxide from the metal magnesium.
Ans: The following steps are involved in preparing the base magnesium hydroxide from the metal magnesium:
(a) Take a small piece of magnesium ribbon.
(b) Clean its tip with sandpaper.
(c) Hold it with a pair of tongs and bring it near the flame. It burns with a dazzling flame and forms a powdery ash (magnesium oxide).
(e) Collect the ash in a test tube and mix it with a small amount of water and stir.
When magnesium oxide dissolves in water, it forms magnesium hydroxide. The equation involved is:
2MgO + 2H2O → 2Mg(OH)2
Magnesium oxide Water Magnesium hydroxide
Q.15. A decomposition reaction is the opposite of a combination reaction. Explain with the help of an example.
Ans: The reaction in which a compound breaks down to give two or more simpler substances is called a decomposition reaction.
For example, on passing an electric current through water in which a small amount of sulphuric acid has been added, water decomposes to give hydrogen and oxygen gases.
2H2O → 2H2 + O2
The reaction in which two or more elements combine to form a single compound is called a combination reaction.
For example, hydrogen combines with oxygen to form water.
2H2 + O2 → 2H2O
From the above examples, it is clear that a decomposition reaction is the opposite of a combination reaction.
Q.16. In which city do you expect rusting to be a greater problem-in Delhi or in Mumbai? Why?
Ans: Two conditions are required for rust formation: oxygen and moisture content in the atmosphere. The climate of Delhi is hot and dry, whereas the climate of Mumbai is humid. Hence, rusting is a greater problem in Mumbai because there is a greater presence of moisture and oxygen in the atmosphere.
Q.17. When a candle burns, both physical and chemical change occur. What are these changes?
Ans: Melting of wax is a physical change and formation of carbon dioxide and water vapour is a chemical change. Therefore, both physical and chemical changes occur when a candle burns.
Q.18. LPG (liquefied petroleum gas) in a gas cylinder is in the form of a liquid. Which physical and chemical changes occur when it comes out of the cylinder and burns?
Ans: LPG in a cylinder exists as a liquid. When it comes out of the cylinder, it becomes a gas, which is a physical change. Burning of LPG is a chemical change.
Q.19. When a soda water bottle is opened, a gas is released. Does a chemical reaction occur? Explain.
Ans: No, the chemical reaction does not occur. When a soda bottle is opened, carbon dioxide is released. The carbon dioxide is already dissolved inside the soda bottle. On removing the cap, the pressure inside is released and carbon dioxide escapes.
Q.20. Why should we not put stainless steel spoons in pickle jars?
Ans: Steel reacts with acids present in the pickle and forms rust. Hence, we should never put stainless steel spoons in pickle jars.
Q.21. Nancy's mother complained that white deposits form on all her utensils used for cooking. Nancy knows that the deposits are of calcium carbonate that gets left behind when water in the utensils evaporates. She also knows that calcium carbonate reacts with hydrochloric acid. However, she realises that hydrochloric acid is not easily available and is too dangerous for her to use in the kitchen to remove the stains.
Suggest to Nancy a method that uses the same principle as she thought of, but solves the two problems she has with hydrochloric acid.
Ans: As hydrochloric acid is not easily available and is too dangerous to use in normal daily use, so, in place of hydrochloric acid, Nancy can use citric acid which is easily available and does not cause much harm. Citric acid can easily be obtained from the lemon.
Q.22. Under which two conditions do decomposition reactions normally take place? Give one example of each.
Ans: The two conditions under which decomposition reaction normally takes place are as follows:
(a) Heating: When calcium hydroxide is heated, calcium oxide and steam are produced.
Ca(OH)2 → CaO + H2O
Calcium hydroxide Calcium oxide Water
(b) Electrolysis: When an electric current is passed through a molten aluminium oxide, it undergoes decomposition to form aluminium and oxygen gas.
2Al2O3 → 4Al + 3O2
Q.23. What is a displacement reaction? Which rule governs displacement of an element in a compound by another element? Explain with an example.
Ans: In a displacement reaction, one element replaces another element from a compound. Generally, a more reactive element replaces a less reactive element from its compound. For example, iron is more reactive element than copper. If a piece of iron is added to a solution of copper sulphate, iron sulphate is formed. Copper metal gets deposited on the iron piece.
CuSO4 + Fe → FeSO4 + Cu
Copper sulphate Iron Iron sulphate Copper
Q.24. What is a double decomposition reaction? How many types of double decomposition reaction do you know of? Give one example of each.
Ans: A reaction in which two compounds react to form two other compounds by mutual exchange of atoms or group of atoms is called double displacement reaction. There are two types of double displacement reaction.
(a) Precipitation reaction: In this reaction, two compounds react in their aqueous state to form an insoluble product, which appears in the form of a precipitate. For example, aqueous sodium chloride and silver nitrate react to form a white precipitate of silver chloride.
NaCl + AgNO3 → AgCl↓ + NaNO3
Sodium chloride Silver nitrate Silver chloride Sodium nitrate
(b) Neutralisation reaction: When a base and an acid react, they exchange their ions and form compounds called salt, water and sometimes a gas. For example, hydrochloric acid reacts with potassium hydroxide to form potassium chloride (salt) and water.
HCl + KOH → KCl + H2O
Hydrochloric acid Potassium hydroxide Potassium chloride Water
Q.25. Explain the terms-oxidation and oxidizing agent.
Ans: Oxidation is a chemical reaction that involves addition of oxygen or removal of hydrogen from a substance. For example, when carbon reacts with oxygen to form carbon dioxide, it is said to be oxidised by gaining oxygen.
C + O2 → CO2
Carbon Oxygen Carbon dioxide
Similarly, when hydrogen sulphide reacts with chlorine, hydrogen sulphide is said to be oxidised to sulphur by loss of hydrogen.
H2S + Cl2 → S + 2HCl
Hydrogen sulphide Chlorine Sulphur Hydrochloric acid
An oxidising agent is one that oxidises other substances either by providing oxygen to them or by removing hydrogen from them.
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