Class 10th science last 10 year papers chapter wise chapter 1 chemical reaction and equation part 2

21.Describe an activity to observe what happens when quick lime is added to water taken in a beaker. State two important observations and name the type of reaction taking place.
Answer.
Aim: To observe what happens when quicklime is added to water taken in a beaker.
Materials Required:- Quicklime (calcium oxide), water, beaker.
chemical-reactions-and-equations-chapter-wise-important-questions-class-10-science-15
Procedure:
1. Take 5 g of calcium oxide in a beaker.
2. Add water to it slowly.
3. Touch the beaker.
4. Note down the observations.
Observation: Calcium oxide reacts with water
vigorously to form calcium hydroxide with the evolution of heat.
Chemical Reaction:
chemical-reactions-and-equations-chapter-wise-important-questions-class-10-science-16
Conclusion: The reaction between CaO (Calcium oxide) and H2O is a combination reaction. It is an exothermic process because heat is evolved.

22.What is the colour of ferrous sulphate crystals? How does this colour change after heating?
Answer.The colour of ferrous sulphate is pale green. The colour changes to reddish brown on heating due to formation of iron (III) oxide.
Give an example each for thermal decomposition and photochemical decomposition reactions. Write relevant balanced chemical equations also.
Thermal decomposition reaction:
chemical-reactions-and-equations-chapter-wise-important-questions-class-10-science-17
Photochemical decomposition reaction:
chemical-reactions-and-equations-chapter-wise-important-questions-class-10-science-18

24. Why does the colour of copper sulphate solution change when an iron nail is dipped in it? Write two observations.
Answer. It is because displacement reaction takes place.
Iron displaces copper from copper sulphate solution and forms pale green
coloured solution of FeS04 and reddish brown copper metal gets deposited.
Fe(s) + CuS04(aq) ——–> FeS04(aq) + Cu(s)

25. Translate the following statement into chemical equation and then balance it Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate. State the two types in which this reaction can be classified.
Answer. 3BaCl2(aq) + A12(S04)3(aq) ——–> 3BaS04(s) + 2AlCl3(aq)
It can be classified as double displacement as well as precipitation reaction.

26. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer. In decomposition reaction, a compound is broken down into simpler compounds or elements, e.g.
chemical-reactions-and-equations-chapter-wise-important-questions-class-10-science-19
Combination reaction is a reaction in which two or more elements or compounds combine to form a new compound, e.g.
chemical-reactions-and-equations-chapter-wise-important-questions-class-10-science-20
Thus, decomposition and combination reactions are opposite to each other.

Short Answer Type Questions[ll] [3 Marks]  -Year 2012

27. The following diagram displays a chemical reaction. Observe carefully and answer the following questions
chemical-reactions-and-equations-chapter-wise-important-questions-class-10-science-21
(a) Identify the type of chemical reaction that will take place and define it. How will the colour of the salt change?
Write the chemical equation of the reaction that takes place.
(c) Mention one commercial use of this salt.
Answer. (a) Photochemical decomposition reaction: Those reactions in which a compound breaks down into simple substances in presence of light are called photochemical decomposition reaction. The colour of salt will change from white to grey.
chemical-reactions-and-equations-chapter-wise-important-questions-class-10-science-22
(c) Silver chloride is used in photography.

28. What is rancidity? Mention any two ways by which rancidity can be prevented.
Answer. The process in which taste and smell of food gets spoiled is called rancidity. It happens due to oxidation.
Prevention from rancidity:
(i) Antioxidants are added to fatty acids to prevent oxidation, e.g. chips are packed in presence of nitrogen gas which prevents spoilage by oxidation.
(ii)Food should be kept in airtight container in refrigerator.

29.Write balanced chemical equation for the reactions that take place during respiration. Identify the type of combination reaction that takes place during this process and justify the name. Give one more example of this type of reaction.
Answer. CgH1206 + 6O2 —————> 6CO2 + 6H20 + heat
It is an exothermic combination reaction because heat is evolved.
CH4(g) + 2O2(g) ————–>CO2 (g) + 2H20
Combustion of methane is another example of exothermic combination reaction.

30. What is redox reaction? Identify the substance oxidised and the substance reduced in the following reactions.
(i)2PbO + C —–> 2Pb + CO2
(ii)MnO2 + 4HCl —–> MnCl2 + 2H20 + Cl2
Answer. Those reactions in which oxidation and reduction takes place simultaneously are called redox reactions.
(i) PbO is getting reduced and C is getting oxidised.
(ii) MnOs is getting reduced and HCl is getting oxidised.

31. Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.
Thermite reaction, iron (III) oxide reacts with aluminium and gives molten iron and aluminium oxide.
Answer.
chemical-reactions-and-equations-chapter-wise-important-questions-class-10-science-23
It is a displacement reaction because A1 is displacing Fe from Fe2O3.
Molten iron is used’for repairing broken railway tracks.

32. A solution of potassium chloride when mixed with silver nitrate solution, an insoluble white substance is formed. Write the chemical reaction involved and also mention the type of the chemical reaction?
Answer.

chemical-reactions-and-equations-chapter-wise-important-questions-class-10-science-24.
It is a double displacement reaction. It is also a precipitation reaction as AgCl is a white precipitate.

Very Short Answer Type Questions [1 Mark] -Year 2011

33.State one basic difference between a physical change and a chemical change.
Answer. In physical change, no new substance is formed, whereas in a chemical change, new substance(s) is/are formed.

34 What is meant by a chemical reaction?
Answer. The reaction representing a chemical change is called a chemical reaction.

35.AgN03(aq) + NaCl(aq)——————– > AgCl(s)4↓ + NaN03(aq)
FeS + H2S04————- > FeS04 + H2S↑
Consider the above mentioned two chemical equations with two different kinds of arrows (↑and ↓) along with product. What do these two different arrows indicate?
Ans,↑shows the gas is evolved whereas ↓shows insoluble substance (precipitate) is formed.

36. Hydrogen being a highly inflammable gas and oxygen being a supporter of combustion, yet water which is a compound made up of hydrogen and oxygen is used to extinguish fire. Why?
Answer. It is because properties of compound (H2O) are different from properties of its constituting elements, i.e. H2and O2.

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