Q  1. Zn + 2HCl → ZnCl2 + H2 In the given equation, how will you depict that it is accompanied by evolution of gas and heat?

SolutionWe can show heat produced as “+ Heat” on the product side and gas produced with an upward arrow ↑.

Zn + 2HCl ➜ ZnCl2 + H2 ↑ + Heat


Q  2. Give an example of an endothermic and exothermic reaction.

SolutionEndothermic reaction: The reaction between barium hydroxide and ammonium chloride is an example of an endothermic reaction. Exothermic reactions: Burning of L.P.G in the presence of air is an example of the exothermic reaction since it is accompanied by the evolution of a large amount of heat.


Q  3. Why do we need to clean magnesium ribbon before burning it? Why is it advised to burn magnesium ribbon at a distance far from eyes?

SolutionWe need to clean magnesium ribbon before burning it to remove the protective layer of basic magnesium carbonate which is formed by the reaction of magnesium with moist air. The bright white light given out during the burning of magnesium ribbon is harmful to the eyes so, the magnesium ribbon is burned by keeping it as far as possible from the eyes.


Q  4. Why decomposition reactions are called the opposite of combination reactions?

SolutionIn combination reactions, two substances combine to form one compound and in decomposition reactions, a compound breaks down into two or more substances, so they are opposite to each other.


Q  5. How does a chemical reaction result in the formation of new products?

SolutionThe rearrangement of atoms results in the formation of new substances with properties, entirely different from those of the reactants. During a chemical reaction, the already existing bonds between reactants are broken and new bonds are formed leading to the formation of products.


Q  6. Give two merits of chemical equation.

SolutionThe two merits of chemical equations are: (i) Representation of a chemical reaction becomes easy as equations save time and space. Since there is uniformity in symbols all over the world, scientists can exchange data and understand each other’s work easily. (ii) From a balanced equation, we can easily calculate the amount of reactants and products.


Q  7. What is a skeletal equation?

SolutionA chemical equation expressed in symbols and formulae such that number of atoms of different elements on the side of reactants is not equal to the number of atoms on the side of products.


Q  8. Define oxidizing agent and reducing agent.

SolutionIn redox reactions, the substance which gives oxygen or takes away hydrogen is called oxidizing agent. On the other hand, the substance which takes away oxygen or gives hydrogen is called reducing agent.


Q  9. Name and state the law which is followed when we balance any chemical equation.

SolutionChemical equations are balanced to satisfy the Law of conservation of mass and this law states that, mass can neither be created nor can be destroyed during a chemical reaction.


Q  10. How can we make equations more informative?

SolutionThe equations can be made more informative in three ways:

1. by indicating the physical states of the reactants and products.

2. by indicating the heat changes taking place in the reaction.

3. By indicating the conditions under which the reaction takes place like temperature, pressure, presence of catalyst etc.


Q  11. Which one is a chemical change – rusting of iron or melting of iron? Explain.

SolutionRusting of iron is a chemical change, it involves the formation of a new product, rust. Rust cannot be converted back to iron by any process.

Q  12. Give example of reactions accompanied by:

(a) Formation of precipitate.

(b) Change in state.

Solution

(a) When a solution of silver nitrate is added to a solution of sodium chloride, a white precipitate of silver chloride is formed. AgNO3  +  NaCl  →  AgCl ↓ +  NaNO3  

(b) Ammonia gas reacts with hydrogen chloride gas to form ammonium chloride, which is in a solid state.

NH3(g)  +  HCl(g ➜ NH4Cl(s)


Q  13. Define the term rancidity.

SolutionThe oils and fats react with oxygen and become oxidized or turn rancid. This process is called rancidity.


Q  14. Can we store iron sulphate solution in a zinc container? Give reason for your answer.

SolutionZinc being more reactive than iron will displace it and form a colorless solution of zinc sulphate. This could even lead to appearance of holes in zinc container. Hence, we cannot store iron sulphate in a zinc container.


Q  15. What are double displacement reactions? Give one special feature of such reactions.

SolutionReactions in which two compounds react with each other and exchange their ions to form two new compounds, are called double displacement reaction. In these reactions, two compounds swap components, in the format:

AB + CD → AD + CB

The reactants of double displacement reaction are in aqueous form i.e. solution and are often accompanied by precipitation. In other words, a solid product separates or settles at the bottom of the solution.


Q  16. When magnesium metal is reacted with sulphuric acid, magnesium sulphate and hydrogen gas are formed with the evolution of heat. Mention the observations that will help to determine that reaction has taken place.

SolutionThe chemical reaction between magnesium and sulphuric acid can be observed by:

(a) Evolution of hydrogen gas

(b) Change in temperature

i.e., there is evolution of heat during the reaction.


Q  17. How do we represent a chemical reaction?

SolutionThe chemical reactions are represented with the help of symbols or formulae of various reactants and products, with a (+) sign between them. The reactants are written on the left side and products on right side. The transformation of reactants into products is shown by an arrow (→).


Q  18. Why is electrolysis of water an endothermic reaction?

SolutionElectrolysis of water to form oxygen and hydrogen is an endothermic reaction because electrical energy is absorbed during this reaction.


Q  19. What are Decomposition reactions? How do they take place?

SolutionDecomposition reactions are those reactions in which a compound breaks or splits into two or more simpler substances. In such reactions, one reactant breaks up to form many i.e. at least two products. These reactions require energy to break the already existing bonds, therefore they take place in presence of heat, light or electricity.

Q  20. FeSO4(aq) + 2NaOH(aq) → Fe(OH)2 ↓ + Na2SO4(aq) 

What information is conveyed from the above equation?

SolutionAn aqueous solution of ferrous sulphate reacts with aqueous solution of sodium hydroxide to form a precipitate of ferrous hydroxide and sodium sulphate remains in the solution.


Q  21.

(a) Oil and fat containing food items are flushed with nitrogen gas while packing them, why?

(b) What is rust? What is its formula?

Solution

(a) Oil and fat containing food items are flushed or surrounded with an inert gas such as nitrogen while packing them to prevent their contact with oxygen present in the air. This is done to avoid rancidity of fats and oils.

(b) When iron objects are exposed to moisture, they combine with oxygen from the air in the presence of water or moisture to form a red-brown flaky substance called rust. This process is commonly called  rusting. Its formula is Fe2O3.xH2O.


Q  22. State one basic difference between a physical change and a chemical change.

SolutionPhysical change: No new substance is formed. Chemical change: New substance is formed.


Q  23. Name two anti-oxidants which are usually added to fats and oils containing foods to prevent rancidity.SolutionBHA and BHT are the common anti-oxidants used in foods to prevent rancidity.


Q  24. Identify the substance that is oxidised and reduced in the reaction: CuO(s) + Zn(s) → Cu(s) + ZnO(s)

SolutionCuO is reduced and Zn is oxidized.


Q  25. Why is there a change in temperature in a  chemical reaction? What happens when quicklime is added to water filled in a bucket?

SolutionSome chemical reactions are characterised by a change in temperature. When a chemical reaction produces heat energy, then the temperature of reaction mixture rises and it becomes hot. However, many a times, chemical reactions absorb heat energy, then the temperature of reaction mixture falls and it becomes cold. So, a change in temperature in a chemical reaction can be ‘a rise in temperature’ or ‘fall in temperature’. When quicklime reacts with water filled in a bucket, slaked lime is formed and a lot of heat is produced. The bucket feels to be quite hot on touching.


Q  26. X + YSO4 → XSO4+ Y Y + XSO4 → No reaction Explain the reason for the above:

Solution‘X’ is more reactive than ‘Y’ hence ‘X’ is able to displace ‘Y’ from its salt whereas ‘Y’ is not able to displace ‘X’ from its salt because it is less reactive than ‘X’.


Q  27. Mention two chemical reactions that are characterised by the evolution of gas.

Solution

1. When zinc granules react with dilute sulphuric acid, then bubbles of hydrogen gas are produced.         

2. The chemical reaction between sodium carbonate and dilute hydrochloric acid is characterised by the evolution of carbon dioxide gas.


Q  28.

(a) What are anti-oxidants? State two examples.

(b) How is rancidity prevented using anti-oxidants?

Solution

(a) Anti-oxidant is a substance which prevents oxidation. These are reducing agents. Examples: BHA and BHT.

(b) When anti-oxidants are added to foods, then the fats and oils present in them do not get oxidised easily and hence do not turn rancid. So, the food remains good to eat for a much longer time.


Q  29. State the main difference between an endothermic reaction and an exothermic reaction.

SolutionExothermic reaction: reactions involving the release of heat energy. Endothermic reaction: reactions involving absorption of heat energy.


Q  30. Identify the substance oxidized, substance reduced, oxidizing agent and reducing agent in the following reaction:

CuO + H2 → Cu + H2O

Solution Oxidised: H2 Reduced : CuO Oxidising agent : CuO Reducing agent : H2


Q  31. Cu + AgNO3 → CuNO3 + Ag

Fe + CuSO4 → FeSO4 + Cu

Zn + FeSO4 → ZnSO4 + Fe

From above information arrange the given metals in increasing order of reactivity. Give reason for your choice.

SolutionSince zinc displaces iron, it is more reactive than iron but iron displaces copper therefore iron comes above copper in reactivity series. Similarly, copper displaces silver that is silver is least reactive.

Ag < Cu < Fe < Zn


Q  32. How will you incorporate the following information into an equation? (i) Presence of a catalyst (ii) Temperature and pressure conditions (iii) Evolution of a gas

Solution

(i) Catalyst is written above the arrow.  

(ii) The temperature and pressure conditions are written above or below the arrow.

(iii) Evolution of a gas is depicted by an upward arrow ↑.


Q  33. Giving one example of each, define the following terms:

(i) Corrosion (ii) Rancidity

Solution

(i) The eating up of metals by the action of air, moisture or a chemical (such as an acid) on their surface is called corrosion. For example: When an iron object is left in damp air (or water) for a considerable time, it gets covered with a red-brown flaky substance called rust. This is called rusting of iron.

(ii) The condition produced by aerial oxidation of fats and oils in foods marked by unpleasant smell and taste is called rancidity. It spoils the food materials prepared in fats and oils which have been kept for a considerable time and makes them unfit for eating. For example: Potato chips are made in oil. On keeping them exposed for a long time, these chips start giving unpleasant smell and taste. They turn rancid.


Q  34. What are displacement reactions? Explain with an example.

SolutionDisplacement reactions are those reactions in which one element takes the place of another element in a compound. The more reactive element displaces the less reactive element from its compound.

AB + C → AC + B

Example: Zn + CuSO4  → ZnSO + Cu Here Zinc displaces Copper from copper sulphate solution to form zinc sulphate and Copper.


Q  35. What is meant by a chemical reaction?

SolutionA process in which two or more substances react to form some other new substances with new set of properties is called a chemical reaction.


Q  36. Give three uses of decomposition reactions.

SolutionUses of decomposition reactions are as follows:  

1. Electrolytic decomposition is used to obtain metals from their chloride or oxide ores.  

2. In digestion, the process of the breakup of starch and other carbohydrates into simple sugars such as glucose in the presence of enzymes is a catalytic decomposition reaction.

3. Decomposition reactions are used in photography. The photographic film has a coating of silver bromide which on exposure to light splits into silver and bromine.  


Q  37. 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?

Solution During a chemical combination, the product formed has its own set of properties and the reactants fail to retain their properties. So, water has different properties from its reactants and is hence used for extinguishing fires.


Q  38. Why do we balance a chemical equation?

SolutionAs per law of conservation of mass, “matter cannot be created or destroyed”. Therefore, total mass of reactants should be equal to the total mass of products. To have that, we must have equal number of atoms on reactant side and product side. In this way, we balance a chemical equation.


Q  39. Describe three reactions to show that chemical reactions are characterised by a change in colour. 

Solution

a. The chemical reaction between citric acid and purple coloured potassium permanganate solution is characterised by a change in colour from purple to colourless.

b. The chemical reaction between sulphur dioxide gas and acidified potassium dichromate solution is characterised by a change in colour from orange to green.

c. The chemical reaction between iron and blue coloured copper sulphate is characterised by a change in colour from blue to light green.


Q    40. On heating a colourless powder, it forms yellow crystals and brown gas is evolved. Identify the reactants and products. Give a balanced equation to represent the reaction.

SolutionThe colourless reactant is lead nitrate which on heating decomposes into yellow precipitate of lead monoxide and brown fumes of nitrogen dioxide gas is evolved. Colourless and odourless oxygen gas is also produced. It is a decomposition reaction.

2Pb(NO)→ 2PbO + 4NO↑ + O

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