Chemical Reactions and Equations NCERT Exemplar Problems

Short Answer Type

Part 2

Question 29: Grapes hanging on the plant do not ferment but after being plucked from the plant can be fermented. Under what conditions do these grapes ferment? Is it a chemical or a physical change?

Answer: We know that fermentation generally takes place because of action of a fungus called yeast over sugar. Fermentation of sugar results in formation of ethanol and carbon dioxide. When grapes are on plant, the action of fungi is nullified by self-defense mechanism of plant. So, grapes do not undergo fermentation when still on plant.

Question 30: Which among the following are physical or chemical changes?

Evaporation of petrol
Answer: Physical change
Burning of Liquefied Petroleum Gas (LPG)
Answer: Chemical change
Heating of an iron rod to red hot.
Answer: Physical change
Curdling of milk
Answer: Chemical change
Sublimation of solid ammonium chloride
Answer: Physical change

Question 31: During the reaction of some metals with dilute hydrochloric acid, following observations were made. Explain these observations giving suitable reasons.

(a) Silver metal does not show any change

Answer: Silver is a noble metal and hence it does not react with dilute hydrochloric acid.

(b) The temperature of the reaction mixture rises when aluminium (Al) is added.

Answer: Reaction between hydrochloric acid and aluminium is exothermic.

Answer: Sodium is at the top ranking in reactivity series of metals, which means sodium is a highly reactive metal. Tat is why, the reaction between sodium and hydrochloric acid is highly explosive.

(d) Some bubbles of a gas are seen when lead (Pb) is reacted with the acid.

Answer: You have seen in activity with zinc ganules that effervescence is seen when acid is poured over zinc garnules. Same process takes place in reaction between lead and hydrochloric acid.

Question 32: A substance X, which is an oxide of a group 2 element, is used intensively in the cement industry. This element is present in bones also. On treatment with water it forms a solution which turns red litmus blue. Identify X and also write the chemical reactions involved.

Answer: Element ‘X’ is calcium. When calcium oxide reacts with water it forms calcium hydroxide. Since calcium hydroxide is a base, thus it turns red litmus blue. Reaction involved in the reaction of calcium oxide with water can be written as follows:

CaO + H₂O ⇨ Ca(OH)₂ + Heat

Calcium belongs to group 2nd in periodic table. It is used in cement industry and also found in bones.

Question 33: Write chemical equations for following reactions and identify types of reaction involved.

(a) Lead acetate solution is treated with dilute hydrochloric acid to form lead chloride and acetic acid solution.

Answer: Pb(CH₃COO)₂ + 2HCl ⇨ PbCl₂ + CH₃COOH

(b) A piece of sodium metal is added to absolute ethanol to form sodium ethoxide and hydrogen gas.

Answer: 2Na + 2C₂H₅OH ⇨ 2C₂H₅ONa + H₂

Answer: Fe₂O₃ + 3CO ⇨ 2Fe + 3CO₂

(d) Hydrogen sulphide gas reacts with oxygen gas to form solid sulphur and liquid water.

Answer: 2H₂S + O₂ ⇨ 2S + 2H₂O

(a) is double displacement reaction, (b) is displacement reaction and (c) and (d) are redox reactions.

Question 34: Why do we store silver chloride in dark coloured bottles?

Answer: Silver chloride shows photo-decomposition. This means when silver chloride is exposed to light it decomposes to silver metal and chlorine gas. That’s why silver chloride is stored in a dark coloured bottle to prevent photodecomposition.

Question 35: Balance the following chemical equations and identify the type of chemical reaction.

(a) Mg + Cl₂ ⇨ MgCl₂

Answer: Mg + Cl₂ ⇨ MgCl₂

(b) HgO ⇨ Hg + O₂

Answer:2HgO ⇨ 2Hg + O₂

Answer: 2Na + S ⇨ Na₂S

(d) TiCl₄ + Mg ⇨ Ti + MgCl₂

Answer: TiCl₄ + 2Mg ⇨ Ti + 2MgCl₂

(e) CaO + SiO₂ ⇨ CaSiO₃

Answer: CaO + SiO₂ ⇨ CaSiO₃

(f) H₂O₂ ⇨ H₂O + O₂

Answer: 2H₂O₂ ⇨ 2H₂O + O₂

Question 36: A magnesium ribbon is burnt in oxygen to give a white compound X accompanied by emission of light. If the burning ribbon is now placed in an atmosphere of nitrogen, it continues to burn and forms a compound Y.

(a) Write the chemical formulae of X and Y.

Answer: X is MgO and Y is Mg₃N₂

(b) Write a balanced chemical equation, when X is dissolved in water.

Answer: MgO + H₂O ⇨ Mg(OH)₂

Question 37: Zinc liberates hydrogen gas when reacted with dilute hydrochloric acid, whereas copper does not. Explain why?

Answer: Reaction depends upon the reactivity of metals. Copper is less reactive than zinc. Copper is also known as noble metal along with gold, silver, etc. that are very less reactive under normal conditions. This is the cause that copper does not react with dilute hydrochloric acid and consequently does not liberate hydrogen gas while zinc does.

Question 38: Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved.

Answer: A layer of black substance, called silver oxide, deposited over silver because of exposure of oxygen present in air. Finally it forms silver sulphide with the reaction of oxygen and hydrogen sulphide gas present in atmosphere, because of which silver metal turns black. This phenomenon is called tarnish or tarnishing of silver. Reaction involved in this is given here:

4Ag + O₂ + 2H₂S ⇨ 2Ag₂S + 2H₂O

The silver sulphide so formed can be cleaned using toothpaste. Toothpastes have many abrasive substance. Aluminium hydroxide - Al(OH)3 is one of them. When toothpaste is rubbed over tarnished silver metal, the silver sulphide reacts with aluminium and aluminium sulphide is formed leaving silver metal shiny. The cleansing speed ups with use of hot water and sodium hydrogen carbonate. The reaction involve in cleansing is as follows:

3Ag₂S + 2Al ⇨ 6Ag + Al₂S₃

Name the black substance formed and gives its chemical formula.

Answer: The black substance formed over the silver metal is silver sulphide. The chemical formula of silver sulphide is Ag₂S.