Class 10 Chemistry

Chemical Reactions and Equations

NCERT Exemplar Problems Short Answer Type Part 2

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

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: Because of the action of a fungi, called yeast over the sugar fermentation takes place. Ethanol and carbon dioxide formed because of fermentation of sugar. Sugar found in grapes ferment because of the action of fungi, yeast when it is plucked from plant, but while grapes are on plants the action of fungi, is nullified by the self defense mechanism of plants and grapes. Thus, grapes do not ferment in the case when hanging on plant.

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

  • Silver metal does not show any change
    Answer: No change takes place because silver metal categorized as noble metal and does not react with hydrochloric acid in normal situations.
  • The temperature of the reaction mixture rises when aluminium (Al) is added.
    Answer: The reaction between hydrochloric acid and aluminium is exothermic, thus the temperature of the reaction mixture rises when aluminium is added.
  • The reaction of sodium metal is found to be highly explosive
    Answer: Since, sodium is a highly reactive metal, thus it reacts with hydrochloric acid vigorously and produces large amount of heat. Thus the reaction between hydrochloric acid and sodium is highly explosive. Even sodium reacts with water vigrouly.
  • Some bubbles of a gas are seen when lead (Pb) is reacted with the acid.
    Answer: Bubbles of hydrogen gas are formed when lead reacts with dilute hydrochloric acid, which is seen in the course of reaction.

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.

  • Lead acetate solution is treated with dilute hydrochloric acid to form lead chloride and acetic acid solution.
    Answer: Pb(CH3COO)2 + 2HCl ⇨ PbCl2 + CH3COOH
  • A piece of sodium metal is added to absolute ethanol to form sodium ethoxide and hydrogen gas.
    Answer: 2Na + 2C2H5OH ⇨ 2C2H5ONa + H2
  • Iron (III) oxide on heating with carbon monoxide gas reacts to form solid iron and liberates carbon dioxide gas.
    Answer: Fe2O3 + 3CO ⇨ 2Fe + 3CO2
  • 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.

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 to prevent photodecomposition silver chloride is stored in a dark coloured bottle.

Balance the following chemical equations and identify the type of chemical reaction.

  • Mg + Cl2 ⇨ MgCl2
    Answer: Mg + Cl2 ⇨ MgCl2
  • HgO ⇨ Hg + O2
    Answer:2HgO ⇨ 2Hg + O2
  • Na + S ⇨ Na2S
    Answer: 2Na + S ⇨ Na2S
  • TiCl4 + Mg ⇨ Ti + MgCl2
    Answer: TiCl4 + 2Mg ⇨ Ti + 2MgCl2
  • CaO + SiO2 ⇨ CaSiO3
    Answer: CaO + SiO2 ⇨ CaSiO3
  • H2O2 ⇨ H2O + O2
    Answer: 2H2O2 ⇨ 2H2O + O2

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.

  • Write the chemical formulae of X and Y.
    Answer: X is MgO and Y is Mg3N2
  • Write a balanced chemical equation, when X is dissolved in water.
    Answer: MgO + H2O ⇨ Mg(OH)2

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.

A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.

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.

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