Class 10 Science answer 1
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Class 10 Science Chapter 1 Chemical Reactions and Equations

Here are Class 10 Science Chapter 1 Chemical Reactions and Equations Question 12 to 20 -Answer

12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light, or electricity.

Examples of Decomposition Reactions

Here are examples of decomposition reactions where energy is supplied in the form of heat, light, or electricity:

  1. Decomposition reaction with heat:

    2HgO(s) → 2Hg(l) + O2(g)

    In this reaction, solid mercury(II) oxide (HgO) decomposes upon heating to produce liquid mercury (Hg) and oxygen gas (O2).

  2. Decomposition reaction with light:

    2AgBr(s) → 2Ag(s) + Br2(g)

    In this reaction, silver bromide (AgBr) decomposes when exposed to light to form silver metal (Ag) and bromine gas (Br2).

  3. Decomposition reaction with electricity (electrolysis):

    2H2O(l) → 2H2(g) + O2(g)

    In this reaction, water (H2O) undergoes electrolysis when an electric current is passed through it, resulting in the decomposition of water into hydrogen gas (H2) and oxygen gas (O2).

These examples demonstrate different methods by which decomposition reactions can occur with the supply of energy in the form of heat, light, or electricity.

13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.

The Difference Between Displacement and Double Displacement Reactions

The difference between displacement and double displacement reactions lies in the number of compounds involved and the manner in which the ions exchange.

Displacement Reaction:

  • In a displacement reaction, one element displaces another element from a compound.
  • Generally, a more reactive element displaces a less reactive element from its compound.
  • The reaction involves one compound and one element.
  • The general form of a displacement reaction is: A + BC → AC + B

Example of a displacement reaction:

Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)

In this reaction, zinc (Zn) displaces hydrogen (H) from hydrochloric acid (HCl), forming zinc chloride (ZnCl2) and hydrogen gas (H2).

Double Displacement Reaction:

  • In a double displacement reaction, the positive and negative ions of two compounds switch places to form two new compounds.
  • Double displacement reactions usually occur in aqueous solutions and involve the exchange of ions.
  • The reaction involves two compounds.
  • The general form of a double displacement reaction is: AB + CD → AD + CB

Example of a double displacement reaction:

NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

In this reaction, sodium hydroxide (NaOH) reacts with hydrochloric acid (HCl) to form sodium chloride (NaCl) and water (H2O).

In summary, the main difference between displacement and double displacement reactions is the number of compounds involved and the manner in which the chemical species interact. Displacement reactions involve the displacement of one element from a compound, while double displacement reactions involve the exchange of ions between two compounds.

14. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

Recovery of Silver from Silver Nitrate Solution

In the refining of silver, the recovery of silver from silver nitrate solution involves the displacement of silver ions by copper metal. The reaction involved can be represented as follows:

Cu(s) + 2AgNO3(aq) → 2Ag(s) + Cu(NO3)2(aq)

In this reaction, solid copper (Cu) displaces silver ions (Ag⁺) from silver nitrate solution (AgNO₃) to form solid silver (Ag) and copper(II) nitrate solution (Cu(NO₃)₂).

This reaction is an example of a displacement reaction, where a more reactive metal (copper) displaces a less reactive metal (silver) from its compound (silver nitrate).

15. What do you mean by a precipitation reaction? Explain by giving examples.

Precipitation Reaction

A precipitation reaction is a type of chemical reaction in which two soluble compounds in aqueous solution react to form an insoluble solid compound, known as a precipitate. This solid compound forms as a result of the reaction between the ions of the two soluble compounds.

Explanation:

  • In a precipitation reaction, when two soluble compounds are mixed, they may undergo a chemical reaction in which one or more insoluble products, called precipitates, are formed.
  • The formation of the insoluble precipitate occurs when the positive and negative ions in the reactants combine to form an insoluble compound that separates from the solution as a solid.
  • The precipitate formed is usually a compound with low solubility in water, which means it cannot remain dissolved and separates out as a solid.
  • Precipitation reactions typically occur in aqueous solutions and are commonly used in qualitative analysis to identify the presence of certain ions in solution.

Example:

AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)

In this example, when aqueous silver nitrate (AgNO₃) is mixed with aqueous sodium chloride (NaCl), a white precipitate of silver chloride (AgCl) forms. This reaction occurs because the silver ions (Ag⁺) from silver nitrate react with the chloride ions (Cl⁻) from sodium chloride to form insoluble silver chloride (AgCl), which precipitates out of the solution. The sodium ions (Na⁺) and nitrate ions (NO₃⁻) remain in solution as soluble sodium nitrate (NaNO₃).

Precipitation reactions are important in various fields such as chemistry, environmental science, and medicine, and they play a crucial role in the formation of minerals, purification of water, and laboratory analysis.

16. Explain the following in terms of gain or loss of oxygen with two examples each.

  1. (a) Oxidation
  2. (b) Reduction

Oxidation and Reduction

(a) Oxidation:

  • Oxidation is a chemical process in which a substance loses electrons or gains oxygen.
  • When a substance undergoes oxidation, it increases its oxidation state or becomes more positive.
  • Examples of oxidation include:
    1. Rusting of iron:

      [4Fe(s) + 3O2(g) → 2Fe2O3(s)]

      In this reaction, iron (Fe) loses electrons to oxygen (O2) to form iron(III) oxide (Fe2O3), indicating oxidation of iron.

    2. Burning of methane:

      [CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)]

      In this reaction, methane (CH4) loses hydrogen and gains oxygen to form carbon dioxide (CO2) and water (H2O), indicating oxidation of methane.

(b) Reduction:

  • Reduction is a chemical process in which a substance gains electrons or loses oxygen.
  • When a substance undergoes reduction, it decreases its oxidation state or becomes more negative.
  • Examples of reduction include:
    1. Reduction of copper(II) oxide by hydrogen:

      [CuO(s) + H2(g) → Cu(s) + H2O(g)]

      In this reaction, copper(II) oxide (CuO) gains electrons from hydrogen (H2) to form copper (Cu) and water (H2O), indicating reduction of copper(II) oxide.

    2. Reduction of potassium permanganate by iron(II) sulfate:

      [5FeSO4(aq) + 2KMnO4(aq) + 8H2SO4(aq) → 5Fe2(SO4)3(aq) + K2SO4(aq) + 2MnSO4(aq) + 8H2O(l)]

      In this reaction, iron(II) sulfate (FeSO4) loses electrons to potassium permanganate (KMnO4) to form iron(III) sulfate (Fe2(SO4)3), indicating reduction of potassium permanganate.

In summary, oxidation involves the loss of electrons or gain of oxygen, while reduction involves the gain of electrons or loss of oxygen. These processes often occur together in redox reactions, where one substance is oxidized while another is reduced.

17. A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.

Identification of Element X

The shiny brown colored element 'X' that becomes black in color upon heating in air is likely to be copper. When copper is heated in air, it undergoes oxidation to form copper oxide, which is black in color.

Therefore, the element 'X' is copper (Cu), and the black colored compound formed is copper oxide (CuO).

18. Why do we apply paint on iron articles?

Importance of Painting Iron Articles

We apply paint on iron articles primarily to prevent them from rusting. Rusting is a chemical process in which iron reacts with oxygen and moisture from the air to form iron oxide, commonly known as rust. Painting iron articles creates a protective barrier that prevents oxygen and moisture from coming into direct contact with the iron surface, thereby inhibiting the oxidation process and preventing rust formation.

Additionally, paint provides aesthetic appeal and enhances the appearance of iron articles. It also serves as a protective coating against other environmental factors such as corrosion, abrasion, and weathering, thereby extending the lifespan of the iron articles.

In summary, applying paint on iron articles helps to protect them from rusting, improve their appearance, and prolong their durability by providing a protective barrier against environmental factors.

19. Oil and fat containing food items are flushed with nitrogen. Why?

Benefits of Flushing Oil and Fat-containing Food Items with Nitrogen

Oil and fat-containing food items are flushed with nitrogen to prevent oxidation and rancidity. When oils and fats are exposed to oxygen in the air, they can undergo oxidation, leading to the formation of off-flavors, odors, and potentially harmful compounds. This process is known as rancidity.

Nitrogen is an inert gas, meaning it does not react chemically with the food or its components. By flushing oil and fat-containing food items with nitrogen, the oxygen in the packaging is displaced, creating an oxygen-free environment. This reduces the likelihood of oxidation reactions occurring, thereby helping to preserve the freshness, flavor, and quality of the food products.

In summary, flushing oil and fat-containing food items with nitrogen helps to prevent oxidation and rancidity, preserving the quality and shelf life of the food products.

20. Explain the following terms with one example each.

  1. (a) Corrosion
  2. (b) Rancidity

Corrosion

Corrosion is the gradual deterioration of a metal due to chemical reactions with the environment, leading to the formation of undesirable compounds, such as oxides or hydroxides of the metal. One example of corrosion is the rusting of iron. When iron is exposed to oxygen and moisture in the air, it undergoes oxidation to form iron oxide, commonly known as rust. The presence of rust weakens the structural integrity of iron objects, leading to their degradation over time.

Rancidity

Rancidity is the development of off-flavors and odors in fats and oils due to the breakdown of their chemical structure. This process occurs when fats and oils undergo oxidation, hydrolysis, or microbial degradation, resulting in the formation of unpleasant-tasting compounds. One example of rancidity is the rancid smell and taste that can develop in cooking oils when exposed to air and light for an extended period. This can occur in oils used for frying or cooking, especially if they are not stored properly in airtight containers and away from heat and light sources.