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The melting points and… (Class 10 Science 2023, Q37)

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ScienceCBSE Class 10 · Board exam 2023, set 31/4/1 · 4 marks · Case-based

CBSE Class 10 Science 2023: question 37

Set in the CBSE Class 10 board exam of 2023, paper 31/4/1, section E, question 37, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q37 · 4 marks · Case-basedCompetency-basedMetals and Non-metals

The melting points and boiling points of some ionic compounds are given below :

CompoundMelting Point (K)Boiling Point (K)
NaCl10741686
LiCl8871600
CaCl_210451900
CaO28503120
MgCl_29811685

These compounds are termed ionic because they are formed by the transfer of electrons from a metal to a non-metal. The electron transfer in such compounds is controlled by the electronic configuration of the elements involved. Every element tends to attain a completely filled valence shell of its nearest noble gas or a stable octet.

  1. (i)

    Show the electron transfer in the formation of magnesium chloride.

    1 mark
  2. (ii)

    List two properties of ionic compounds other than their high melting and boiling points.

    1 mark
  3. (iii)(A)

    While forming an ionic compound say sodium chloride how does sodium atom attain its stable configuration ?

    2 marks

  4. OR

    (iii)(B)

    Give reasons : (i) Why do ionic compounds in the solid state not conduct electricity ? (ii) What happens at the cathode when electricity is passed through an aqueous solution of sodium chloride ?

    2 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 31/4/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Write the electronic configurations of Mg (12) and Cl (17) first.

Hint 2

Think of solubility, hardness and conduction in different states.

Hint 3

Count sodium's valence electrons. Is it easier to lose one or gain seven?

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. (i) Mg (2,8,2) loses its two valence electrons, one to each of two Cl atoms (2,8,7): . This gives (2,8) and two (2,8,8) ions.

    1 mark

  2. (ii) Ionic compounds are hard, brittle solids, and they are generally soluble in water but insoluble in solvents like kerosene and petrol.

    ½ mark

  3. They also conduct electricity in the molten state or in aqueous solution.

    ½ mark

  4. (iii)(A) Sodium (atomic number 11) has the configuration 2,8,1, so it has only one electron in its outermost shell.

    1 mark

  5. It loses this one electron to chlorine and forms with configuration 2,8: . This is the stable octet of its nearest noble gas, neon.

    1 mark

  6. (iii)(B)(i) In the solid state the ions are held tightly in a rigid crystal structure and cannot move. Without mobile ions, no current can flow.

    1 mark

  7. (iii)(B)(ii) Hydrogen gas is liberated at the cathode (sodium hydroxide forms near it, as in the chlor-alkali process).

    1 mark

Answer: (i) Mg → Mg²⁺ + 2e⁻, with each Cl gaining 1e⁻ to form Cl⁻, giving Mg²⁺(Cl⁻)₂; (ii) hard solids, soluble in water, conduct when molten/aqueous (any two); (iii)(A) Na (2,8,1) loses 1e⁻ to form Na⁺ (2,8), the neon configuration; (iii)(B) ions are immobile in the rigid solid; H₂ gas is evolved at the cathode

Where the marks are

  • Shows the transfer of two electrons from Mg to two Cl atoms forming Mg²⁺ and 2Cl⁻ (1 mark)
  • Gives any two properties of ionic compounds (½ + ½ mark)
  • States that Na has one valence electron, 2,8,1 (1 mark)
  • Shows that Na loses it to form Na⁺ (2,8), the stable noble gas configuration (1 mark)
  • OR: explains that ions cannot move in the rigid solid (1 mark)
  • States that H₂ gas is liberated at the cathode (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Metals and Non-metals, one at a time, marked against the answer key and marking points.Back to the full paper2023 board paper, set 31/4/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.