SolveItClass 9 · NCERT

NCERT Solutions · Class 9 Science Atomic Foundations of Matter

39 questions · 21 still being checked

Revise, Reflect, Refine 9.14–9.15 (part 21 of 22)

  1. Exercise 9.14

    Assertion (A): Copper sulfate conducts electricity in the molten state but not in the solid state. Reason (R): Copper and sulfate ions are fixed in the lattice in molten state, while in solid state they can move freely. Choose the correct option: (i) Both A and R are true, and R is the correct explanation of A. (ii) Both A and R are true, but R is not the correct explanation of A. (iii) A is true, but R is false. (iv) A is false, but R is true.

    Not cross-checked

    NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.

    NCERT’s answer
    (iii)
    (iii) A is true, but R is false.
    NCERT_Solution_Class9_Science_Ch9_RRR_Q9-14
    A is true. Copper sulfate is ionic; it does not conduct as a solid but does conduct once molten.
    R is false because it has the two states the wrong way round. The ions are held in fixed positions in the solid lattice, and they become free to move in the molten state.
    The chapter's rule is that ions must be free to move for a compound to conduct — which is why solid \(\displaystyle CuSO_4\) does not conduct, while its melt (and its solution in water) does.
    Since R is false, it cannot explain A, and options (i), (ii) and (iv) are all ruled out.

  2. Exercise 9.15

    The species 27Al, 80Br How many electrons and neutrons do they have? y Design and perform an experiment to show and compare that water always contains hydrogen and oxygen in the same ratio, regardless of its source. y Compare atoms and ions of any three elements. Show the number of electrons before and after ion formation using bar graphs. y Make a card game with cations and anions. Possible ideas may include — Š Pick cards from the pile or from open cards and match them with cards in your hand to form compounds. You may discard any unwanted cards. Š Ask other players for cards and form compounds using the cards they already have. y To learn more about molecules you can explore the link given below — Š https://phet.colorado.edu/sims/html/build-a-molecule/latest/build-a-molecule_all.html The Quest Continues … Are there any chemical changes that do not obey the Law of Conservation of Mass?
    NCERT’s answer
    Electrons $\displaystyle 13$, Neutrons $\displaystyle 14$; Electrons $\displaystyle 36$, Neutrons $\displaystyle 45$; Electrons $\displaystyle 78$, Neutrons $\displaystyle 121$
    \(\displaystyle ^{27}Al\): $\displaystyle 13$ electrons and $\displaystyle 14$ neutrons.
    It is a neutral atom, so electrons = protons = $\displaystyle 13$; neutrons \(\displaystyle = 27 - 13 = 14\).
    \(\displaystyle ^{80}Br^-\): $\displaystyle 36$ electrons and $\displaystyle 45$ neutrons.
    The $\displaystyle 1$− charge means one extra electron: \(\displaystyle 35 + 1 = 36\); neutrons \(\displaystyle = 80 - 35 = 45\).
    \(\displaystyle ^{201}Hg^{2+}\): $\displaystyle 78$ electrons and $\displaystyle 121$ neutrons.
    The $\displaystyle 2$+ charge means two electrons lost: \(\displaystyle 80 - 2 = 78\); neutrons \(\displaystyle = 201 - 80 = 121\).
    In every case neutrons = mass number − number of protons, and the charge changes only the electron count — the nucleus is untouched when an ion forms.