SolveItClass 9 · NCERT

NCERT Solutions · Class 9 Science Atomic Foundations of Matter

39 questions · 21 still being checked

Pause and Ponder 9.1–9.2 (part 2 of 22)

  1. Exercise 9.1

    A student burns 10\displaystyle 10 g of ethanol in an open beaker. reaction, no residue is left in the beaker. Does this mean the Law of Conservation of Mass is violated? Explain.

    Not cross-checked

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

    No — the Law of Conservation of Mass is not violated.
    NCERT_Solution_Class9_Science_Ch9_PP_Q9-1
    Burning is a chemical reaction: ethanol + oxygen (from the air) → carbon dioxide + water vapour.
    Both products are gases, and the beaker is open, so they escape into the air — that is why no residue is left behind.
    Nothing was destroyed; the mass simply left the beaker. Mass of ethanol + mass of oxygen used = mass of carbon dioxide + mass of water vapour formed.
    This is Experimental set-up $\displaystyle 1$ of Activity $\displaystyle 9.2$ again: the balance read less only because the \(\displaystyle CO_2\) escaped. Sealing the system (set-up $\displaystyle 2$) made the initial and final readings match.
    To test it properly, burn the ethanol in a closed vessel and weigh the vessel with everything inside it, before and after.

  2. Exercise 9.2

    When 20\displaystyle 20 g of hydrogen reacts completely with 160\displaystyle 160 g of oxygen, how much water is formed according to the Law of Conservation Mass? 166\displaystyle 166 Exploration|Grade 9\displaystyle 9
    NCERT’s answer
    $\displaystyle 180$ g
    Water formed = $\displaystyle 180$ g.
    NCERT_Solution_Class9_Science_Ch9_PP_Q9-2
    Mass is neither created nor destroyed in a chemical reaction, so mass of product = total mass of reactants.
    \(\displaystyle 20 + 160 = 180\) g of water.
    The ratio agrees with the chapter too: \(\displaystyle 20 : 160 = 1 : 8\), which is the fixed hydrogen : oxygen mass ratio in water.