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NCERT Exemplar · Class 10 Science Chemical Reactions and Equations

44 questions · 44 still being checked

Short Answer Questions 29–38 (part 4 of 5)

  1. Exercise 29

    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?

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    NCERT’s answer
    Grapes when attached to the plants are living and therefore their own immune system prevents fermentation. The microbes can grow in the plucked grapes and under anaerobic conditions these can be fermented. This is a chemical change.
    On the vine the grape's cells are alive and intact, so yeast does not act on its sugar. After plucking, yeast (zymase) acts on the sugar when it is warm and away from air. \[\mathrm{C_6H_{12}O_6 \xrightarrow[\text{anaerobic}]{\text{zymase}} 2C_2H_5OH + 2CO_2} \] Answer: Plucked grapes ferment by yeast under warm, anaerobic conditions; it is a chemical change, since new substances (ethanol, \(\displaystyle \mathrm{CO_2}\)) form.

  2. Exercise 30

    Which among the following are physical or chemical changes?
    (a)
    Evaporation of petrol
    (b)
    Burning of Liquefied Petroleum Gas (LPG)
    (c)
    Heating of an iron rod to red hot.
    (d)
    Curdling of milk
    (e)
    Sublimation of solid ammonium chloride

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    NCERT’s answer
    (a), (c) and (e) — are physical changes. (b) and (d) are chemical changes
    (a)
    Liquid to vapour, same substance: Physical
    (b)
    New substances form:
    \[\mathrm{2C_4H_{10}(g) + 13O_2(g) \rightarrow 8CO_2(g) + 10H_2O(g)} \]
    Chemical
    (c)
    Only colour and temperature change; it is still iron: Physical
    (d)
    Bacteria turn milk sugar into lactic acid, a new substance: Chemical
    (e)
    On cooling, the vapour returns to solid \(\displaystyle \mathrm{NH_4Cl}\); no new substance remains: Physical
    Answer: Physical: (a), (c), (e); chemical: (b), (d).

  3. Exercise 31

    During the reaction of some metals with dilute hydrochloric acid, following observations were made.
    (a)
    Silver metal does not show any change
    (b)
    The temperature of the reaction mixture rises when aluminium (Al) is added.
    (c)
    The reaction of sodium metal is found to be highly explosive
    (d)
    Some bubbles of a gas are seen when lead (Pb) is reacted with the acid.
    Explain these observations giving suitable reasons.

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    NCERT’s answer
    Hint— (a) Silver metal does not react with dilute HCl
    (b)
    The temperature of the reaction mixture rises when aluminium is added because it is an exothermic reaction.
    (c)
    Reaction of sodium metal is found to be highly explosive because it is an exothermic reaction
    (d)
    When lead is treated with hydrochloric acid, bubbles of hydrogen gas are evolved
    \[\mathrm{Pb}+2 \mathrm{HCl} \rightarrow \mathrm{PbCl}_2+\mathrm{H}_2 \]
    (a)
    Ag lies below \(\displaystyle \mathrm{H}\) in the reactivity series: no displacement, no reaction.
    (b)
    \[\mathrm{2Al(s) + 6HCl(aq) \rightarrow 2AlCl_3(aq) + 3H_2(g)} \]
    Exothermic reaction: the mixture warms up.
    (c)
    \[\mathrm{2Na(s) + 2HCl(aq) \rightarrow 2NaCl(aq) + H_2(g)} \]
    Na is very reactive: the reaction is fast and highly exothermic, so explosive.
    (d)
    \[\mathrm{Pb(s) + 2HCl(aq) \rightarrow PbCl_2(s) + H_2(g)} \]
    Pb is weakly reactive and \(\displaystyle \mathrm{PbCl_2}\) coats the metal: only a few bubbles.
    Answer: Rate follows reactivity, Na > Al > Pb; Ag, below \(\displaystyle \mathrm{H}\), does not react.

  4. Exercise 32

    A substance X, which is an oxide of a group 2\displaystyle 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.

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    NCERT’s answer
    Calcium oxide \[\mathrm{CaO}(\mathrm{~s})+\mathrm{H}_2 \mathrm{O}(\mathrm{l}) \longrightarrow \mathrm{Ca}(\mathrm{OH})_2(\mathrm{aq}) \]
    The group $\displaystyle 2$ element present in bones is calcium; its oxide used heavily in the cement industry is calcium oxide. \[\mathrm{X = CaO} \] \[\mathrm{CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq)} \] The resulting solution (limewater) is basic, turning red litmus blue. Answer: X is calcium oxide (quicklime), CaO.

  5. Exercise 33

    Write a balanced chemical equation for each of the following reactions and also classify them.
    (a)
    Lead acetate solution is treated with dilute hydrochloric acid to form lead chloride and acetic acid solution.
    (b)
    A piece of sodium metal is added to absolute ethanol to form sodium ethoxide and hydrogen gas.
    (c)
    Iron (III) oxide on heating with carbon monoxide gas reacts to form solid iron and liberates carbon dioxide gas.
    (d)
    Hydrogen sulphide gas reacts with oxygen gas to form solid sulphur and liquid water.

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    (a)
    \[\mathrm{Pb(CH_3COO)_2(aq)} + 2\mathrm{HCl(aq)} \rightarrow \mathrm{PbCl_2(s)} + 2\mathrm{CH_3COOH(aq)} \]
    Double displacement (precipitation) reaction.
    (b)
    \[2\mathrm{Na(s)} + 2\mathrm{C_2H_5OH(l)} \rightarrow 2\mathrm{C_2H_5ONa} + \mathrm{H_2(g)} \]
    Displacement reaction.
    (c)
    \[\mathrm{Fe_2O_3(s)} + 3\mathrm{CO(g)} \rightarrow 2\mathrm{Fe(s)} + 3\mathrm{CO_2(g)} \]
    Redox (reduction) reaction.
    (d)
    \[2\mathrm{H_2S(g)} + \mathrm{O_2(g)} \rightarrow 2\mathrm{S(s)} + 2\mathrm{H_2O(l)} \]
    Redox (oxidation) reaction.
    Answer: (a) double displacement (b) displacement (c) redox/reduction (d) redox/oxidation.

  6. Exercise 34

    Why do we store silver chloride in dark coloured bottles?

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    NCERT’s answer
    Silver chloride on exposure to sunlight may decompose as per the following rection. \[2 \mathrm{AgCl} \longrightarrow 2 \mathrm{Ag}+\mathrm{Cl}_2 \] Therefore, it is stored in dark coloured bottles.
    Silver chloride slowly decomposes on exposure to light (a photolytic decomposition), turning grey: \[2\mathrm{AgCl(s)} \xrightarrow{\text{sunlight}} 2\mathrm{Ag(s)} + \mathrm{Cl_2(g)} \] A dark bottle blocks light and prevents this breakdown, keeping the compound stable.Answer: Stored in dark bottles to prevent light-induced (photochemical) decomposition of \(\displaystyle \mathrm{AgCl}\) into \(\displaystyle \mathrm{Ag}\) and \(\displaystyle \mathrm{Cl_2}\).

  7. Exercise 35

    Balance the following chemical equations and identify the type of chemical reaction.
    (a)
    Mg(s)+Cl2( g)⟶MgCl2( s)\displaystyle \mathrm{Mg}(\mathrm{s})+\mathrm{Cl}_2(\mathrm{~g}) \longrightarrow \mathrm{MgCl}_2(\mathrm{~s})
    (b)
    HgO(s)→ Heat Hg(l)+O2( g)\displaystyle \mathrm{HgO}(\mathrm{s}) \xrightarrow{\text { Heat }} \mathrm{Hg}(\mathrm{l})+\mathrm{O}_2(\mathrm{~g})
    (c)
    Na(s)+S(s)→ Fuse Na2 S( s)\displaystyle \mathrm{Na}(\mathrm{s})+\mathrm{S}(\mathrm{s}) \xrightarrow{\text { Fuse }} \mathrm{Na}_2 \mathrm{~S}(\mathrm{~s})
    (d)
    TiCl4(l)+Mg(s)⟶Ti(s)+MgCl2( s)\displaystyle \mathrm{TiCl}_4(\mathrm{l})+\mathrm{Mg}(\mathrm{s}) \longrightarrow \mathrm{Ti}(\mathrm{s})+\mathrm{MgCl}_2(\mathrm{~s})
    (e)
    CaO(s)+SiO2( s)⟶CaSiO3( s)\displaystyle \mathrm{CaO}(\mathrm{s})+\mathrm{SiO}_2(\mathrm{~s}) \longrightarrow \mathrm{CaSiO}_3(\mathrm{~s})
    (f)
    H2O2(l)→UVH2O(l)+O2( g)\displaystyle \mathrm{H}_2 \mathrm{O}_2(\mathrm{l}) \xrightarrow{\mathrm{UV}} \mathrm{H}_2 \mathrm{O}(\mathrm{l})+\mathrm{O}_2(\mathrm{~g})

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    NCERT’s answer
    (a)
    Balanced; Combination reaction
    (b)
    \(\displaystyle 2 \mathrm{HgO}(\mathrm{s}) \xrightarrow{\text { Heat }} 2 \mathrm{Hg}(\mathrm{l})+\mathrm{O}_2(\mathrm{~g})\); Decomposition reaction
    (c)
    \(\displaystyle 2 \mathrm{Na}(\mathrm{s})+\mathrm{S}(\mathrm{s}) \xrightarrow{\text { Fuse }} \mathrm{Na}_2 \mathrm{~S}(\mathrm{~s})\); Combination reaction
    (d)
    \(\displaystyle \mathrm{TiCl}_4(\mathrm{l})+2 \mathrm{Mg}(\mathrm{s}) \longrightarrow \mathrm{Ti}(\mathrm{s})+2 \mathrm{MgCl}_2(\mathrm{~s})\); Displacement reaction
    (e)
    Balanced; Combination reaction
    (f)
    \(\displaystyle 2 \mathrm{H}_2 \mathrm{O}_2(\mathrm{l}) \xrightarrow{\mathrm{UV}} 2 \mathrm{H}_2 \mathrm{O}(\mathrm{l})+\mathrm{O}_2(\mathrm{~g})\); Decomposition reaction
    (a)
    \[\mathrm{Mg(s)} + \mathrm{Cl_2(g)} \rightarrow \mathrm{MgCl_2(s)} \] Combination reaction.
    (b)
    \[2\mathrm{HgO(s)} \xrightarrow{\text{Heat}} 2\mathrm{Hg(l)} + \mathrm{O_2(g)} \] Decomposition (thermal, redox) reaction.
    (c)
    \[2\mathrm{Na(s)} + \mathrm{S(s)} \xrightarrow{\text{Fuse}} \mathrm{Na_2S(s)} \] Combination reaction.
    (d)
    \[\mathrm{TiCl_4(l)} + 2\mathrm{Mg(s)} \rightarrow \mathrm{Ti(s)} + 2\mathrm{MgCl_2(s)} \] Displacement reaction.
    (e)
    \[\mathrm{CaO(s)} + \mathrm{SiO_2(s)} \rightarrow \mathrm{CaSiO_3(s)} \] Combination reaction.
    (f)
    \[2\mathrm{H_2O_2(l)} \xrightarrow{\text{UV}} 2\mathrm{H_2O(l)} + \mathrm{O_2(g)} \] Decomposition (photolytic) reaction.
    Answer: (a),(c),(e) combination; (b),(f) decomposition; (d) displacement.

  8. Exercise 36

    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.
    (a)
    Write the chemical formulae of X and Y.
    (b)
    Write a balanced chemical equation, when X is dissolved in water.

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    NCERT’s answer
    \(\displaystyle 2 \mathrm{Mg}+\mathrm{O}_2 \longrightarrow 2 \mathrm{MgO}\)
    \[3 \mathrm{Mg}+\mathrm{N}_2 \longrightarrow \mathrm{Mg}_3 \mathrm{~N}_2 \]
    (a)
    X is \(\displaystyle \mathrm{MgO} ; \mathrm{Y}\) is \(\displaystyle \mathrm{Mg}_3 \mathrm{~N}_2\)
    (b)
    \(\displaystyle \mathrm{MgO}+\mathrm{H}_2 \mathrm{O} \longrightarrow \mathrm{Mg}(\mathrm{OH})_2\)
    (a)
    Burning in oxygen gives the white oxide; continuing in nitrogen gives the nitride:
    \[2\mathrm{Mg(s)} + \mathrm{O_2(g)} \rightarrow 2\mathrm{MgO(s)} \quad (X) \]
    \[3\mathrm{Mg(s)} + \mathrm{N_2(g)} \rightarrow \mathrm{Mg_3N_2(s)} \quad (Y) \]
    (b)
    \[\mathrm{MgO(s)} + \mathrm{H_2O(l)} \rightarrow \mathrm{Mg(OH)_2(aq)} \]
    Answer: \(\displaystyle X=\mathrm{MgO}\), \(\displaystyle Y=\mathrm{Mg_3N_2}\); \(\displaystyle \mathrm{MgO}+\mathrm{H_2O}\rightarrow \mathrm{Mg(OH)_2}\).

  9. Exercise 37

    Zinc liberates hydrogen gas when reacted with dilute hydrochloric acid, whereas copper does not. Explain why?

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    Zinc lies above hydrogen in the activity series, so it displaces \(\displaystyle \mathrm{H_2}\) from the acid; copper lies below hydrogen, so it cannot. \[\mathrm{Zn(s)} + 2\mathrm{HCl(aq)} \rightarrow \mathrm{ZnCl_2(aq)} + \mathrm{H_2(g)} \] \[\mathrm{Cu(s)} + \mathrm{HCl(aq)} \rightarrow \text{No reaction} \]Answer: \(\displaystyle \mathrm{Zn}\) is above hydrogen and displaces \(\displaystyle \mathrm{H_2}\); \(\displaystyle \mathrm{Cu}\) is below and cannot.

  10. Exercise 38

    A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.
    (a)
    Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved.
    (b)
    Name the black substance formed and give its chemical formula.

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    NCERT’s answer
    (a)
    Metals such as silver when attacked by substances around it such as moisture, acids, gases etc, are said to corrode and this phenomenon is called corrosion.
    (b)
    The black substance is formed because silver (Ag) reacts with \(\displaystyle \mathrm{H}_2 \mathrm{~S}\) present in air. It forms thin black coating of silver sulphide \(\displaystyle \left(\mathrm{Ag}_2 \mathrm{~S}\right)\).
    (a)
    Trace \(\displaystyle \mathrm{H_2S}\) in air reacts with the silver surface (with atmospheric \(\displaystyle \mathrm{O_2}\)); the phenomenon is corrosion (tarnishing):
    \[4\mathrm{Ag(s)} + 2\mathrm{H_2S(g)} + \mathrm{O_2(g)} \rightarrow 2\mathrm{Ag_2S(s)} + 2\mathrm{H_2O(l)} \]
    (b)
    The black coating is silver sulphide, \(\displaystyle \mathrm{Ag_2S}\).
    Answer: Corrosion by atmospheric \(\displaystyle \mathrm{H_2S}\) forms black \(\displaystyle \mathrm{Ag_2S}\) on the surface.