Class 10 Science Chapter 1 Chemical Reactions and Equations NCERT Solutions

Q. Using these steps, can you balance Eq. (1.2) given in the text earlier?

Answer: Eq. (1.2) is the skeletal equation $\ce{Mg + O2 -> MgO}$. Oxygen: 2 atoms on the LHS, 1 on the RHS, so put the coefficient 2 before MgO. Magnesium: now 2 atoms on the RHS and 1 on the LHS, so put 2 before Mg. Balanced equation: $\ce{2Mg + O2 -> 2MgO}$ (Mg 2 = 2, O 2 = 2).

Q. Why should a magnesium ribbon be cleaned before burning in air?

Answer: On standing in air, the surface of a magnesium ribbon gets covered with a dull layer of magnesium oxide (formed by its slow reaction with oxygen of the air). This layer does not burn and stops the magnesium underneath from reaching the oxygen, so the ribbon does not burn properly. Rubbing it with sandpaper removes the layer, so the clean metal burns readily with a dazzling white flame.

Q. Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen

Answer: (i) $\ce{H2 + Cl2 -> 2HCl}$
(ii) $\ce{3BaCl2 + Al2(SO4)3 -> 3BaSO4 + 2AlCl3}$
(iii) $\ce{2Na + 2H2O -> 2NaOH + H2}$

Q. Write a balanced chemical equation with state symbols for the following reactions.
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Answer: (i) $\ce{BaCl2(aq) + Na2SO4(aq) -> BaSO4(s) + 2NaCl(aq)}$
(ii) $\ce{NaOH(aq) + HCl(aq) -> NaCl(aq) + H2O(l)}$

Q. Identify the type of the reaction taking place in Activity 1.1, where heat is given out along with the formation of a single product.

Answer: $\ce{2Mg(s) + O2(g) -> 2MgO(s)}$. Magnesium and oxygen combine to form a single product, magnesium oxide, so it is a combination reaction; since heat is given out along with the product, it is also an exothermic reaction.

Q. Take about 2 g barium hydroxide in a test tube. Add 1 g of ammonium chloride and mix with the help of a glass rod. Touch the bottom of the test tube with your palm. What do you feel? Is this an exothermic or endothermic reaction?

Answer: The bottom of the test tube feels cold. The reaction absorbs heat from its surroundings (including your palm), so it is an endothermic reaction: $\ce{Ba(OH)2 + 2NH4Cl -> BaCl2 + 2NH3 + 2H2O}$.

Q. A solution of a substance ‘X’ is used for whitewashing.
(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.

Answer: (i) ‘X’ is calcium oxide (quicklime), formula $\ce{CaO}$.
(ii) $\ce{CaO(s) + H2O(l) -> Ca(OH)2(aq)}$ + heat. The calcium hydroxide (slaked lime) formed is the solution used for whitewashing.

Q. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.

Answer: In the electrolysis of water, $\ce{2H2O(l) -> 2H2(g) + O2(g)}$: every two molecules of water give two molecules of hydrogen but only one molecule of oxygen. So the volume of hydrogen collected is double the volume of oxygen. The gas collected in double amount is hydrogen.

Q. Recall Activity 1.2, where you have mixed the solutions of lead(II) nitrate and potassium iodide.
(i) What was the colour of the precipitate formed? Can you name the compound precipitated?
(ii) Write the balanced chemical equation for this reaction.
(iii) Is this also a double displacement reaction?

Answer: (i) The precipitate is yellow; the compound precipitated is lead iodide ($\ce{PbI2}$).
(ii) $\ce{Pb(NO3)2(aq) + 2KI(aq) -> PbI2(s) + 2KNO3(aq)}$
(iii) Yes. Lead(II) nitrate and potassium iodide exchange ions (the lead ions pair with iodide ions and the potassium ions with nitrate ions), so this is a double displacement reaction; since a precipitate forms, it is also a precipitation reaction.

Q. Recall Activity 1.1, where a magnesium ribbon burns with a dazzling flame in air (oxygen) and changes into a white substance, magnesium oxide. Is magnesium being oxidised or reduced in this reaction?

Answer: Magnesium is being oxidised, because it gains oxygen to form magnesium oxide: $\ce{2Mg + O2 -> 2MgO}$.

Q. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Answer: Iron is more reactive than copper, so it displaces copper from copper sulphate solution: $\ce{Fe(s) + CuSO4(aq) -> FeSO4(aq) + Cu(s)}$. As blue copper sulphate is used up and iron sulphate forms in its place, the blue colour of the solution fades, and brown copper is deposited on the nail.

Q. Give an example of a double displacement reaction other than the one given in Activity 1.10.

Answer: Mixing lead nitrate solution with potassium iodide solution (Activity 1.2) gives a yellow precipitate of lead iodide: $\ce{Pb(NO3)2(aq) + 2KI(aq) -> PbI2(s) + 2KNO3(aq)}$. The two compounds exchange ions, so this is a double displacement (precipitation) reaction.

Q. Identify the substances that are oxidised and the substances that are reduced in the following reactions.
(i) $\ce{4Na(s) + O2(g) -> 2Na2O(s)}$
(ii) $\ce{CuO(s) + H2(g) -> Cu(s) + H2O(l)}$

Answer: (i) Sodium (Na) gains oxygen, so it is oxidised; oxygen ($\ce{O2}$) is reduced.
(ii) Copper oxide (CuO) loses oxygen, so it is reduced; hydrogen ($\ce{H2}$) gains oxygen to form water, so it is oxidised.

Q. $2\,\ce{PbO(s) + C(s) -> 2Pb(s) + CO2(g)}$
(a) Lead is getting reduced. (b) Carbon dioxide is getting oxidised. (c) Carbon is getting oxidised. (d) Lead oxide is getting reduced.
(i) (a) and (b)   (ii) (a) and (c)   (iii) (a), (b) and (c)   (iv) all

Answer: (i) (a) and (b). Carbon gains oxygen and is oxidised, forming CO2, so statement (c) “carbon is getting oxidised” and statement (d) “lead oxide is getting reduced” are both correct. Statement (a) “lead is getting reduced” is wrong, because the reactant is lead oxide, not lead: it is lead oxide that gets reduced to lead. Statement (b) “carbon dioxide is getting oxidised” is also wrong, since CO2 is the final product of this reaction; it is not itself being oxidised any further. So the incorrect statements are (a) and (b), answer (i).

(a) combination reaction (b) double displacement reaction (c) decomposition reaction (d) displacement reaction

Answer: (d) displacement reaction. Aluminium displaces iron from its oxide.

(a) Hydrogen gas and iron chloride are produced. (b) Chlorine gas and iron hydroxide are produced. (c) No reaction takes place. (d) Iron salt and water are produced.

Answer: (a) Hydrogen gas and iron chloride are produced. $\ce{Fe(s) + 2HCl(aq) -> FeCl2(aq) + H2(g)}$. Iron is more reactive than hydrogen, so it displaces hydrogen from the acid.

Answer: An equation in which the number of atoms of every element is equal on both sides of the arrow is called a balanced chemical equation. By the law of conservation of mass, mass can neither be created nor destroyed in a chemical reaction, so an equation must be balanced, otherwise it would go against this basic law of nature.

(a) Hydrogen gas combines with nitrogen to form ammonia. (b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide. (c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate. (d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Answer (a): Hydrogen combines with nitrogen to form ammonia: $\ce{N2(g) + 3H2(g) -> 2NH3(g)}$
Answer (b): Hydrogen sulphide gas burns in air to give water and sulphur dioxide: $2\,\ce{H2S(g) + 3O2(g) -> 2H2O(l) + 2SO2(g)}$
Answer (c): Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate: $3\,\ce{BaCl2(aq) + Al2(SO4)3(aq) -> 2AlCl3(aq) + 3BaSO4(s)}$
Answer (d): Potassium metal reacts with water to give potassium hydroxide and hydrogen gas: $2\,\ce{K(s) + 2H2O(l) -> 2KOH(aq) + H2(g)}$

(a) $\ce{HNO3 + Ca(OH)2 -> Ca(NO3)2 + H2O}$ (b) $\ce{NaOH + H2SO4 -> Na2SO4 + H2O}$ (c) $\ce{NaCl + AgNO3 -> AgCl + NaNO3}$ (d) $\ce{BaCl2 + H2SO4 -> BaSO4 + HCl}$

Answer (a): $2\,\ce{HNO3 + Ca(OH)2 -> Ca(NO3)2 + 2H2O}$
Answer (b): $2\,\ce{NaOH + H2SO4 -> Na2SO4 + 2H2O}$
Answer (c): $\ce{NaCl + AgNO3 -> AgCl + NaNO3}$ (this equation is already balanced)
Answer (d): $\ce{BaCl2 + H2SO4 -> BaSO4 + 2HCl}$

(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water (b) Zinc + Silver nitrate → Zinc nitrate + Silver (c) Aluminium + Copper chloride → Aluminium chloride + Copper (d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

Answer (a): $\ce{Ca(OH)2 + CO2 -> CaCO3 + H2O}$
Answer (b): $\ce{Zn + 2AgNO3 -> Zn(NO3)2 + 2Ag}$
Answer (c): $2\,\ce{Al + 3CuCl2 -> 2AlCl3 + 3Cu}$
Answer (d): $\ce{BaCl2 + K2SO4 -> BaSO4 + 2KCl}$

(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s) (b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g) (c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g) (d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)

Answer (a): $2\,\ce{KBr(aq) + BaI2(aq) -> 2KI(aq) + BaBr2(s)}$: double displacement reaction
Answer (b): $\ce{ZnCO3(s) -> ZnO(s) + CO2(g)}$: (thermal) decomposition reaction
Answer (c): $\ce{H2(g) + Cl2(g) -> 2HCl(g)}$: combination reaction
Answer (d): $\ce{Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g)}$: displacement reaction

Answer: A reaction in which heat is given out along with the product is called an exothermic reaction: examples are burning of natural gas and respiration. A reaction in which energy (heat/light/electricity) is absorbed is called an endothermic reaction: for example, the thermal decomposition of $\ce{CaCO3}$, or the reaction of barium hydroxide with ammonium chloride.

Answer: In respiration, glucose reacts with oxygen to give carbon dioxide, water and energy: $\ce{C6H12O6(aq) + 6O2(aq) -> 6CO2(aq) + 6H2O(l)}$ + energy. Because this reaction releases energy (which the body then uses for its activities), respiration is considered an exothermic reaction.

Answer: In a combination reaction, two or more substances combine to form a single product, $\ce{CaO + H2O -> Ca(OH)2}$. In a decomposition reaction, a single reactant breaks down into two or more simpler products, $\ce{CaCO3 -> CaO + CO2}$. Since substances join together in one and break apart in the other, decomposition is called the opposite of combination.

Answer (heat): $\ce{CaCO3(s) ->[heat] CaO(s) + CO2(g)}$ (thermal decomposition)
Answer (light): $2\,\ce{AgCl(s) ->[sunlight] 2Ag(s) + Cl2(g)}$
Answer (electricity): $2\,\ce{H2O(l) ->[electricity] 2H2(g) + O2(g)}$ (electrolysis of water)

Answer: In a displacement reaction, a more reactive element displaces a less reactive element from its compound, $\ce{Zn + CuSO4 -> ZnSO4 + Cu}$. In a double displacement reaction, there is an exchange of ions between the reactants, $\ce{Na2SO4 + BaCl2 -> BaSO4 v + 2NaCl}$ (here the exchange of ions also produces a precipitate of barium sulphate).

Answer: $2\,\ce{AgNO3(aq) + Cu(s) -> Cu(NO3)2(aq) + 2Ag(s)}$. Copper is more reactive than silver, so it displaces silver from silver nitrate solution.

Answer: A reaction in which an insoluble solid (a precipitate) is formed is called a precipitation reaction. Example: $\ce{Na2SO4(aq) + BaCl2(aq) -> BaSO4(s) + 2NaCl(aq)}$, where insoluble white barium sulphate is formed as a precipitate.

Answer (a) Oxidation: $2\,\ce{Cu + O2 -> 2CuO}$ and $\ce{C + O2 -> CO2}$: oxygen is gained in both.
Answer (b) Reduction: $\ce{CuO + H2 -> Cu + H2O}$ and $\ce{ZnO + C -> Zn + CO}$: oxygen is lost in both.

Answer: Element ‘X’ is copper (Cu), and the black compound formed is copper(II) oxide (CuO). $2\,\ce{Cu(s) + O2(g) -> 2CuO(s)}$.

Answer: So that the iron surface does not come into direct contact with moisture and oxygen from the air, which stops or slows down corrosion (rusting).

Answer: Nitrogen is an inert gas. Flushing it into the packet pushes out the oxygen present, which slows down oxidation of the fats (rancidity), so the food does not turn stale as quickly.

Answer (a) Corrosion: When a metal is attacked by substances around it, such as moisture and acids, it is said to corrode, and this process is called corrosion. Example: rusting of iron (a reddish-brown coating forms on it).
Answer (b) Rancidity: Fat/oil-containing food gets oxidised, changing its taste and smell, example: fried food kept for many days going stale.

Q. Take four beakers and label them as A, B, C and D. Put 25 mL of water in A, B and C beakers and copper sulphate solution in beaker D. Measure and record the temperature of each liquid. Add two spatulas of potassium sulphate, ammonium nitrate, anhydrous copper sulphate and fine iron fillings to beakers A, B, C and D respectively and stir. Finally measure and record the temperature of each mixture. Find out which reactions are exothermic and which ones are endothermic in nature.

Answer: Beaker A (potassium sulphate): the temperature falls slightly, so the change is endothermic. Beaker B (ammonium nitrate): the temperature falls noticeably, so it is endothermic. Beaker C (anhydrous copper sulphate): the solution becomes warm, so it is exothermic. Beaker D (iron fillings in copper sulphate solution): iron displaces copper, $\ce{Fe(s) + CuSO4(aq) -> FeSO4(aq) + Cu(s)}$, and the mixture becomes warm, so this reaction is exothermic. (In A, B and C the salt is only dissolving in water; D is the true chemical reaction.)