Mix Examples - Chemical Reactions and Equations Questions in English

(A) physical change is a change in which no new substance is formed and the chemical composition remains the same.
$A$. Combustion of $LPG$ is a chemical change because it involves a chemical reaction with oxygen to produce new substances like carbon dioxide and water,releasing energy.
$B$. Melting of ice is a physical change as it only involves a change in the state of water from solid to liquid.
$C$. Dissolution of salt in water is a physical change because the salt can be recovered by evaporation,and no new chemical substance is formed.
$D$. Boiling of water is a physical change as it only involves a change in the state of water from liquid to gas.
Therefore,the combustion of $LPG$ is not a physical change.

(B) $1$. Analyze the oxidation states of the elements in the reaction:
- In $NH_{3}$,the oxidation state of $N$ is $-3$ and $H$ is $+1$.
- In $O_{2}$,the oxidation state of $O$ is $0$.
- In $NO$,the oxidation state of $N$ is $+2$ and $O$ is $-2$.
- In $H_{2}O$,the oxidation state of $H$ is $+1$ and $O$ is $-2$.
$2$. Observe the changes:
- Nitrogen $(N)$ changes from $-3$ to $+2$ (oxidation).
- Oxygen $(O)$ changes from $0$ to $-2$ (reduction).
Since both oxidation and reduction occur,it is a redox reaction $(iii)$.
$3$. Identify the reaction type:
- This is a displacement reaction because $O_{2}$ displaces $H$ from $NH_{3}$ to form $NO$ and $H_{2}O$,which is a type of oxidation-reduction displacement $(i)$.
Therefore,the reaction is both a displacement reaction and a redox reaction.

(C) In the given reaction: $3 Fe (s) + 4 H_2 O (g) \rightarrow Fe_3 O_4 (s) + 4 H_2 (g)$
$(i)$ Iron $(Fe)$ gains oxygen to form $Fe_3 O_4$,so it is getting oxidised.
$(ii)$ Water $(H_2 O)$ loses oxygen to form $H_2$,so it is getting reduced.
$(iii)$ Since water provides oxygen to iron,it acts as an oxidising agent.
$(iv)$ Therefore,statement $(iv)$ is correct while statement $(iii)$ is incorrect.
Thus,statements $(i), (ii)$ and $(iv)$ are correct.

(D) An exothermic process is a process that releases energy in the form of heat.
$(i)$ The reaction of water with quick lime $(CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat})$ is highly exothermic, releasing a large amount of heat.
$(ii)$ The dilution of an acid is a highly exothermic process, which is why it is recommended to add acid to water slowly with constant stirring.
$(iii)$ Evaporation of water is an endothermic process because it requires the absorption of heat from the surroundings to change the state from liquid to gas.
$(iv)$ Sublimation of camphor is an endothermic process as it requires heat energy to change the state from solid to gas.
Therefore, $(i)$ and $(ii)$ are exothermic processes.

(A) $1$. When $NaOH$ and anhydrous $CuSO_4$ are dissolved in water,the temperature of the solution increases. This indicates that heat is released during the dissolution process,which is characteristic of an exothermic process.
$2$. When $NaCl$ is dissolved in water,the temperature of the solution decreases. This indicates that heat is absorbed from the surroundings,which is characteristic of an endothermic process.
$3$. Therefore,statement $(i)$ is correct because beakers $A$ and $B$ involve exothermic processes.
$4$. Statement $(iv)$ is correct because beaker $C$ involves an endothermic process.
$5$. Thus,the correct combination is $(i)$ and $(iv)$.

(B) The reaction between acidified potassium permanganate $(KMnO_4)$ and ferrous sulphate $(FeSO_4)$ is a classic redox reaction.
In this reaction,$KMnO_4$ acts as a strong oxidising agent.
It oxidises ferrous ions $(Fe^{2+})$ to ferric ions $(Fe^{3+})$.
During this process,the purple-coloured permanganate ions $(MnO_4^-)$ are reduced to colourless manganese ions $(Mn^{2+})$.
Therefore,the purple colour of the solution fades and eventually disappears as the reaction proceeds.

(C) double displacement reaction is a chemical reaction where two reactant compounds exchange ions to form two new compounds.
$(i)$ $Pb + CuCl_{2} \rightarrow PbCl_{2} + Cu$ is a displacement reaction where $Pb$ displaces $Cu$.
$(ii)$ $Na_{2}SO_{4} + BaCl_{2} \rightarrow BaSO_{4} + 2NaCl$ is a double displacement reaction where $Na^+$ and $Ba^{2+}$ ions exchange places.
$(iii)$ $C + O_{2} \rightarrow CO_{2}$ is a combination reaction.
$(iv)$ $CH_{4} + 2O_{2} \rightarrow CO_{2} + 2H_{2}O$ is a combustion reaction.
Therefore,only $(ii)$ is a double displacement reaction.

(B) When silver chloride $(AgCl)$ is exposed to sunlight, it undergoes a photochemical decomposition reaction.
The chemical equation for this reaction is: $2AgCl(s) \xrightarrow{\text{sunlight}} 2Ag(s) + Cl_2(g)$.
In this reaction, white silver chloride decomposes into grey-colored metallic silver $(Ag)$ and chlorine gas $(Cl_2)$.
Statement $(i)$ is true because the grey color is due to the formation of metallic silver.
Statement $(iii)$ is also true because the decomposition of silver chloride results in the release of chlorine gas.
Therefore, both $(i)$ and $(iii)$ are correct.

(A) The reaction between calcium oxide $(CaO)$ and water $(H_2O)$ is represented as: $CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq) + \text{Heat}$.
$1$. Since heat is released during this process, it is an exothermic reaction. Thus, statement $(ii)$ is correct.
$2$. Calcium hydroxide $(Ca(OH)_2)$ is a base. When it dissolves in water to form lime water, the resulting solution is basic in nature.
$3$. Basic solutions have a $pH$ value greater than $7$. Thus, statement $(iii)$ is correct.
Therefore, the correct statements are $(ii)$ and $(iii)$.

(B) The chemical equation for the reaction is:
$BaCl_2(aq) + (NH_4)_2SO_4(aq) \rightarrow BaSO_4(s) + 2NH_4Cl(aq)$
$1$. In this reaction, the ions $Ba^{2+}$ and $NH_4^+$ are exchanged between the reactants, which characterizes a $\text{Double displacement reaction}$ $(iv)$.
$2$. Barium sulphate $(BaSO_4)$ is formed as an insoluble white solid, which is known as a precipitate. Therefore, this is also a $\text{Precipitation reaction}$ $(ii)$.
Thus, both $(ii)$ and $(iv)$ are correct.

(C) The chemical equation for the electrolysis of water is: $2H_2O(l) \rightarrow 2H_2(g) + O_2(g)$.
According to the balanced chemical equation,$2$ moles of water decompose to produce $2$ moles of hydrogen gas and $1$ mole of oxygen gas.
Therefore,the mole ratio of hydrogen gas to oxygen gas liberated is $2:1$.

(D) An endothermic process is one that absorbs heat from the surroundings.
$(i)$ Dilution of sulphuric acid is an exothermic process because it releases a large amount of heat.
$(ii)$ Sublimation of dry ice ($CO_2$ solid to gas) requires the absorption of heat,making it an endothermic process.
$(iii)$ Condensation of water vapours (gas to liquid) releases heat,making it an exothermic process.
$(iv)$ Evaporation of water (liquid to gas) requires the absorption of heat,making it an endothermic process.
Therefore,both $(ii)$ and $(iv)$ are endothermic processes.

(A) double displacement reaction requires soluble ionic compounds to provide ions in the solution.
Lead nitrate is soluble in water and provides $Pb^{2+}$ ions,which react with $I^-$ ions from potassium iodide to form a yellow precipitate of lead iodide $(PbI_2)$.
Lead acetate is also a soluble lead salt that dissociates to provide $Pb^{2+}$ ions,making it a suitable substitute for lead nitrate in this reaction.
Lead sulphate is insoluble in water and will not provide the necessary $Pb^{2+}$ ions for the reaction to occur.

(B) Oils and fats are prone to rancidity,which is the oxidation of fats and oils when exposed to air.
To prevent this,food items containing oils and fats are often flushed with inert gases.
Nitrogen is an inert gas commonly used to replace oxygen in food packaging to prevent oxidation.
Helium is also an inert gas that does not react with oils.
Therefore,helium or nitrogen can be used to store fresh samples of oil for a long time to prevent rancidity.

(C) $1$. The given reaction is $2KClO_{3(s)} \xrightarrow[Catalyst]{Heat} 2KCl_{(s)} + 3O_{2(g)}$.
$2$. In this reaction,a single reactant $(KClO_3)$ breaks down into simpler products ($KCl$ and $O_2$),which characterizes it as a decomposition reaction.
$3$. The reaction requires the supply of heat (thermal energy) to proceed,which indicates that it is an endothermic reaction.
$4$. Therefore,the correct statement is that it is a decomposition reaction and endothermic in nature.

(D) chemical reaction involves the formation of new substances with different chemical properties.
$A$. Storing oxygen gas under pressure is a physical change as it only changes the state of the gas.
$B$. Liquefaction of air is a physical process involving changes in temperature and pressure.
$C$. Keeping petrol in an open dish is a physical process of evaporation.
$D$. Heating copper wire in the presence of air at high temperature leads to the formation of copper$(II)$ oxide $(2Cu + O_2 \rightarrow 2CuO)$. This is a chemical reaction because a new substance is formed.

(A) The reaction between hydrogen gas $(H_2)$ and oxygen gas $(O_2)$ to form water $(H_2O)$ is a standard chemical reaction.
At room temperature and standard reaction conditions,hydrogen and oxygen exist as gases $(g)$.
When they react to form water,the product formed is liquid water $(l)$.
Therefore,the correct chemical equation representing the physical states is $2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$.

(B) combination reaction is a reaction in which two or more reactants combine to form a single product.
$(i) 2 KClO_3 \xrightarrow{\text{Heat}} 2 KCl + 3 O_2$: This is a decomposition reaction because a single reactant breaks down into simpler products.
$(ii) MgO + H_2O \longrightarrow Mg(OH)_2$: This is a combination reaction because two reactants ($MgO$ and $H_2O$) combine to form a single product $(Mg(OH)_2)$.
$(iii) 4 Al + 3 O_2 \longrightarrow 2 Al_2O_3$: This is a combination reaction because two reactants ($Al$ and $O_2$) combine to form a single product $(Al_2O_3)$.
$(iv) Zn + FeSO_4 \longrightarrow ZnSO_4 + Fe$: This is a displacement reaction because $Zn$ displaces $Fe$ from its salt solution.
Therefore, $(ii)$ and $(iii)$ are combination reactions.

(N/A) The balanced chemical equation is: ${N_2}(g) + 3{H_2}(g) \xrightarrow[{773 \ K}]{{\text{Catalyst}}} 2NH_3(g)$.
This is a combination reaction because two reactants combine to form a single product.
$(b)$ The balanced chemical equation is: $NaOH(aq) + CH_3COOH(aq) \longrightarrow CH_3COONa(aq) + H_2O(l)$.
This is a double displacement reaction,specifically a neutralisation reaction,as an acid reacts with a base to form salt and water.

(N/A) $C_2H_5OH(l) + CH_3COOH(l) \xrightarrow{conc. H_2SO_4} CH_3COOC_2H_5(l) + H_2O(l)$
This is an esterification reaction (a type of condensation reaction).
$(b)$ $C_2H_4(g) + 3O_2(g) \longrightarrow 2CO_2(g) + 2H_2O(g) + \text{Heat} + \text{Light}$
This is a combustion reaction (a type of redox reaction).

(N/A) $Fe_{2}O_{3}(s) + 2Al(s) \longrightarrow Al_{2}O_{3}(s) + 2Fe(l) + \text{Heat}$
This is a displacement reaction (specifically a thermit reaction) and also a redox reaction.
$(b)$ $3Mg(s) + N_{2}(g) \longrightarrow Mg_{3}N_{2}(s)$
This is a combination reaction.

(N/A) $2KI(aq) + Cl_2(g) \longrightarrow 2KCl(aq) + I_2(s)$
This is a displacement reaction because chlorine displaces iodine from potassium iodide.
$(b)$ $C_2H_5OH(l) + 3O_2(g) \longrightarrow 2CO_2(g) + 3H_2O(l) + \text{Heat}$
This is a combustion reaction,which is also a type of redox reaction,as ethanol reacts with oxygen to release energy.

(A) In chemical equations,the physical states of substances are represented as $(s)$ for solid,$(l)$ for liquid,$(g)$ for gas,and $(aq)$ for aqueous solution.
$(a)$ In the reaction $Pb(NO_3)_2(aq) + 2KI(aq) \longrightarrow PbI_2(s) + 2KNO_3(aq)$,$PbI_2$ is a precipitate formed,so $x = (s)$. $KNO_3$ remains dissolved in the solution,so $y = (aq)$.
$(b)$ In the reaction $Cu(s) + 2AgNO_3(aq) \longrightarrow Cu(NO_3)_2(aq) + 2Ag(s)$,silver is displaced as a solid metal,so $x = 2Ag$.

(A) In the reaction between zinc and sulfuric acid, $ZnSO_4$ is formed as an aqueous solution, so $x = (aq)$. Hydrogen gas is released as a product, so $y = (g)$.
$(b)$ The decomposition of calcium carbonate into calcium oxide and carbon dioxide requires the application of heat, so $x = \text{Heat}$.

(B) An exothermic process is one that releases heat into the surroundings, while an endothermic process absorbs heat from the surroundings.
$(a)$ Decomposition of ferrous sulphate $(2FeSO_4(s) \xrightarrow{\Delta} Fe_2O_3(s) + SO_2(g) + SO_3(g))$ requires heat input, so it is endothermic.
$(b)$ Dilution of sulphuric acid is a highly exothermic process as it releases a large amount of heat.
$(c)$ Dissolution of sodium hydroxide in water $(NaOH(s) \rightarrow Na^+(aq) + OH^-(aq) + \text{Heat})$ is an exothermic process.
$(d)$ Dissolution of ammonium chloride in water $(NH_4Cl(s) + \text{Heat} \rightarrow NH_4^+(aq) + Cl^-(aq))$ absorbs heat, so it is endothermic.
Therefore, $(b)$ and $(c)$ are exothermic, while $(a)$ and $(d)$ are endothermic.

(N/A) In $4 NH_{3} + 5 O_{2} \longrightarrow 4 NO + 6 H_{2}O$,the oxidation state of nitrogen in $NH_{3}$ increases from $-3$ to $+2$. Thus,$NH_{3}$ is the reducing agent.
$(b)$ In $H_{2}O + F_{2} \longrightarrow HF + HOF$,the oxidation state of oxygen in $H_{2}O$ increases from $-2$ to $0$ (in $HOF$). Thus,$H_{2}O$ is the reducing agent.
$(c)$ In $Fe_{2}O_{3} + 3 CO \longrightarrow 2 Fe + 3 CO_{2}$,the oxidation state of carbon in $CO$ increases from $+2$ to $+4$. Thus,$CO$ is the reducing agent.
$(d)$ In $2 H_{2} + O_{2} \longrightarrow 2 H_{2}O$,the oxidation state of hydrogen in $H_{2}$ increases from $0$ to $+1$. Thus,$H_{2}$ is the reducing agent.
$A$ reducing agent is a substance that donates electrons or increases its own oxidation state during a chemical reaction.

(A) An oxidising agent is a substance that gains electrons or provides oxygen (or removes hydrogen) in a chemical reaction.
$(a) Pb_{3}O_{4}$: Lead is reduced from $+8/3$ to $+2$,so $Pb_{3}O_{4}$ acts as the oxidising agent.
$(b) O_{2}$: Oxygen is reduced from $0$ to $-2$,so $O_{2}$ acts as the oxidising agent.
$(c) CuSO_{4}$: Copper is reduced from $+2$ to $0$,so $CuSO_{4}$ acts as the oxidising agent.
$(d) V_{2}O_{5}$: Vanadium is reduced from $+5$ to $0$,so $V_{2}O_{5}$ acts as the oxidising agent.
$(e) H_{2}O$: Hydrogen is reduced from $+1$ to $0$,so $H_{2}O$ acts as the oxidising agent.
$(f) CuO$: Copper is reduced from $+2$ to $0$,so $CuO$ acts as the oxidising agent.

(N/A) $Na_2CO_3 + HCl \longrightarrow NaCl + NaHCO_3$
$(b)$ $NaHCO_3 + HCl \longrightarrow NaCl + H_2O + CO_2$
$(c)$ $2CuSO_4 + 4KI \longrightarrow Cu_2I_2 + 2K_2SO_4 + I_2$

(N/A) The chemical reaction between potassium chloride $(KCl)$ and silver nitrate $(AgNO_3)$ is as follows:
$KCl(aq) + AgNO_3(aq) \longrightarrow AgCl(s) + KNO_3(aq)$
In this reaction,silver chloride $(AgCl)$ is formed as an insoluble white precipitate.
This is a double displacement reaction because there is an exchange of ions between the reactants.
It is also classified as a precipitation reaction because an insoluble solid (precipitate) is formed.

(N/A) The chemical reaction for the thermal decomposition of ferrous sulphate is:
$2FeSO_4(s) \xrightarrow{\text{Heat}} Fe_2O_3(s) + SO_2(g) + SO_3(g)$
In this reaction,green ferrous sulphate crystals lose water on heating and then decompose to form ferric oxide,sulphur dioxide,and sulphur trioxide. The gas evolved $(SO_2)$ has the characteristic smell of burning sulphur. Since a single reactant breaks down into multiple products upon heating,it is classified as a thermal decomposition reaction.

(B) Fireflies possess a specialized protein called luciferin,which,in the presence of an enzyme called luciferase and oxygen,undergoes a chemical reaction known as bioluminescence.
This reaction involves the oxidation of luciferin,which releases energy in the form of visible light.
Therefore,fireflies glow at night as a result of this chemical process.

(N/A) Grapes attached to the plant are living,and their internal immune system and metabolic processes prevent fermentation.
Once plucked,the grapes are no longer connected to the parent plant's supply of nutrients and defense mechanisms.
Fermentation occurs when microbes (like yeast) act on the sugars present in the grapes under anaerobic conditions (absence of oxygen).
This process results in the formation of new substances like ethanol and carbon dioxide,which is a chemical change.

(A) ,$(c)$,and $(e)$ are physical changes because they do not involve the formation of new substances and are often reversible.
$(b)$ and $(d)$ are chemical changes because they involve the formation of new substances with different chemical properties and are generally irreversible.

(N/A) Silver $(Ag)$ is less reactive than hydrogen in the reactivity series,so it does not displace hydrogen from dilute $HCl$.
$(b)$ The reaction between aluminium and dilute $HCl$ is exothermic,meaning it releases heat energy,which causes the temperature of the reaction mixture to rise.
$(c)$ Sodium is a highly reactive alkali metal. Its reaction with dilute $HCl$ is extremely exothermic and rapid,releasing a large amount of heat that ignites the hydrogen gas produced,making it highly explosive.
$(d)$ Lead reacts with dilute $HCl$ to form lead$(II)$ chloride and hydrogen gas. The bubbles observed are due to the evolution of hydrogen gas: $Pb(s) + 2HCl(aq) \rightarrow PbCl_2(aq) + H_2(g)$.

(A) The substance $X$ is Calcium oxide $(CaO)$, commonly known as quicklime.
$1$. It is an oxide of Calcium, which is a group $2$ element.
$2$. Calcium is a major component of bones in the form of calcium phosphate.
$3$. It is used extensively in the cement industry.
$4$. When $CaO$ reacts with water, it forms Calcium hydroxide $(Ca(OH)_2)$, which is a strong base and turns red litmus blue.
The chemical reaction is:
$CaO(s) + H_2O(l) \longrightarrow Ca(OH)_2(aq) + \text{Heat}$

(N/A) $Pb(CH_3COO)_2(aq) + 2HCl(aq) \longrightarrow PbCl_2(s) + 2CH_3COOH(aq)$ : Double displacement reaction.
$(b)$ $2Na(s) + 2C_2H_5OH(l) \longrightarrow 2C_2H_5ONa(alc) + H_2(g)$ : Displacement reaction (specifically,a single displacement or redox reaction).

(N/A) The balanced chemical equation is: $Fe_{2}O_{3}(s) + 3CO(g) \longrightarrow 2Fe(s) + 3CO_{2}(g)$.
This is a redox reaction because iron $(III)$ oxide is reduced to iron,and carbon monoxide is oxidized to carbon dioxide.
$(b)$ The balanced chemical equation is: $2H_{2}S(g) + O_{2}(g) \longrightarrow 2S(s) + 2H_{2}O(l)$.
This is a redox reaction because hydrogen sulphide is oxidized to sulphur,and oxygen is reduced to water.

(N/A) Silver chloride $(AgCl)$ is a light-sensitive compound.
When exposed to sunlight,it undergoes a photochemical decomposition reaction to form metallic silver and chlorine gas.
The chemical equation for this reaction is:
$2AgCl(s) \xrightarrow{\text{sunlight}} 2Ag(s) + Cl_2(g)$
To prevent this decomposition and preserve the chemical,it is stored in dark-coloured bottles that block light.

(N/A) Balanced equation: $Mg(s) + Cl_2(g) \to MgCl_2(s)$. This is a Combination reaction because two reactants combine to form a single product.
$(b)$ Balanced equation: $2HgO(s) \xrightarrow{\text{Heat}} 2Hg(l) + O_2(g)$. This is a Decomposition reaction because a single reactant breaks down into two simpler products upon heating.
$(c)$ Balanced equation: $2Na(s) + S(s) \xrightarrow{\text{Fuse}} Na_2S(s)$. This is a Combination reaction because two reactants combine to form a single product.

(N/A) Balanced equation: $TiCl_4(l) + 2Mg(s) \to Ti(s) + 2MgCl_2(s)$. This is a displacement reaction because $Mg$ displaces $Ti$ from its chloride.
$(b)$ Balanced equation: $CaO(s) + SiO_2(s) \to CaSiO_3(s)$. This is a combination reaction because two reactants combine to form a single product.
$(c)$ Balanced equation: $2H_2O_2(l) \to 2H_2O(l) + O_2(g)$. This is a decomposition reaction because a single reactant breaks down into simpler products.

(A) When magnesium ribbon burns in oxygen,it forms magnesium oxide $(MgO)$:
$2Mg(s) + O_2(g) \longrightarrow 2MgO(s)$ (Compound $X$)
When magnesium ribbon continues to burn in nitrogen,it forms magnesium nitride $(Mg_3N_2)$:
$3Mg(s) + N_2(g) \longrightarrow Mg_3N_2(s)$ (Compound $Y$)
$(a)$ The chemical formula of $X$ is $MgO$ and the chemical formula of $Y$ is $Mg_3N_2$.
$(b)$ When magnesium oxide $(X)$ is dissolved in water,it forms magnesium hydroxide:
$MgO(s) + H_2O(l) \longrightarrow Mg(OH)_2(aq)$

(N/A) In the reactivity series of metals,zinc is placed above hydrogen,which means it is more reactive than hydrogen. Therefore,zinc can displace hydrogen from dilute hydrochloric acid to form zinc chloride and hydrogen gas.
$Zn(s) + 2HCl(aq) \longrightarrow ZnCl_2(aq) + H_2(g)$
Conversely,copper is placed below hydrogen in the reactivity series,meaning it is less reactive than hydrogen. Consequently,copper cannot displace hydrogen from dilute hydrochloric acid,and no reaction occurs.
$Cu(s) + HCl(aq) \longrightarrow \text{No reaction}$

(N/A) Metals such as silver,when exposed to substances in their surroundings like moisture,acids,or gases,undergo a process called corrosion. In the case of silver,it reacts with sulfur compounds in the air,leading to the formation of a black layer. This phenomenon is known as the corrosion of metals.
$(b)$ The black substance formed is silver sulphide. Its chemical formula is $Ag_2S$.

(N/A) Balanced chemical equation:
$2Cu(NO_3)_2(s) \xrightarrow{\text{Heat}} 2CuO(s) + O_2(g) + 4NO_2(g)$
$(b)$ The brown gas $X$ evolved is nitrogen dioxide $(NO_2)$.

(N/A) This is a thermal decomposition reaction.
$(b)$ The brown gas $X$ is nitrogen dioxide $(NO_2)$. When nitrogen dioxide dissolves in water,it forms nitric acid $(HNO_3)$ and nitrous acid $(HNO_2)$,resulting in an acidic solution. Since the solution is acidic,the $pH$ range of the aqueous solution of gas $X$ is less than $7$.

(N/A) Carbon dioxide $(CO_2)$ gas turns lime water milky when passed through it due to the formation of insoluble calcium carbonate.
$Ca(OH)_2 + CO_2 \to CaCO_3 + H_2O$
$(b)$ Sulphur dioxide $(SO_2)$ gas,when passed through an acidic potassium permanganate solution (purple in colour),turns it colourless because $SO_2$ is a strong reducing agent.
$2KMnO_4 + 2H_2O + 5SO_2 \to K_2SO_4 + 2MnSO_4 + 2H_2SO_4$
Alternatively,sulphur dioxide gas,when passed through an acidic potassium dichromate solution (orange in colour),turns it green because sulphur dioxide is a strong reducing agent.

(N/A) The evolution of oxygen $(O_2)$ gas during a reaction can be confirmed by bringing a burning candle near the mouth of the test tube containing the reaction mixture. The intensity of the flame increases because oxygen supports combustion.
$(b)$ Hydrogen $(H_2)$ gas burns with a 'pop' sound when a burning candle or matchstick is brought near it.

(A) Zinc,being more reactive than copper,displaces copper from its solution,and a solution of zinc sulphate is obtained.
$Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)$
This is an example of a displacement reaction.
$(b)$ Aluminium,being more reactive,displaces hydrogen from dilute hydrochloric acid solution,and hydrogen gas is evolved.
$2Al(s) + 6HCl(aq) \rightarrow 2AlCl_3(aq) + 3H_2(g)$
$(c)$ Silver metal,being less reactive than copper,cannot displace copper from its salt solution. Therefore,no reaction occurs.
$Ag(s) + CuSO_4(aq) \rightarrow \text{No reaction}$

(N/A) The reaction of $Zn$ granules with:
$(a)$ Dilute $H_{2}SO_{4}$:
$Zn(s) + H_{2}SO_{4}(aq) \rightarrow ZnSO_{4}(aq) + H_{2}(g)$
$(b)$ Dilute $HCl$:
$Zn(s) + 2HCl(aq) \rightarrow ZnCl_{2}(aq) + H_{2}(g)$
$(c)$ Dilute $HNO_{3}$:
Reaction with dilute $HNO_{3}$ is different compared to other acids because nitric acid is a strong oxidizing agent. It oxidizes the $H_{2}$ gas evolved to $H_{2}O$ and gets reduced itself to nitrogen oxides (like $N_{2}O$,$NO$,or $NO_{2}$ depending on concentration). For very dilute $HNO_{3}$,the reaction is:
$4Zn(s) + 10HNO_{3}(aq) \rightarrow 4Zn(NO_{3})_{2}(aq) + 5H_{2}O(l) + N_{2}O(g)$

(N/A) The reaction of $Zn$ granules with:
$(a)$ $NaCl$ solution:
$Zn(s) + NaCl(aq) \rightarrow$ No reaction.
Zinc is less reactive than sodium,so it cannot displace sodium from its salt solution.
$(b)$ $NaOH$ solution:
$Zn(s) + 2NaOH(aq) \rightarrow Na_2ZnO_2(aq) + H_2(g)$
When zinc reacts with a hot concentrated solution of sodium hydroxide,it forms sodium zincate and releases hydrogen gas.