Mix Examples - Chemical Reactions and Equations Questions in English

(N/A) $(i)$ The reaction between barium chloride and sodium sulphate is a double displacement reaction:
$BaCl_{2(aq)} + Na_2SO_{4(aq)} \to BaSO_{4(s)} + 2NaCl_{(aq)}$
$(ii)$ The reaction between sodium hydroxide and hydrochloric acid is a neutralization reaction:
$NaOH_{(aq)} + HCl_{(aq)} \to NaCl_{(aq)} + H_2O_{(l)}$

(A) The reaction is $H_{2(g)} + N_{2(g)} \to NH_{3(g)}$. Balancing gives $3H_{2(g)} + N_{2(g)} \to 2NH_{3(g)}$.
$(b)$ The reaction is $H_{2}S_{(g)} + O_{2(g)} \to H_{2}O_{(l)} + SO_{2(g)}$. Balancing gives $2H_{2}S_{(g)} + 3O_{2(g)} \to 2H_{2}O_{(l)} + 2SO_{2(g)}$.
$(c)$ The reaction is $BaCl_{2(aq)} + Al_{2}(SO_{4})_{3(aq)} \to AlCl_{3(aq)} + BaSO_{4(s)}$. Balancing gives $3BaCl_{2(aq)} + Al_{2}(SO_{4})_{3(aq)} \to 2AlCl_{3(aq)} + 3BaSO_{4(s)}$.
$(d)$ The reaction is $K_{(s)} + H_{2}O_{(l)} \to KOH_{(aq)} + H_{2(g)}$. Balancing gives $2K_{(s)} + 2H_{2}O_{(l)} \to 2KOH_{(aq)} + H_{2(g)}$.

(A-D) $2KBr(aq) + BaI_2(aq) \rightarrow 2KI(aq) + BaBr_2(s)$; Double displacement reaction.
$(b)$ $ZnCO_3(s) \rightarrow ZnO(s) + CO_2(g)$; Decomposition reaction.
$(c)$ $H_2(g) + Cl_2(g) \rightarrow 2HCl(g)$; Combination reaction.
$(d)$ $Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)$; Displacement reaction.

(N/A) Respiration is considered an exothermic reaction because energy is released during the process.
$1$. Living organisms require energy to survive, which is obtained from the food we eat.
$2$. During digestion, complex food molecules (like carbohydrates) are broken down into simpler substances like glucose.
$3$. This glucose combines with oxygen in the cells of our body to provide energy.
$4$. Since energy is released in the form of heat (or $ATP$) during this process, it is classified as an exothermic reaction.
The chemical equation for respiration is:
$C_{6}H_{12}O_{6(aq)} + 6O_{2(g)} \to 6CO_{2(g)} + 6H_{2}O_{(l)} + \text{Energy}$

(N/A) Decomposition reactions are those in which a single compound breaks down to form two or more simpler substances. These reactions typically require an input of energy in the form of heat, light, or electricity to proceed.
Combination reactions are those in which two or more substances combine to form a single new substance, often releasing energy in the process.
Therefore, they are considered opposites because decomposition involves breaking down a substance (requiring energy), while combination involves forming a substance (releasing energy).
Decomposition reaction: $2H_2O(l) \xrightarrow{\text{Electrolysis}} 2H_2(g) + O_2(g)$
Combination reaction: $2H_2(g) + O_2(g) \to 2H_2O(l) + \text{Energy}$

(N/A) In a displacement reaction,a more reactive element replaces a less reactive element from its compound.
$A + BX \longrightarrow AX + B$; where $A$ is more reactive than $B$.
In a double displacement reaction,two atoms or groups of atoms exchange their positions to form two new compounds.
$AB + CD \longrightarrow AD + CB$
Examples:
Displacement reaction:
$CuSO_{4(aq)} + Zn_{(s)} \longrightarrow ZnSO_{4(aq)} + Cu_{(s)}$
Double displacement reaction:
$Na_2SO_{4(aq)} + BaCl_{2(aq)} \longrightarrow BaSO_{4(s)} + 2NaCl_{(aq)}$

(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.

(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.

(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 $(iii)$.

(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.

(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) $Fe_{2}O_{3}(s) + 2Al(s) \longrightarrow Al_{2}O_{3}(s) + 2Fe(l) + \text{Heat}$
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) $Na_2CO_3 + HCl \longrightarrow NaCl + NaHCO_3$
$(b)$ $NaHCO_3 + HCl \longrightarrow NaCl + H_2O + CO_2$
$(c)$ $2CuSO_4 + 4KI \longrightarrow Cu_2I_2 + 2K_2SO_4 + I_2$

(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) $Pb(CH_3COO)_2(aq) + 2HCl(aq) \longrightarrow PbCl_2(s) + 2CH_3COOH(aq)$ : 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) 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) 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.

(N/A) When $Zn$ reacts with a dilute solution of a strong acid (such as $HCl$),it forms a salt and hydrogen gas $(H_2)$ is evolved.
The chemical equation is: $Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g) \uparrow$.
Test to detect the gas: When a burning splinter is brought near the mouth of the test tube containing the gas,it burns with a characteristic 'pop' sound,which confirms the presence of hydrogen gas.

(C) When we exhale,the air we breathe out contains a higher concentration of $CO_2$ compared to the air we inhale.
Lime water is a solution of calcium hydroxide,$Ca(OH)_2$.
When $CO_2$ is passed through lime water,it reacts to form calcium carbonate $(CaCO_3)$,which is insoluble in water and forms a white precipitate.
This reaction is represented as: $Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)$.
The formation of this white precipitate of calcium carbonate makes the lime water appear milky.

(A) The balanced chemical equation is:
$Na_{2}SO_{3}(aq) + BaCl_{2}(aq) \rightarrow BaSO_{3}(s) + 2NaCl(aq)$
$(b)$ This reaction is also known as a double displacement reaction.
$(c)$ $BaSO_{3}$ is a salt of a weak acid,sulphurous acid $(H_{2}SO_{3})$. When dilute hydrochloric acid $(HCl)$ is added,it reacts with the insoluble barium sulphite to form soluble barium chloride,water,and sulphur dioxide gas. The reaction is:
$BaSO_{3}(s) + 2HCl(aq) \rightarrow BaCl_{2}(aq) + H_{2}O(l) + SO_{2}(g)$
Since $BaCl_{2}$ is soluble in water,the white precipitate disappears.

(N/A) The element $X$ is sulphur $(S)$, which is a yellow solid at room temperature and exhibits catenation and allotropy.
$(b)$ The atomic number of sulphur is $16$. Its electronic configuration is $2, 8, 6$ or $1s^2 2s^2 2p^6 3s^2 3p^4$.
$(c)$ The thermal decomposition of ferrous sulphate crystals $(FeSO_4 \cdot 7H_2O)$ is represented by the following balanced chemical equation:
$2FeSO_4(s) \xrightarrow{\Delta} Fe_2O_3(s) + SO_2(g) + SO_3(g)$

(A) The correct answer is $A$. Burning of wax.
Reason: Burning of wax is a chemical change because candle wax is a hydrocarbon that reacts with oxygen to produce new substances like carbon dioxide $(CO_{2})$ and water vapor $(H_{2}O)$. This process is irreversible,as the original wax cannot be recovered from the combustion products.
In contrast,the melting of wax is a physical change because it only involves a change in the state of matter from solid to liquid,and the wax can be solidified again by cooling.

(B) In a physical change,the chemical composition of the substance remains the same and no new substance is formed.
In a chemical change,the chemical composition of the substance changes,resulting in the formation of one or more new substances with different properties.

(N/A) $(i)$ Decomposition reaction: The breakdown of complex organic matter into simpler substances by microorganisms is a decomposition process.
$(ii)$ Oxidation (Combustion) reaction: Burning of natural gas $(CH_4)$ in the presence of oxygen is an exothermic oxidation reaction, represented as: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{Heat}$.

(N/A) $(i)$ Corrosion (Oxidation reaction): Silver reacts with sulfur compounds in the air to form silver sulfide $(Ag_2S)$,which is black in color.
$(ii)$ Displacement reaction: Iron $(Fe)$ is more reactive than copper $(Cu)$,so it displaces copper from the copper sulphate $(CuSO_4)$ solution to form ferrous sulphate $(FeSO_4)$,which is green in color.

(N/A) To test for hydrogen gas,bring a burning matchstick or a burning splinter near the mouth of the test tube from which the gas is being evolved.
If the gas is hydrogen,it will burn with a characteristic '$pop$' sound,which is caused by the rapid combustion of hydrogen in the presence of oxygen.

(N/A) $FeSO_{4} \cdot 7H_{2}O$ crystals are green in colour. Upon heating,they first lose their water of crystallisation and turn white (anhydrous $FeSO_{4}$). On further heating,they decompose to form ferric oxide $(Fe_{2}O_{3})$,which is reddish-brown in colour,along with sulphur dioxide $(SO_{2})$ and sulphur trioxide $(SO_{3})$ gases.
Step $1$: Loss of water of crystallisation:
$FeSO_{4} \cdot 7H_{2}O(s) \xrightarrow{\Delta} FeSO_{4}(s) + 7H_{2}O(g)$
(Green) (White)
Step $2$: Decomposition of anhydrous ferrous sulphate:
$2FeSO_{4}(s) \xrightarrow{\Delta} Fe_{2}O_{3}(s) + SO_{2}(g) + SO_{3}(g)$
(Reddish-brown)

(N/A)

Displacement reactions	Double displacement reactions
$1.$ In these reactions, a more active element displaces a less active element from its salt solution.	$1.$ In these reactions, there is an exchange of ions between two reactants.
$2.$ These reactions are usually slow and take longer time for their completion.	$2.$ These reactions are usually fast and take place instantaneously.
$3.$ During these reactions, a change of color usually takes place. Example: $Fe(s) + CuSO_4(aq) \to FeSO_4(aq) + Cu(s)$ (Blue to Light green).	$3.$ During these reactions, precipitates are usually formed. Example: $AgNO_3(aq) + NaCl(aq) \to AgCl(s) \downarrow + NaNO_3(aq)$ (White precipitate).

(N/A) An example of an exothermic combination reaction is the formation of water from hydrogen and oxygen: $2H_{2}(g) + O_{2}(g) \rightarrow 2H_{2}O(l) + \text{Heat}$.
$(b)$ In the reaction $H_{2}S + Cl_{2} \rightarrow 2HCl + S$:
- The oxidizing agent is $Cl_{2}$ because it gains electrons (or helps in the removal of hydrogen).
- The reducing agent is $H_{2}S$ because it loses hydrogen (or provides electrons).
$(c)$ The phenomenon is known as Rancidity. It can be prevented by storing food in airtight containers to prevent oxidation.

(N/A) $(i)$ The metal is copper $(Cu)$.
The black coloured substance formed is copper$(II)$ oxide $(CuO)$.
$(ii)$ The balanced chemical equations are:
Reaction $1$ (Oxidation of copper): $2Cu(s) + O_2(g) \xrightarrow{\Delta} 2CuO(s)$
Reaction $2$ (Reduction of copper oxide): $CuO(s) + H_2(g) \xrightarrow{\Delta} Cu(s) + H_2O(g)$

(N/A) It is a photochemical decomposition reaction. It can be defined as the decomposition of a substance in the presence of sunlight in which a single reactant breaks down to give simpler products.
$(b)$ The white silver chloride changes to grey,as it decomposes into silver and chlorine in the presence of sunlight.
The chemical equation is:
$2 AgCl (s) \xrightarrow{\text{Sunlight}} 2 Ag (s) + Cl_2 (g)$
$(c)$ This salt is used in black and white photography.

(N/A) $(i)$ Decomposition reactions require energy in the form of heat,light,or electricity to break down the reactants,meaning energy is absorbed.
$(ii)$ Iron is more reactive than copper. When an iron nail is dipped in a copper sulphate solution,iron displaces copper from the solution to form iron sulphate,which is light green in colour.
$(iii)$ During respiration,glucose $(C_6H_{12}O_6)$ combines with oxygen in the cells of our body to produce carbon dioxide,water,and energy. Since energy is released during this process,it is considered an exothermic reaction.

(N/A) $(i)$ Electrolysis of water is an endothermic reaction because it requires electrical energy to break the bonds of water molecules.
$(ii)$ Burning of natural gas (methane) is an exothermic reaction as it releases a large amount of heat energy during combustion.
$(iii)$ Decomposition of calcium carbonate is an endothermic reaction because it requires heat energy to decompose into calcium oxide and carbon dioxide.
$(iv)$ Burning of magnesium ribbon in air is an exothermic reaction as it releases heat and light energy during the formation of magnesium oxide.

(N/A) To balance the equation $Fe(s) + H_2O(g) \rightarrow Fe_3O_4(s) + H_2(g)$:
$1$. Balance $Fe$: $3Fe(s) + H_2O(g) \rightarrow Fe_3O_4(s) + H_2(g)$
$2$. Balance $O$: $3Fe(s) + 4H_2O(g) \rightarrow Fe_3O_4(s) + H_2(g)$
$3$. Balance $H$: $3Fe(s) + 4H_2O(g) \rightarrow Fe_3O_4(s) + 4H_2(g)$
The balanced equation is $3Fe(s) + 4H_2O(g) \rightarrow Fe_3O_4(s) + 4H_2(g)$.
$(b)$ The reaction $Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s) + NaCl(aq)$ is a double displacement reaction because there is an exchange of ions between the reactants to form new products.

(N/A) $(i)$ The balanced chemical equation is:
$Na_{2}SO_{4}(aq) + BaCl_{2}(aq) \longrightarrow BaSO_{4}(s) \downarrow + 2NaCl(aq)$
$(ii)$ This is a double displacement reaction (specifically a precipitation reaction).
$(iii)$ Yes,a precipitate is formed.
$(iv)$ The precipitate formed is barium sulphate $(BaSO_{4})$,which is white in colour.

(N/A) $(i)$ $CaCO_{3} + 2HCl \longrightarrow CaCl_{2} + H_{2}O + CO_{2}$
$(ii)$ $N_{2} + 3H_{2} \xrightarrow{\text{pressure}} 2NH_{3}$
$(iii)$ $CS_{2} + 3O_{2} \longrightarrow CO_{2} + 2SO_{2}$

(N/A) $(i)$ The reaction between calcium carbonate and hydrochloric acid produces calcium chloride,water,and carbon dioxide: $CaCO_{3} + 2HCl \longrightarrow CaCl_{2} + H_{2}O + CO_{2}$
$(ii)$ The reaction between aluminium and hydrochloric acid produces aluminium chloride and hydrogen gas: $2Al + 6HCl \longrightarrow 2AlCl_{3} + 3H_{2}$
$(iii)$ The reaction between manganese dioxide and hydrochloric acid produces manganese$(II)$ chloride,chlorine gas,and water: $MnO_{2} + 4HCl \longrightarrow MnCl_{2} + Cl_{2} + 2H_{2}O$