(i)A colourless gas is evolved.
(ii) Test-tube becomes hot.

Black layer was deposited due to coating of silver, because copper being more reactive than silver, displaced silver from silver nitrate solution.
2AgNO₃(aq) + Cu \(\rightarrow \) Cu(NO₃)₂(aq) + 2Ag(s)

In a physical change, no new substance is formed. In a chemical change, new substance is formed.

Oxidation the reaction seen during rancidity of food.

Magnesium powder particles provide more surface area for contact with the molecules of sulphuric acid as compared to that of magnesium ribbon.

Iron reacts with HCl to release hydrogen gas and iron chloride is formed. This reaction is called as displacement reaction.

(a) \(\underset{Lead\ acetate}{Pb(CH_3COO)_2}+\underset{Hydrochloric\ acid}{2HCl}\rightarrow\underset{Lead\ chloride}{PbCl_2}+\underset{Aceti}{2CH_3COOH}\)
It is a double displacement reaction .
(b) \(\underset{sodium}{2Na}+\underset{Ethanol}{2C_2H_5ONa}+\underset{Hydrogen}{H_2}\)
It is a displacement reaction.
(c) \(\underset{Ferric\ oxiside}{Fe_2O_3(s)}+\underset{Carbon\ monoxide}{3CO(s)}\rightarrow\underset{Iron}{2Fe(s)}+\underset{Carbon\ dioside}{3CO_2(g)}\)
It is a redox reaction.
(d) \(\underset{Hydrogen\ sulphate}{2H_2S(g)}+\underset{Oxygen}{O_2(g)}\rightarrow\underset{sulphur}{2S(s)}+\underset{Water}{2H_2O(l)}\)
It is a redox reaction.

\(\underset{Ferrous\ sulphate}{2FeSO_4(s)}\xrightarrow{Heat/Decompostion}\underset{Ferric\ oxide}{Fe_2O_3}+\underset{sulphur\ dioxide}{SO_2(g)}+\underset{sulphur\ trioxide}{SO_3}\)
Sulphur dioxide evolves in this smells like burning of sulphur. This reaction is a thermal decomposition reaction.

(a) NH₃
(b) F₂
(c) CO
(d) H₂

(a)\(\underset{Nitrogen}{N_2}+\underset{Hydrogen}{3H_2} \xrightarrow{}Catalyst /773K\underset{Ammonia}{2NH_3}\)
It is a combination reaction
(b)\(\underset{Sodium Hydroxide}{NaOH}+\underset{Acetic\ acid}{CH_3COOH}\rightarrow\underset{Sodium\ acetate}{CH_3COONa}+ \underset{Water}{H_2O}\)
Since, number of various atoms present in both side is equal, thus it is balanced equation. It is a double displacement reaction.
(c)\(\underset{Ethanol}{C_2H_5OH}+\underset{Ethanoic\ acid}{CH_3COOH}\xrightarrow{Warmed/Conc.H_2SO_4}\underset{Ethyl\ Acetate}{CH_3COOC_2H_5}+\underset{Water}{H_2O}\)
Since, the number of various atoms present in both side is equal, thus it is a balanced equation. It is a combination reaction
(d)\(\underset{Ethene}{C_2H_4}+\underset{Oxygen}{3O_2}\rightarrow\underset{Carbon\ dioxide}{2CO_2}+\underset{Water}{2H_2O}+Heat+Light\)
It is redox reaction.

By applying paint on iron articles, they can be prevented from corrosion (rusting). Paint does not allow oxygen (from air) and water (moisture) to come in contact with the surface of iron.

(a) 2HNO₃   +   Ca(OH)₂            \(\rightarrow\) Ca(NO₃)₂          + 2H₂O
Nitric acid      Calcium hydroxide     Calcium nitrate     Water
(b) 2NaOH          + H₂SO₄             \(\rightarrow\) Na₂SO₄           +    2H₂O
Sodium hydroxide  Sulphuric acid     Sodium sulphate   Water
(c) NaCl            +    AgNO₃         \(\rightarrow\)  AgCl            +  NaNO₃
Sodium chloride    Silver nitrate      Silver chloride    Sodium nitrate
(d) BaCl₂          + H₂SO₄             \(\rightarrow\) BaSO₄              + 2HCl
Barium chloride    Sulphuric acid     Barium sulphate   Hydrochloric acid

(i) BaCl₂(aq) + Na₂SO₄ (aq) \(\rightarrow\) BaSO₄(s)\(\downarrow\) + 2NaCl(aq)
(ii) NaOH (aq) + HCl (aq) \(\rightarrow\) NaCl (aq) + H₂O(l)

(a) Zinc sulphate and copper is formed when zinc reacts with copper sulphate,
\(\underset{Zinc}{Zn( s)} + \underset{Copper\ sulphate}{CuSO_4} (aq) \rightarrow \underset{Zinc\ sulphate}{ZnSO_4(aq)} + \underset{Copper}{Cu}\)
(b) Hydrochloric acid forms aluminium chloride and hydrogen gas when reacts with aluminium metal.
\(2AI + 6HCI\rightarrow 2AlCl_3 + 3H_2\)
(c) When silver metal is added to copper sulphate solution, no reaction takes place because silver is less reactive than copper

(i) CaCO₃(s) + 2HCl(aq) \(\longrightarrow \) CaCl₂(s)+ H₂O(I) + CO₂(g)
(ii) 2Al(s) + 6HCl(aq)\(\longrightarrow \) 2AlCI₃(aq) + 3H₂(g)
(iii) MnO₂ + 4HCl(aq) \(\longrightarrow \) MnCl₂ + 2H₂0(I) +Cl₂(g)

(i) Na₂CO₃(s) + 2HCl(aq) \(\longrightarrow \) 2NaCI(aq) + H₂O(I) + CO₂(g)
(ii) CaO(s) + H₂O(I) \(\longrightarrow \)Ca(OH)₂(aq) + Heat
(iii) Pb(NO₃)₂(aq) + NaCl(aq) \(\longrightarrow \) PbCI₂(s) + 2NaNO₃(aq)

(i) Substance X-Calcium Hydroxide.
Ca(OH)₂(aq) + CO₂(g)\(\longrightarrow \) CaCO₃(s) + H₂Cl
                                             (White ppt.)
(ii) Calcium hydroxide is obtained by reaction of calcium oxide and water.
CaO(s) + H₂O(l) \(\longrightarrow \)Ca(OH)₂(aq) + Heat

(i) Copper is more reactive than silver. Hence, when copper wire is dipped in silver nitrate solution, it displaces silver from AgNO₃ solution forming copper nitrate which is bluish green in colour.
\((ii)\quad Cu+2AgN{ O }_{ 3 }\longrightarrow \underset { Copperr\quad (II)\quad nitrate\\ bluish\quad green }{ Cu{ (N{ O }_{ 3 }) }_{ 2 } } +\underset { Silver }{ 2Ag } \)

(a) (i) Physical state of reactants and products.
(ii) Conditions such as temperature, pressure, heat etc.
(iii) Catalyst involved.
(iv) Change in state. 
(b) Total mass of the elements present in the products in a chemical reaction has to be equal to the total mass of elements present in the reactants or Mass can neither be created nor. destroyed in a chemical reaction.