Reagents/chemicals like silver bromide decompose when exposed to light. Hence, they are kept in dark bottles in labs, to prevent exposure to light and their decomposition.

(i) 2KBr(aq) + BaI₂(aq) ) \(\longrightarrow\)2KI(aq) + BaBr₂(aq)
It is a double displacement reaction.
(ii) H₂(g) + CI₂(g) \(\longrightarrow\) 2HCl(g)

The volume of gas collected over one electrode is double than that of the gas collected over the other electrode as the ratio of H : O in water is 2:1.

The reaction which is accompanied by the formation of an insoluble solid mass (called precipitate) is known as precipitation reaction, e.g.
(i) When barium chloride solution is added to an aqueous solution of sodium sulphate, a white precipitate of barium sulphate is obtained.
BaCl₂(aq) + Na₂SO₄(aq) \(\rightarrow\) BaSO₄(s) \(\downarrow\) + 2NaCl (aq)
                                               (White ppt.)
(ii) When silver nirate is added to an aqueous solution of sodium chloride, a white precipitate of silver chloride (AgCl), which is soluble in NH₄OH is obtained.
AgNO₃(aq) + NaCl(aq) \(\rightarrow\) AgCl(s) \(\downarrow\) + NaNO₃(aq)
                                           (White ppt.)

In a displacement reaction, a more reactive element displaces a less reactive element from its salt solution. But in a double displacement reaction, two atoms or groups from different compounds displace each other. Chemical equation for single displacement,
 Zn(s) + CuSO₄(aq) \( \longrightarrow \) ZnSO₄(aq) + Cu(s)
Here, Zn displaces Cu from its salt solution (CuSO₄).
Chemical equation for double displacement,
BaCl₂(aq) + K₂SO₄(aq) \( \longrightarrow \) BaSO₄ (s) + 2KCl(aq)
Here, Ba and K displace each other.

Exothermic reaction.

\(\underset { (Lead\quad nitrate) }{ Pb({ { NO }_{ 3 } })_{ 2 } } +\underset { )Potassium\quad iodide) }{ 2KI } \longrightarrow \quad \underset { (Pottassime\quad nitrate) }{ 2KN{ O }_{ 3 } } +\underset { (Lead\quad iodide) }{ Pb{ I }_{ 2 } } \)

Quick lime reacts with water vigorously to produce slaked lime and a large amount of heat.
\(\underset { (Quick\quad lime) }{ CaO(s) } +{ H }_{ 2 }O\rightarrow \underset { (Slaked\quad lime) }{ Ca(O{ H) }_{ 2 } } +Heat\)

Law of conservation of mass.
Mass can neither be created nor destroyed during a chemical reaction.

Combination reaction.

Hydrogen gas.

Calcium oxide (CaO), commonly known as quicklime 

Nitrogen dioxide gas (reddish brown in colour) and Oxygen gas (colourless).

Silver bromide.

Hydrogen gas is evolved.

(i) 4Na (s) + O₂ (g) \(\rightarrow\) 2Na₂O (s)
Na has gained oxygen and forms Na₂O.
So, Na is oxidised and O₂ is reduced
(ii) CuO(s) + H₂ (g)  \(\rightarrow\) Cu(s) + H₂ O (l)
CuO has lost oxygen and forms Cu.
So, Cu is reduced while H₂ has gained oxygen, hence, it is oxidised.

The following reaction is an example of a double displacement reaction:
2NaOH (aq)     +    H₂SO₄ (aq)      \(\rightarrow\)  Na₂SO₄(aq)       +   2H₂O (l)
Sodium hydroxide  Sulphuric acid          Sodium sulphate     Water

The colour of copper sulphate solution changes when an iron nail is dipped in it because iron being more reactive than copper, displaces copper metal from aqueous copper sulphate solution. Thus, blue colour of copper sulphate fades away to give green colour solution of ferrous sulphate.
Fe (s) + CuSO₄(aq) \(\rightarrow\) FeSO₄(aq) + Cu(s)
Grey       Blue                  Green         Brown       

The green colour of ferrous sulphate crystals is due to the presence of 7 water molecules (water of crystallization). :It loses the water of crystallization on heating, thus leading to change in colour.

(a) Photochemical decomposition reaction: Those reactions in which a compound breaks down into simple substances in presence of light are called photochemical decomposition reaction. The colour of salt will change from white to grey.
(b)  \( 2 \mathrm{AgCl}(\mathrm{s}) \stackrel{\text { Sunlight }}{\longrightarrow} 2 \mathrm{Ag}(\mathrm{s})+\mathrm{Cl}_{2}(\mathrm{~g}) \)
(c) Silver chloride is used in photography.

(i) Before heating, it is pale green.
After heating, it is brown or reddish brown.
Two observations are:
(a) Change in state and colour
(b) Evolution of gas
(ii) Decomposition reaction
2FeSO₄(s) \(\overset { Heat }{ \rightarrow } \) Fe₂O₃(s) + S0₂(g) + SO₃(g) 

(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.
\((ii)\quad Cu+2AgN{ O }_{ 3 }\longrightarrow \underset { Copperr\quad (II)\quad nitrate\\ bluish\quad green }{ Cu{ (N{ O }_{ 3 }) }_{ 2 } } +\underset { Silver }{ 2Ag } \)

The food taken by the living beings is ultimately broken down to glucose by the digestive system. The glucose so formed is slowly oxidised to carbon dioxide and water with the release of heat energy. Thus, respiration is an exothermic reaction.
C₆H₁₂O₆(aq) + 6O₂(g) \(\rightarrow\) 6CO₂(g) + 6H₂O(l) + energy
   (Glucose)