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

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

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

An example of photochemical reactions is the process of photosynthesis occurring in the leaves of green plants.
\(\underset { Carbon\\ dioxide }{ { 6CO }_{ 2 } } +\underset { Water}{ { 6H }_{ 2 } O} \overset { sunlight }{ \underset { Chlorophyll }{ \rightleftharpoons } } \underset {Glucose }{ C_6H_{12}O_6 } +\underset {Oxygen }{6{ O }_{ 2 }}\)

CuO + H₂ → Cu + H₂O

Quick lime is CaO. On adding water to it, large amount of heat is released with hissing sound. Carbon dioxide gas is released and calcium oxide is formed.

\(\underset{Potassium\ chloride}{KCl(aq)}+\underset{Silver\ nitrate}{AgNO_3(aq)}\rightarrow\underset{Silver\ chlorie\\ (white\ ppt)}{AgCl(s)\downarrow}+\underset{Pottassium\ nitrate}{KNO_3(aq)}\)
The is Precipitation as well as double displacement reaction.

(a) Ca(OH)₂ + CO₂ \(\rightarrow\) CaCO₃ + H₂O
(b) Zn + 2AgNO₃ \(\rightarrow\) Zn(NO₃)₂ + 2Ag
(c) 2Al + 3CuCl₂ \(\rightarrow\) 2AlCl₃ + 3Cu
(d) BaCl₂ + K₂SO₄ \(\rightarrow\) BaSO₄ + 2KCl

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 composition of water, i.e. the chemical formula H₂O, suggests that the molar ratio of hydrogen and oxygen is 2 : 1. Therefore, when water is electrically decomposed, the constituent gases hydrogen and oxygen are produced in the same molar ratio, 2 : 1. Thus, the amount (volume) of hydrogen gas is double than that of oxygen gas. So, this gas is hydrogen.

Magnesium ribbon reacts with oxygen present in air to form a protective and inert layer of magnesium oxide on its surface. This layer is unreactive and prevents the ribbon from burning. Hence, it needs to be cleaned with sand paper before burning in air.

N₂ + H₂ \(\rightarrow \)  NH₃ (Unbalanced equation).
(i) Examine the number of atoms of different elements present in the unbalanced equation.
(ii) In the above reaction, left hand N₂ and H₂ of both the sides are unbalanced.
(iii) To balance hydrogen, H₂ is multiplied by 3 on side. It makes 6H-atoms on the left hand side.
(iv) Now to balance hydrogen atoms on the right hand side, NH₃ should be multiplied by 2. It makes 6H-atoms on this side.
(v) Now to balance nitrogen atoms, they are counted separately for both the sides and we will find that, nitrogen atoms are 2 on both the sides.
(vi) Balanced chemical equation will be :
N₂ + H₂ \(\rightarrow \)  NH₃.

Reaction in which a single reactant breaks down to give simpler products.
Thermal decomposition-
\(CaCO_{ 2 }\overset { \triangle }{ \longrightarrow } CaO+C{ O }_{ 2 }\)
(Or any other)
Electrolytic decomposition-
2H₂O\(\overset { Electric\quad current }{ \rightarrow } \) 2H₂+O₂
(or)
In a decomposition reaction, a single reactant breaks down into two or more simpler products.
When a decomposition reaction is carried out by heating, it is called thermal decomposition reaction.
\(\underset { (Calucium\quad carbonate) }{ CaCO_{ 3 }(s) } \overset { Heat }{ \rightarrow } \underset { Calcium\quad oxide) }{ CaO(s) } +\underset { (Carbon\quad dioxide) }{ { CO }_{ 2 }(g) } \)
When a decomposition reaction is carried out with the help of electric current, the process is called electrolysis.
\(\underset { Water }{ 2{ H }_{ 2 }O(I) } \overset { Electric\quad current }{ \longrightarrow } \underset { (Hydrogen\quad gas) }{ 2{ H }_{ 2 }(g) } +\underset { (Oxygen\quad gas) }{ { O }_{ 2 }(g) } \)