Magnesium combines with atmospheric oxygen to form magnesium oxide.
2Mg+O₂ \(\rightarrow \) 2MgO

Example of a decomposition reaction:
\(\underset { limestone }{ { CaCO }_{ 3 }(s) } \overset { heat }{ \rightarrow } \underset { quick\quad lime }{ CaO(s) } +C{ O }_{ 2 }(g)\)
Here, when limestone (calcium carbonate) is heated, it decomposes into quick lime (calcium oxide) and carbon di oxide (gas).

Photosynthesis is a reaction in which energy is required to form glucose from carbon dioxide and water. Energy in the form of sunlight is also required to break the bonds of hydrogen and oxygen. hence it is termed as endothermic reaction.

Reaction depends upon the reactivity of metals. Copper is less reactive than zinc. Copper is also known as noble metal along with gold, silver, etc. that are very less reactive under normal conditions. This is the cause that copper does not react with dilute hydrochloric acid and consequently does not liberate hydrogen gas while zinc does.

(a) No change takes place because silver metal categorized as noble metal and does not react with hydrochloric acid in normal situations.
(b) The reaction between hydrochloric acid and aluminium is exothermic, thus the temperature of the reaction mixture rises when aluminium is added.
(c) Since, sodium is a highly reactive metal, thus it reacts with hydrochloric acid vigorously and produces, large amount of heat. Thus the reaction between
hydrochloric acid and sodium is highly explosive. Even sodium reacts with water vigorously.
(d) Bubbles of hydrogen gas are formed when lead reacts with dilute hydrochloric acid, which is seen in the course of reaction.

(a) \(\underset{Iron(III)oxide}{Fe_2O_3}+\underset{Aluminium}{2Al} \rightarrow\underset{Iron}{2Fe}+\underset{Aluminium\ oxide}{Al_2O_3}+Heat+Light\)
It is an exothermic redox reaction.
(b) \(\underset{Magnesium}{3Mg(s)}+\underset{Nitrogen\ gas}{N_2}\rightarrow\underset{Magnesium\ nitride}{{}Mg_3N_2(s)}\)
It is a combination reaction.
(c) \(\underset{Potassium\ Iodide}{2K(aq)}+\underset{Chlorine}{Cl_2(g)}\rightarrow\underset{Potassium\ chloride}{2KCl(aq)}+\underset{Iodine}{I_2(s)}\)
It is a redox reaction.
Potassium chloride Iodine
(d)\(\underset{Ethanal}{C_2H_5OH}+\underset{Oxygen}{3O_2}\rightarrow\underset{Carbon\ dioxide}{2CO_2}+\underset{Water}{3H_2O}+Heat\)
It is an exothermic - redox reaction.

The yellowish compound is silver bromide [AgBr].
When silver bromide is exposed to sun light, it produces a grey coloured silver metal.
\(2AgBr\longrightarrow2Ag+Br_2\)
Thus 'A' is silver bromide [AgBr], 'B' is silver metal [Ag] and 'C' is bromine [Br₂]
The nitrate of 'B' is silver nitrate [AgNO3]
Silver nitrate when treated with sodium bromide, it forms silver bromide.
AgNO₃+NaBr\(\rightarrow\)AgBr+NaNO₃

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) 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

(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

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.