A reaction in which two or more reactants combine to form a single product is called a combination reaction. For example, calcium oxide reacts vigorously with water to form calcium hydroxide. The reaction is highly exothermic in nature, as lots of heat is produced during the reaction.
\(\mathrm{CaO}_{(s)}+\mathrm{H}_{2} \mathrm{O}_{(l)} \longrightarrow \mathrm{Ca}(\mathrm{OH})_{2(a q)}+\text { Heat }\)
Calcium oxide        Water              Calcium hydroxide
Solution of Ca(OH)₂ is used for white wash the walls. Calcium hydroxide reacts slowly with carbon dioxide in air to form a thin layer of calcium carbonate on the wall which gives a shiny appearance to wall. Calcium carbonate will form after two or three days of white wash.
(i) What is the chemical name of quick lime?

(ii) When carbon dioxide is passed through lime water,

(iii) Following observations are observed when calcium oxide reacts vigorously with water.

Identify the incorrect observations
(I) It is an endothermic reaction                       (II) Slaked lime is produced.
(III) Quick lime is produced.                             (IV) It is an exothermic reaction.
(V) It is a combination reaction

(iv) Quick lime combines Vigorously with water to form (A) which reacts slowly with the carbon dioxide in air to form (B)
Identify the compounds(A) and (B)

(v) Among the following, the endothermic reaction is

Reactions in which one element takes place of another element in a compound, are known as displacement reactions. In general, more reactive elements displaces a less reactive element from its compound. In all single displacement reactions, only one element displaces another element from its compound. The single displacement reactions are, however, written as just displacement reactions. The displacement reaction between iron (III) oxide and powdered aluminium produces so much heat that iron metal obtained is in molten form.
(i) Copper displaces which of the following metals from its salt solution?

(ii) When zinc reacts with dilute sulphuric acid, the gas evolved is

(iii) When dry hydrogen is passed over a heated oxide of metal X using the apparatus shown below, a reddish-brown residue is obtained

The reddish -brown residue could be

(iv) Which of the following reactions is a displacement reaction?

(v) When dilute hydrochloric acid is added to granulated zinc placed in a test tube, the observation made is

Those reactions in which two compounds react by an exchange of ions to form two new compounds are called double displacement reactions. A double displacement reaction usually occurs in solution and one of the products, being insoluble, precipitate out (separates as a solid). Any reaction in which an insoluble solid (called precipitate) is formed that separates from the solution is called a precipitation reaction. The reaction in which acid or acidic oxide reacts with base or basic oxide to form salt and water is called neutralisation reaction.
For example, \(2 \mathrm{NaOH}+\mathrm{H}_{2} \mathrm{SO}_{4} \longrightarrow \mathrm{Na}_{2} \mathrm{SO}_{4}+\mathrm{H}_{2} \mathrm{O}\)
(i) When hydrogen sulphide gas is passed through a blue solution of copper sulphate, a black precipitate of copper sulphide is obtained and the sulphuric acid so formed remains in the solution. The reaction is an example of a

(ii) Which of the following is not a double displacement reaction?

(iii) Barium chloride on reaction with ammonium sulphate forms barium sulphate and ammonium chloride. Which of the following correctly represents the type of the reaction involved?
(I) Displacement reaction 
(II) Precipitation reaction
(III) Combination reaction
(IV) Double displacement reaction

(iv) Identify A in the following reaction.
\(\mathrm{AlCl}_{3(a q)}+3 \mathrm{NH}_{4} \mathrm{OH}_{(a q)} \longrightarrow A+3 \mathrm{NH}_{4} \mathrm{Cl}_{(a q)}\)

(v) Consider the following reaction,
\(\mathrm{BaCl}_{2}+\mathrm{Na}_{2} \mathrm{SO}_{4} \longrightarrow \mathrm{BaSO}_{4}+2 \mathrm{NaCl}\)
identify the precipitate in the reaction,

The earlier concept of oxidation and reduction is based on the addition or removal of oxygen or hydrogen elements so, in terms of oxygen and hydrogen, oxidation is addition of oxygen to a substance and removal of hydrogen from a substance. On the other hand, reduction is addition of hydrogen to a substance and removal of oxygen from a substance. The substance which gives oxygen to another substance or removes hydrogen from another substance in an oxidation reaction is known as oxidising agent, while the substance which gives hydrogen to another substance or removes oxygen from another substance in a reduction reaction is known as reducing agent. For example,

(i) A redox reaction is one in which

(ii) In the reaction, \(\mathrm{H}_{2} \mathrm{~S}+\mathrm{Cl}_{2} \longrightarrow \mathrm{S}+2 \mathrm{HCl}\)

(iii) Which ofthe following processes does not involve either oxidation or reduction?

\(\text { (iv) } \mathrm{Mg}+\mathrm{CuO} \longrightarrow \mathrm{MgO}+\mathrm{Cu}\)

   Which of the following is wrong relating to the above reaction?

(v) Identify the correct oxidising agent and reducing agent in the following reaction.
\(\mathrm{Fe}_{2} \mathrm{O}_{3}+2 \mathrm{Al} \longrightarrow 2 \mathrm{Fe}+\mathrm{Al}_{2} \mathrm{O}_{3}\)

Oxidation has damaging effect on metals as well as on food. The damaging effect of oxidation on metal is studied as corrosion and that on food is studied as rancidity. The phenomenon due to which metals are slowly eaten away by the reaction of air, water and chemicals present in atmosphere, is called corrosion. For example, iron articles are shiny when new, but get coated with a reddish brown powder when left for sometime. This process is known as rusting of iron. Rancidity is the process of slow oxidation of oil and fat (which are volatile in nature) present in the food materials resulting in the change of smell and taste in them.
(i) Rancidity can be prevented by

(ii) Combination of phosphorus and oxygen is an example of

(iii) A science teacher wrote the following statements about rancidity :
 (I) When fats and oils are reduced, they become rancid.
(II) In chips packet, rancidity is prevented by oxygen.
(III) Rancidity is prevented by adding antioxidants.
Select the correct option.

(iv) Two statements are given below regarding rusting of iron.
(I) The rusting of iron is a redox reaction and reaction occurs as, \(4 \mathrm{Fe}+3 \mathrm{O}_{2} \longrightarrow 4 \mathrm{Fe}^{3+}+6 \mathrm{O}^{2-}\)
(II) The metallic iron is oxidised to \(\mathrm{Fe}^{2+} \text { and } \mathrm{O}_{2} \text { is reduced to } \mathrm{O}^{2-}\)
Select the correct statement(s).

(v) Which of the following measures can be adopted to prevent or slow down rancidity?
(I) Food materials should be packed in air tight container.
(II) Food should be refrigerated.
(III) Food materials and cooked food should be kept away from direct sunlight