Railway Science (Chemistry-Chemical Reaction) (Part-II)

Wasting away metals layer by layer due to the presence of air and moisture in the atmosphere is called Corrosion. It is an Oxidation reaction. Metals lose their luster and color change is observed.
For example, Iron turns reddish brown when it gets rusted after corrosion. Copper turns green. It also loses its mechanical strength.

When a solution of potassium iodide is added to a solution of lead nitrate taken in a test tube, the precipitation of a yellowish solid is observed. This yellowish solid is lead iodide. Potassium nitrate is formed along with lead iodide. This is a double displacement reaction.

Almost all metals combine with oxygen to form metal oxides. Metal + Oxygen → Metal oxide. Magnesium burns in the air with a dazzling white flame. It reacts with the oxygen present in the air to form magnesium oxide.
Reaction - 2Mg + O₂→ 2MgO. Magnesium chloride (MgCl₂). Magnesium Sulphate (MgSO₄). Magnesium carbonate (MgCO₃).

In ZnO + C → Zn + CO reaction, zinc oxide is reduced (due to loss of oxygen) to zinc by reducing agent coke and carbon is oxidized (due to gain of oxygen) to carbon monoxide by oxidizing agent zinc oxide.
Oxidation - Gain of oxygen or loss of hydrogen or loss of electrons. Reduction - Gain of hydrogen or loss of oxygen or gain of electrons.

Silver bromide on exposure to sunlight decomposes into silver and bromine. Reaction - 2AgBr (Silver bromide) + Sunlight → 2Ag (Silver) + Br₂(g). Silver bromide (AgBr) - A soft, pale-yellow, water-insoluble salt. Well known for its unusual sensitivity to light, used in photographic films.

Balanced chemical equation - A chemical equation in which the number of each type of atom is equal on the two sides of the equation. When Magnesium nitride (a greenish yellow powder) reacts with water, it produces Magnesium Hydroxide and Ammonia. The balanced chemical equation is - Mg₃N₂ + 6H₂O →  3Mg(OH)₂ + 2NH₃.

The reaction is - AgNO₃(aq) + NaCl (aq)→ AgCl(s) + NaNO₃ . The reaction of NaCl (Sodium chloride) and AgNO₃ (Silver nitrate) gives AgCl (Silver chloride) and NaNO₃ (Sodium nitrate).
It is an example of a double displacement reaction. NaCl is used as a food preservative and as a seasoning to enhance flavor.
NaNO₃ is used in solid propellants, explosives, fertilizers, and for many other uses. AgCl is used as disinfectant, antiseptic and is also used in the treatment of mercury poisoning.

A precipitation reaction is one in which dissolved substances react to form one (or more) solid products. The reactions occur when cations and anions in aqueous solution combine to form an insoluble ionic solid called a precipitate. Example - AgNO₃(aqueous) + KCl (aqueous) → AgCl (precipitate) + KNO₃(aqueous).

Combination reaction: A chemical reaction where two or more elements or compounds combine to form a single product. Example - H₂(g) + Cl₂(g) → 2HCI.

A double displacement reaction is a type of chemical reaction in which the reactant ions exchange places to form new products. Usually, a double displacement reaction results in precipitate formation. The chemical bonds between the reactants may be either covalent or ionic.