Test: Chemical Reactions - UPSC MCQ

Reaction between Ammonium chloride and Barium hydroxide is an endothermic reaction.An exothermic reaction is a chemical reaction that releases energy through light or heat. It is the opposite of an endothermic reaction. Expressed in a chemical equation: reactants → products + energy. Exothermic Reaction means "exo" meaning releases and "thermic" means heat.

Hydrogen is a highly reactive fuel. Hydrogen molecules violently react with oxygen when the existing molecular bonds break and new bonds are formed between oxygen and hydrogen atoms. As the products of the reaction are at a lower energy level than the reactants(i.e products are more stable), the result is an explosive release of energy and the production of water.

Its an example of exothermic reaction
It is an example of Combination reaction as 2 or more reactants combine to form a single product.

As in the given equation, the electrons are transferred from Hydrogen to Oxygen, hence Oxygen is reduced and electrons are accepted by Oxygen from Hydrogen, hence Hydrogen is oxidised .Now,both oxidation and reduction are going together, therefore it is also a Redox (Reduction- Oxidation reaction) reaction.

Hydrogen gas and heat are produced in the reaction between zinc and hydrochloric acid.
Zn + HCl → ZnCl₂ + H₂

When chemical reactions take place, energy is either absorbed or evolved. As the given reaction is exothermic, heat must have been evolved.

An exothermic reaction is a chemical reaction that releases energy through light or heat. It is the opposite of an endothermic reaction.
Expressed in a chemical equation: reactants → products + energy.

A precipitation reaction refers to the formation of an insoluble salt when two solutions containing soluble salts are combined. The insoluble salt that falls out of solution is known as the precipitate, hence the reaction's name.
Precipitation reactions can help determine the presence of various ions in solution

Sodium metal reacts rapidly with water to form a colorless solution of sodium hydroxide (NaOH) and hydrogen gas (H₂). The reaction is exothermic.
2Na(s) + 2H₂O → 2NaOH(aq) + H₂(g)

Reaction between limestone(CaCO₃₎ and hydochloric acid(HCl) can be represented by the following formula : CaCO₃ + 2HCl → CaCl₂ + CO₂ ↑ + H₂O here, it can be seen that CO₂ gas has evolved.

Freezing water is a physical change

A precipitation reaction occurs. Yellow precipitates of lead iodide are formed.

The equation for this reaction is Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃.

The above reaction is a double displacement reaction as well.

Because in physical change the substance change only it's physical appearance but there is no change  in its basic composition that's why we can say that boiling of water is a physical change.

Preparation of water from hydrogen and oxygen involves a change in state.

2H_2(g) + O_2(g) → 2H₂O_(l)

If the change of state and colour is a result of some chemical reaction, then the change of state and colour are considered as characteristics of a chemical reaction. 
Otherwise the change of state and colour are considered as characteristics of a physical change.

Acids turn blue litmus to red hence dilute hydrochloric acid turns blue litmus paper red.

The reaction in which two compound exchange their ions to form two new compounds is called double displacement reaction.