Three metal samples of magnesium, aluminium and iron were taken and rubbed with sand paper. These samples were then put separately in test tubes containing dilute hydrochloric acid. Thermometers were also suspended in each test tube so that their bulbs dipped in the acid. The rate of formation of bubbles was observed. The above activity was repeated with dilute nitric acid and the observations were recorded. Answer the following questions : (a) When activity was done with dilute hydrochloric acid, then in which one of the test tubes was the rate of formation of bubbles the fastest and the thermometer showed the highest temperature ? (b) Which metal did not react with dilute hydrochloric acid ? Give reason. (c) (i) Why is hydrogen gas not evolved when a metal reacts with dilute nitric acid ? Name the ultimate products formed in the reaction. OR (c) (ii) Name the type of reaction on the basis of which reactivity of metals is decided. You have two metals X and Y. How would you decide which is more reactive than the other ?

(a) When the activity was carried out with dilute hydrochloric acid ($HCl$), the rate of formation of bubbles was the fastest and the thermometer showed the highest temperature in the test tube containing magnesium ($Mg$).

(b) Copper ($Cu$) did not react with dilute hydrochloric acid. This is because copper lies below hydrogen in the reactivity series (Table 3.2). Being less reactive than hydrogen, it cannot displace hydrogen ions from dilute acids like $HCl$ to evolve hydrogen gas.

(c) (i) Hydrogen gas ($H_2$) is not evolved when a metal reacts with nitric acid ($HNO_3$) because nitric acid is a strong oxidising agent. It immediately oxidises the hydrogen gas produced in the reaction to water ($H_2O$) and itself gets reduced to any of the nitrogen oxides, such as nitrous oxide ($N_2O$), nitric oxide ($NO$), or nitrogen dioxide ($NO_2$). The ultimate products formed in this reaction are the metal nitrate salt, water ($H_2O$), and a nitrogen oxide (such as $N_2O$, $NO$, or $NO_2$).

OR

(c) (ii) The reactivity of metals is decided on the basis of displacement reactions.

To decide which of the two metals, $X$ or $Y$, is more reactive, we can perform a displacement experiment:

1. Place a clean strip of metal $X$ into an aqueous salt solution of metal $Y$.
2. Place a clean strip of metal $Y$ into an aqueous salt solution of metal $X$.

If metal $X$ is more reactive than metal $Y$, it will displace $Y$ from its salt solution, resulting in a visible chemical change (such as a color change of the solution or a deposit on the metal strip):

$$X + \text{Salt solution of } Y \rightarrow \text{Salt solution of } X + Y$$

If no reaction occurs when metal $Y$ is placed in the salt solution of $X$, it confirms that $X$ is more reactive than $Y$.