Question

Which of the following statement(s) is(are) true?

a. The alkali metals are found in the earth’s crust in the form of pure elements.

b. Gallium has one of the highest melting points known for metals.

c. When calcium metal reacts with water, one of the products is ${\text{H}}_{\text{2}}\text{(g)}$.

d. When ${\text{AlCl}}_{\text{3}}$is dissolved in water, it produces an acidic solution.

e. Lithium reacts in the presence of excess oxygen gas to form lithium superoxide.

Short answer

(a) It is false that the alkali metals are found in the earth’s crust in the form of pure elements.

(b) It is false that gallium has one of the highest melting points known for metals.

(c) It is true that when calcium metal reacts with water, one of the products is ${\text{H}}_{\text{2}}\left(\text{g}\right)$.

(d) It is true that when ${\text{AlCl}}_{\text{3}}$is dissolved in water, it produces an acidic solution.

(e) It is false that lithium reacts in the presence of excess oxygen gas to form lithium superoxide.

Step-by-step solution

Concept Introduction

Generally speaking, the term "alkali" describes the basic or alkaline nature of their metal hydroxides. The substances are known as alkali metals because, typically, when they interact with water, they produce alkalis, which are merely potent bases that are capable of quickly neutralising acids.

Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Ru), Caesium (Cs), and Francium are alkali elements (Fr).

Alkali metals found on Earth

(a)

Alkali metals are not found in the earth's crust in the form of pure elements.

Alkali metals are never found in nature uncombined because they are unstable and they react fast to other elements. They bond well with all elements except the noble gases.

Therefore, the statement is false.

Highest Melting point of Gallium

(b)

Gallium does not have one of the highest melting points known for metals.

In fact, Gallium has a very low melting point Its low melting point is attributed to its crystal structure, in which each Ga atom has only one nearest neighbour and six next-nearest neighbours: thus, the Ga atoms tend to form Ga-Ga pairs.

Therefore, the statement is false.

Reaction of Calcium with water

(c)

When calcium is added to water, the calcium metal begins to bubble vigorously as it reacts with the water, producing hydrogen gas, and a cloudy white precipitate of calcium hydroxide.

The reaction is as follows –

$\text{Ca}\left(\text{s}\right){\text{+ 2H}}_{\text{2}}\text{O}\left(\text{l}\right)\to \text{Ca}{\left(\text{OH}\right)}_{\text{2}}\left(\text{aq}\right){\text{+H}}_{\text{2}}\left(\text{g}\right)$

Therefore, the statement is true.

Reaction of Aluminium Chloride in water

(d)

When ${\text{AlCl}}_{\text{3}}$; is added to water, it dissociates into ${\text{Al}}^{\text{3 +}}$and ${\text{Cl}}^{\text{-}}$.

The aluminium ions form complexes with water, ${\left[\text{Al}{\left({\text{H}}_{\text{2}}\text{O}\right)}_{\text{6}}\right]}^{\text{3 +}}$. The aluminium complex ion can act as an acid and donate a ${\text{H}}^{\text{+}}$to become ${\left[\text{Al}\left(\text{OH}\right){\left({\text{H}}_{\text{2}}\text{O}\right)}_{\text{5}}\right]}^{\text{2 +}}$. Therefore, when ${\text{AlCl}}_{\text{3}}$is added to water, the result is an increase in the${\text{H}}_3{\text{O}}^{\text{+}}$ concentration, the solution becomes acidic.

Therefore, the statement is true.

Reaction of Lithium in excess oxygen

(e)

The lithium ion is small in size. it cannot stabilize larger peroxide or superoxide ions. Thus, when burnt in the air, lithium forms only the monoxide.

The reaction is as follows –

${\text{4Li +O}}_{\text{2}}\to {\text{2Li}}_{\text{2}}\text{O}$

Therefore, the statement is false.