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Chapter 20: Problem 90

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 \(\mathrm{H}_{2}(g)\). d. When \(\mathrm{AlCl}_{3}\) 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

Expert verified
Statements c and d are true. Calcium reacts with water to produce hydrogen gas, and when aluminum chloride is dissolved in water, it produces an acidic solution.

Step by step solution

01

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

This statement is false. Alkali metals are highly reactive and readily form compounds with other elements. Thus, they are not found as pure elements in nature. For example, sodium (Na) and potassium (K) are found as sodium chloride (NaCl) and potassium salts in the earth's crust.
02

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

This statement is false. Gallium actually has a very low melting point for a metal (around 29.8°C or 85.6°F), which means it can melt easily, even just by holding it in the hand. Metals with high melting points include tungsten (3422°C) and tantalum (3017°C).
03

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

This statement is true. Calcium (Ca) reacts vigorously with water to produce calcium hydroxide (Ca(OH)₂) and hydrogen gas (H₂). The reaction can be represented as follows: \[Ca(s) + 2H_{2}O(l) \rightarrow Ca(OH)_{2}(aq) + H_{2}(g)\]
04

d. When \(\mathrm{AlCl}_{3}\) is dissolved in water, it produces an acidic solution.

This statement is true. When aluminum chloride (AlCl₃) is dissolved in water, it undergoes hydrolysis, producing acidic solutions. The hydrolysis reaction can be represented as follows: \[AlCl_{3}(s) + 3 H_{2}O(l) \rightarrow Al(OH)_{3}(s) + 3 HCl(aq)\] As hydrochloric acid (HCl) is a strong acid, the resulting solution is acidic.
05

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

This statement is false. Lithium (Li) reacts with oxygen (O₂) to form lithium oxide (Li₂O). The reaction can be represented as follows: \[4Li(s) + O_{2}(g) \rightarrow 2Li_{2}O(s)\] Only alkali metals like potassium (K), rubidium (Rb), and cesium (Cs) react with excess oxygen to form superoxides (KO₂, RbO₂, CsO₂). In conclusion, statements c and d are true, while statements a, b, and e are false.

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