State reasons for the following: (a) Zinc oxide is an amphoteric oxide. (b) Sodium metal is stored in bottle filled with kerosene oil. (c) In the reactions of nitric acid with metals, generally hydrogen gas is not evolved.

Q: State reasons for the following: (a) Zinc oxide is an amphoteric oxide. (b) Sodium metal is stored in bottle filled with kerosene oil. (c) In the reactions of nitric acid with metals, generally hydrogen gas is not evolved.

### (a) Zinc oxide is an amphoteric oxide Amphoteric oxides are metal oxides that react with both acids and bases to produce salt and water. Zinc oxide ($ZnO$) exhibits both acidic and basic characteristics: - **As a basic oxide (reacting with acid):** $$ZnO(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2O(l)$$ - **As an acidic oxide (reacting with base):** $$ZnO(s) + 2NaOH(aq) \rightarrow Na_2ZnO_2(aq) + H_2O(l)$$ *(Sodium zincate)* ### (b) Sodium metal is stored in a bottle filled with kerosene oil Sodium is an extremely reactive alkali metal. It reacts so vigorously with the oxygen and moisture present in the air that it catches fire if kept in the open. To protect the metal from reacting with atmospheric moisture and oxygen, and to prevent accidental fires, it is kept completely immersed in kerosene oil. ### (c) In the reactions of nitric acid with metals, generally hydrogen gas is not evolved Nitric acid ($HNO_3$) is a strong oxidising agent. When a metal reacts with it, the hydrogen gas ($H_2$) produced is immediately oxidised to water ($H_2O$) by the acid. In the process, nitric acid itself gets reduced to any of the nitrogen oxides, such as nitrous oxide ($N_2O$), nitric oxide ($NO$), or nitrogen dioxide ($NO_2$). *(Note: Extremely dilute nitric acid reacts with magnesium and manganese to evolve hydrogen gas).*

Metals and Non-metals

Chemical Properties of Metals

Official Answer

(a) Zinc oxide is an amphoteric oxide

Amphoteric oxides are metal oxides that react with both acids and bases to produce salt and water. Zinc oxide ( $ZnO$ ) exhibits both acidic and basic characteristics:

As a basic oxide (reacting with acid):

$ZnO(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2O(l)$

As an acidic oxide (reacting with base):

$ZnO(s) + 2NaOH(aq) \rightarrow Na_2ZnO_2(aq) + H_2O(l)$ (Sodium zincate)

(b) Sodium metal is stored in a bottle filled with kerosene oil

Sodium is an extremely reactive alkali metal. It reacts so vigorously with the oxygen and moisture present in the air that it catches fire if kept in the open. To protect the metal from reacting with atmospheric moisture and oxygen, and to prevent accidental fires, it is kept completely immersed in kerosene oil.

(c) In the reactions of nitric acid with metals, generally hydrogen gas is not evolved

Nitric acid ( $HNO_3$ ) is a strong oxidising agent. When a metal reacts with it, the hydrogen gas ( $H_2$ ) produced is immediately oxidised to water ( $H_2O$ ) by the acid. In the process, nitric acid itself gets reduced to any of the nitrogen oxides, such as nitrous oxide ( $N_2O$ ), nitric oxide ( $NO$ ), or nitrogen dioxide ( $NO_2$ ).

(Note: Extremely dilute nitric acid reacts with magnesium and manganese to evolve hydrogen gas).

amphoteric oxidesodium zincatekerosene oiloxidising agentnitrogen oxideszinc oxide

Marking Scheme

1(a) 1 Mark: Defining amphoteric oxides and writing balanced chemical equations for $ZnO$ reacting with both $HCl$ and $NaOH$ .
2(b) 1 Mark: Explaining sodium's high reactivity with oxygen/moisture and how kerosene acts as a protective barrier.
3(c) 1 Mark: Explaining that $HNO_3$ is a strong oxidising agent that oxidises $H_2$ to $H_2O$ and itself gets reduced to nitrogen oxides.

Hint

Recall the definition of amphoteric oxides. Think about the position of sodium in the reactivity series and how it reacts with air. Remember that nitric acid is a strong oxidising agent.

Quick Oral Answer

Zinc oxide is amphoteric because it reacts with both acids and bases to form salt and water. Sodium is stored in kerosene because it is highly reactive and catches fire in contact with air or moisture. Nitric acid is a strong oxidising agent, so it oxidises the produced hydrogen gas to water.

Analysis & Explanation

This question tests the understanding of key chemical properties of metals and their oxides:

Amphoteric Nature: Most metal oxides are basic, but oxides of zinc and aluminium are amphoteric. They can neutralize both acids (like $HCl$ ) and strong bases (like $NaOH$ ) to form corresponding salts and water.
Reactivity with Air/Moisture: Sodium is highly electropositive and reacts exothermically with atmospheric oxygen and water vapor. Kerosene is a hydrocarbon mixture that does not react with sodium and acts as a physical barrier against air and moisture.
Oxidising Behavior of Nitric Acid: Unlike dilute $HCl$ or $H_2SO_4$ , which release $H_2$ gas upon reaction with metals, $HNO_3$ is a powerful oxidiser. The nascent oxygen released by $HNO_3$ oxidises the evolved $H_2$ to $H_2O$ .

Common Mistakes

1Writing that sodium reacts with kerosene (it is actually inert in kerosene).
2Forgetting to write the chemical equations for the amphoteric reactions of zinc oxide.
3Stating that no metal ever reacts with nitric acid to produce hydrogen (magnesium and manganese do react with very dilute $HNO_3$ ).

Interesting Facts

Gallium and caesium have such low melting points that they will melt if kept on your palm!

Gold and platinum are so unreactive that they do not react with oxygen even at high temperatures and can only be dissolved in a freshly prepared mixture called Aqua Regia (royal water).

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2024?

This question carries 3 marks in the CBSE Class 10 Science 2024 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Metals and Non-metals" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Chemical Properties of Metals" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2024 paper?

This is a Short Answer question from Section C in the CBSE Class 10 Science 2024 paper.