List three differentiating features between the processes of galvanisation and alloying.

OR

Compare in tabular form the reactivities of the following metals with cold and hot water:

(a) Sodium

(b) Calcium

(c) Magnesium

Q: List three differentiating features between the processes of galvanisation and alloying. OR Compare in tabular form the reactivities of the following metals with cold and hot water: (a) Sodium (b) Calcium (c) Magnesium

### Option 1: Differentiating Features between Galvanisation and Alloying | Feature | Galvanisation | Alloying | | :--- | :--- | :--- | | **Definition** | It is the process of coating a thin protective layer of zinc on steel or iron to prevent rusting. | It is the process of preparing a homogeneous mixture of a metal with other metals or non-metals to improve its properties. | | **Nature of Process** | It is a surface treatment process; only the outer surface of the metal is protected. | It is a bulk modification process; the entire composition and structure of the metal are altered. | | **Effect on Properties** | It does not change the physical or mechanical properties (like hardness, tensile strength, melting point) of the base metal. | It significantly modifies the physical and chemical properties (increases hardness, tensile strength, and resistance to corrosion). | --- ### Option 2 (OR): Tabular Comparison of Reactivities of Sodium, Calcium, and Magnesium with Water | Metal | Reaction with Cold Water | Reaction with Hot Water | Key Observations & Chemical Equations | | :--- | :--- | :--- | :--- | | **(a) Sodium** ($Na$) | Reacts violently and vigorously. | Reaction is too violent and explosive to be safely tested. | The reaction is highly exothermic; the evolved hydrogen gas ($H_2$) immediately catches fire with a golden-yellow flame. $$2Na(s) + 2H_2O(l) ightarrow 2NaOH(aq) + H_2(g) + \text{heat energy}$$ | | **(b) Calcium** ($Ca$) | Reacts less violently. | Reacts rapidly. | The heat evolved is not sufficient for hydrogen to catch fire. The metal starts floating because bubbles of $H_2(g)$ stick to its surface. $$Ca(s) + 2H_2O(l) ightarrow Ca(OH)_2(aq) + H_2(g)$$ | | **(c) Magnesium** ($Mg$) | Does not react with cold water. | Reacts to form magnesium hydroxide and hydrogen. | The metal starts floating because the bubbles of $H_2(g)$ formed stick to its surface. $$Mg(s) + 2H_2O(l) ightarrow Mg(OH)_2(aq) + H_2(g)$$ |

Metals and Non-metals

Reactivity of Metals with Water and Prevention of Corrosion

Official Answer

Option 1: Differentiating Features between Galvanisation and Alloying

Feature	Galvanisation	Alloying
Definition	It is the process of coating a thin protective layer of zinc on steel or iron to prevent rusting.	It is the process of preparing a homogeneous mixture of a metal with other metals or non-metals to improve its properties.
Nature of Process	It is a surface treatment process; only the outer surface of the metal is protected.	It is a bulk modification process; the entire composition and structure of the metal are altered.
Effect on Properties	It does not change the physical or mechanical properties (like hardness, tensile strength, melting point) of the base metal.	It significantly modifies the physical and chemical properties (increases hardness, tensile strength, and resistance to corrosion).

Option 2 (OR): Tabular Comparison of Reactivities of Sodium, Calcium, and Magnesium with Water

Metal	Reaction with Cold Water	Reaction with Hot Water	Key Observations & Chemical Equations
(a) Sodium ( $Na$ )	Reacts violently and vigorously.	Reaction is too violent and explosive to be safely tested.	The reaction is highly exothermic; the evolved hydrogen gas ( $H_2$ ) immediately catches fire with a golden-yellow flame.<br> $2Na(s) + 2H_2O(l) ightarrow 2NaOH(aq) + H_2(g) + \text{heat energy}$
(b) Calcium ( $Ca$ )	Reacts less violently.	Reacts rapidly.	The heat evolved is not sufficient for hydrogen to catch fire. The metal starts floating because bubbles of $H_2(g)$ stick to its surface.<br> $Ca(s) + 2H_2O(l) ightarrow Ca(OH)_2(aq) + H_2(g)$
(c) Magnesium ( $Mg$ )	Does not react with cold water.	Reacts to form magnesium hydroxide and hydrogen.	The metal starts floating because the bubbles of $H_2(g)$ formed stick to its surface.<br> $Mg(s) + 2H_2O(l) ightarrow Mg(OH)_2(aq) + H_2(g)$

GalvanisationAlloyingZinc coatingHomogeneous mixtureViolentlyExothermicFloatingHydrogen bubblesHot waterCold water

Marking Scheme

13 marks
21 mark
31 mark
41 mark

Hint

For the OR question, remember that sodium reacts explosively with cold water, calcium reacts gently and floats, while magnesium requires hot water to react and also floats.

Quick Oral Answer

Q: Why does calcium float when added to water? A: Calcium reacts with water to produce hydrogen gas. The bubbles of this hydrogen gas stick to the surface of the calcium metal, carrying it upward and making it float.

Analysis & Explanation

This question tests the understanding of chemical properties of metals (specifically their reaction with water) and methods of preventing corrosion.

Water Reactivity: The reactivity of metals with water decreases down the activity series: $Na > Ca > Mg$ . Sodium reacts violently with cold water, releasing enough heat to ignite hydrogen. Calcium reacts gently with cold water, and the bubbles of hydrogen gas stick to the metal surface, causing it to float. Magnesium is less reactive and requires hot water (or steam) to react, also floating due to adhering hydrogen bubbles.
Corrosion Prevention: Galvanisation is a physical barrier method using a sacrificial zinc coating. Alloying is a chemical modification method that alters the intrinsic properties of the metal to make it naturally resistant to corrosion and mechanically stronger.

Common Mistakes

1Writing that magnesium reacts with cold water.
2Forgetting to mention why calcium and magnesium float (due to hydrogen gas bubbles sticking to their surfaces).
3Confusing galvanisation with alloying by stating that galvanisation changes the internal composition of iron.

Interesting Facts

Gold and silver are so unreactive that they do not react with water at all, even in the form of superheated steam.

Alloying iron with carbon, nickel, and chromium produces stainless steel, which does not rust at all, showing how alloying completely changes a metal's chemical properties.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2020?

This question carries 3 marks in the CBSE Class 10 Science 2020 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 "Reactivity of Metals with Water and Prevention of Corrosion" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2020 paper?

This is a Short Answer question from Section B in the CBSE Class 10 Science 2020 paper.