Give reasons for the following : (i) Element carbon forms compounds mainly by covalent bonding. (ii) Diamond has a high melting point. (iii) Graphite is a good conductor of electricity. (iv) Acetylene burns with a sooty flame. (v) Kerosene does not decolourise Bromine water while cooking oils do. OR (a) What is a detergent ? Name one detergent. (b) Write two advantages and two disadvantages of using detergents over soaps. (c) Why, by using a detergent, can we wash clothes even in hard water ?

Q: Give reasons for the following : (i) Element carbon forms compounds mainly by covalent bonding. (ii) Diamond has a high melting point. (iii) Graphite is a good conductor of electricity. (iv) Acetylene burns with a sooty flame. (v) Kerosene does not decolourise Bromine water while cooking oils do. OR (a) What is a detergent ? Name one detergent. (b) Write two advantages and two disadvantages of using detergents over soaps. (c) Why, by using a detergent, can we wash clothes even in hard water ?

(i) **Covalent Bonding:** Carbon has 4 valence electrons. It cannot gain 4 electrons to form $C^{4-}$ because the nucleus (6 protons) cannot hold 10 electrons. It cannot lose 4 electrons to form $C^{4+}$ because it requires too much energy. Thus, it shares electrons to achieve stability. (ii) **Diamond Melting Point:** In diamond, each carbon atom is bonded to four others in a rigid three-dimensional structure with strong covalent bonds. Breaking this network requires a massive amount of thermal energy. (iii) **Graphite Conductivity:** In graphite, each carbon is bonded to only three others. This leaves one valence electron free (forming a double bond as per text) which allows for the flow of electricity. (iv) **Acetylene Sooty Flame:** Acetylene ($C_2H_2$) is an unsaturated hydrocarbon with a high carbon-to-hydrogen ratio. During combustion in air, the carbon does not burn completely, leaving unburnt carbon particles that glow yellow and form soot. (v) **Bromine Water Test:** Cooking oils are unsaturated fats (containing double bonds) which undergo addition reactions with bromine, decolourising it. Kerosene is a mixture of saturated hydrocarbons (alkanes) which do not react with bromine water under normal conditions. **OR** (a) A **detergent** is generally an ammonium or sulphonate salt of long-chain carboxylic acids. Example: Sodium lauryl sulphate. (b) **Advantages:** (1) Works effectively in hard water. (2) Stronger cleansing action than soap. **Disadvantages:** (1) Most are non-biodegradable. (2) Can cause skin irritation or water pollution. (c) Detergents do not form insoluble precipitates (scum) with the $Ca^{2+}$ and $Mg^{2+}$ ions present in hard water, unlike soaps.

Carbon and its Compounds

Properties of Carbon and its Compounds

Official Answer

(i) Covalent Bonding: Carbon has 4 valence electrons. It cannot gain 4 electrons to form $C^{4-}$ because the nucleus (6 protons) cannot hold 10 electrons. It cannot lose 4 electrons to form $C^{4+}$ because it requires too much energy. Thus, it shares electrons to achieve stability.

(ii) Diamond Melting Point: In diamond, each carbon atom is bonded to four others in a rigid three-dimensional structure with strong covalent bonds. Breaking this network requires a massive amount of thermal energy.

(iii) Graphite Conductivity: In graphite, each carbon is bonded to only three others. This leaves one valence electron free (forming a double bond as per text) which allows for the flow of electricity.

(iv) Acetylene Sooty Flame: Acetylene ( $C_2H_2$ ) is an unsaturated hydrocarbon with a high carbon-to-hydrogen ratio. During combustion in air, the carbon does not burn completely, leaving unburnt carbon particles that glow yellow and form soot.

(v) Bromine Water Test: Cooking oils are unsaturated fats (containing double bonds) which undergo addition reactions with bromine, decolourising it. Kerosene is a mixture of saturated hydrocarbons (alkanes) which do not react with bromine water under normal conditions.

(a) A detergent is generally an ammonium or sulphonate salt of long-chain carboxylic acids. Example: Sodium lauryl sulphate.

(b) Advantages: (1) Works effectively in hard water. (2) Stronger cleansing action than soap. Disadvantages: (1) Most are non-biodegradable. (2) Can cause skin irritation or water pollution.

(c) Detergents do not form insoluble precipitates (scum) with the $Ca^{2+}$ and $Mg^{2+}$ ions present in hard water, unlike soaps.

Covalent bondingsharing electronsrigid 3D structurefree electronsunsaturatedsooty flameaddition reactionhard waterscum

Marking Scheme

1Reason for covalent bonding: 1 mark
2Reason for Diamond/Graphite properties: 2 marks
3Reason for flame/bromine test: 2 marks
4OR: Detergent definition/name: 1 mark, Advantages/Disadvantages: 2 marks, Hard water explanation: 2 marks

Hint

Think about the number of bonds in allotropes and the difference between saturated and unsaturated bonds.

Quick Oral Answer

Carbon forms covalent bonds because it's energetically unfavorable to lose or gain 4 electrons. Graphite conducts electricity because each carbon atom is only bonded to three others, leaving free electrons to move.

Analysis & Explanation

The reasons are grounded in the electronic configuration of carbon (Section 4.1) and the structural differences of its allotropes. Diamond's hardness and high melting point come from its 3D tetrahedral network. Graphite's conductivity arises because only 3 out of 4 valence electrons are used in localized single bonds. The combustion and bromine tests relate to the degree of saturation; unsaturated compounds (like acetylene and oils) have different reactivity profiles compared to saturated ones (like kerosene). Detergents are designed to overcome the limitations of soaps in hard water by preventing the formation of 'scum'.

Common Mistakes

1Saying carbon forms ionic bonds.
2Confusing the structure of diamond and graphite.
3Thinking kerosene is unsaturated.

Interesting Facts

Diamond is the hardest known natural substance, yet it is made of the same element as the soft lead in your pencil (graphite).

The 'Vital Force' theory was disproved when urea was synthesized from inorganic materials.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2011?

This question carries 5 marks in the CBSE Class 10 Science 2011 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Carbon and its Compounds" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Properties of Carbon and its Compounds" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2011 paper?

This is a Long Answer question from Section A in the CBSE Class 10 Science 2011 paper.