Class 10 – Science Extra Questions – Ch. 4 – Carbon and Its Compounds

Ans: B

Explanation: When the exterior bottom of a cooking vessel turns black, it typically indicates the presence of soot. Soot is a byproduct of incomplete combustion, meaning that the fuel is not burning efficiently and is releasing unburnt carbon particles into the air, which then deposit on the vessel. Option B accurately describes this scenario. Option A is incorrect because the color of the vessel’s exterior is related to combustion, not the cooking state of the food. Option C, while wet fuel might affect combustion, the primary indicator of blackening is incomplete burning, not necessarily wetness. Option D is incorrect because complete combustion would result in a cleaner burn, not soot.
Correct Option: B

Ans: B

Explanation: The transformation of ethanol (an alcohol) into ethanoic acid (a carboxylic acid) involves an increase in the oxidation state of the carbon atom bonded to the hydroxyl group. Specifically, ethanol has a carbon atom bonded to one oxygen atom and two hydrogen atoms, while in ethanoic acid, this carbon atom is bonded to two oxygen atoms and no hydrogen atoms. This increase in oxygen or decrease in hydrogen atoms bonded to a carbon indicates an oxidation reaction. Common oxidizing agents like potassium dichromate or acidified potassium permanganate are used for this conversion.
Correct Option: B

Ans: D

Explanation: The question describes a molecule with a central carbon atom double-bonded to two oxygen atoms. Carbon dioxide (CO2) has the chemical formula CO2, meaning it has one central carbon atom and two oxygen atoms. The carbon atom is indeed double-bonded to each oxygen atom (O=C=O). This arrangement leads to a linear molecular geometry, as the two double bonds push the oxygen atoms as far apart as possible from the central carbon. Methane (CH4) has a central carbon bonded to four hydrogen atoms in a tetrahedral geometry. Ammonia (NH3) has a central nitrogen atom bonded to three hydrogen atoms in a trigonal pyramidal geometry. Water (H2O) has a central oxygen atom bonded to two hydrogen atoms in a bent geometry. Therefore, carbon dioxide fits the description.
Correct Option: D

Ans: C

Explanation:
To determine which molecule does not have a double bond, we need to consider the Lewis structure of each molecule.
A. CO2 (Carbon Dioxide): Carbon has 4 valence electrons and Oxygen has 6 valence electrons. The Lewis structure of CO2 shows a double bond between the carbon atom and each oxygen atom (O=C=O).
B. C2H4 (Ethene): Carbon has 4 valence electrons and Hydrogen has 1 valence electron. The Lewis structure of C2H4 shows a double bond between the two carbon atoms and single bonds between each carbon and two hydrogen atoms (H2C=CH2).
C. HCl (Hydrogen Chloride): Hydrogen has 1 valence electron and Chlorine has 7 valence electrons. The Lewis structure of HCl shows a single covalent bond between the hydrogen atom and the chlorine atom.
D. O2 (Oxygen): Each Oxygen atom has 6 valence electrons. The Lewis structure of O2 shows a double bond between the two oxygen atoms (O=O).

Therefore, HCl is the molecule that does not have a double bond.

Correct Option: C

Ans: C

Explanation: CH3Cl is a molecule composed of nonmetal atoms (carbon, hydrogen, and chlorine). In such molecules, atoms achieve stability by sharing electrons to form covalent bonds. Carbon shares electrons with hydrogen and chlorine atoms, and chlorine shares electrons with the carbon atom. Ionic bonding involves the transfer of electrons between a metal and a nonmetal. Metallic bonding occurs between metal atoms. Hydrogen bonding is an intermolecular force that occurs between molecules containing hydrogen bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine, which is not the primary force holding the atoms *within* a CH3Cl molecule together.
Correct Option: C

Ans: C

Explanation: Soap molecules have a hydrophilic head (attracted to water) and a hydrophobic tail (repelled by water, attracted to nonpolar substances like grease). When soap interacts with grease, the hydrophobic tails embed themselves into the grease molecules, breaking them down. The hydrophilic heads then extend outwards, attracting water molecules. This allows the grease, now surrounded by soap molecules, to be suspended in the water and washed away. Option C accurately describes the behavior of the hydrophobic tail in this context.
Correct Option: C

Ans: C

Explanation: Concentrated sulfuric acid acts as a dehydrating agent in this reaction. It removes a molecule of water from ethanol, leading to the formation of ethene. The high temperature also favors the elimination reaction.
Correct Option: C

Ans: B

Explanation: The question describes a hydrocarbon with alternating single and double bonds in a ring structure. This is the characteristic of an aromatic hydrocarbon. Benzene (C6H6) is the simplest and most common example of such a compound, with its delocalized pi electron system forming the alternating double and single bonds within a hexagonal ring. Cyclobutane is a saturated cyclic hydrocarbon with only single bonds. Butene and hexene are open-chain alkenes with double bonds, but they do not form a ring structure with alternating single and double bonds.
Correct Option: B

Ans: D

Explanation: The reaction between sodium acetate and soda lime (a mixture of sodium hydroxide and calcium oxide) is a classic method for producing methane through a process called decarboxylation. Soda lime acts as a strong base that abstracts a proton from the methyl group of sodium acetate, and the acetate ion then loses its carboxylate group as carbon dioxide. The overall reaction is: CH3COONa + NaOH -> CH4 + Na2CO3. This involves the removal of a carboxyl group (COO-) as CO2.
Correct Option: D

Ans: C

Explanation: Butter is a solid at room temperature, meaning its fat molecules are packed in a relatively ordered crystalline or semi-crystalline structure. This ordered arrangement, even at the nanoscale, leads to a firmer texture. Cooking oil, on the other hand, is a liquid. Its fat molecules are in constant motion and are not arranged in any fixed structure, allowing them to flow and giving it a much smoother, less viscous texture. Particle size and arrangement are the primary nanoscale factors that dictate the macroscopic properties of matter, including texture. Surface tension, viscosity, and electrical conductivity are also physical properties, but they are consequences of the molecular arrangement and interactions, rather than the primary cause of the fundamental textural difference between a solid and a liquid.
Correct Option: C