CBSE Class 10 Science (2026–27)
Chapter 4: Carbon and Its Compounds
20 Important Questions and Answers
1. What are covalent compounds? Why does carbon form covalent bonds?
Answer:
Covalent compounds are substances formed when atoms share electrons to achieve a stable electronic configuration. Carbon has four electrons in its outermost shell and needs four more electrons to complete its octet. Gaining or losing four electrons is energetically difficult, so carbon shares electrons with other atoms. This sharing results in the formation of covalent bonds. For example, in methane (CH₄), carbon shares one electron each with four hydrogen atoms. Covalent compounds generally have low melting and boiling points and do not conduct electricity because they lack free ions. Carbon’s ability to form stable covalent bonds makes it the basis of millions of organic compounds.
2. Explain the phenomenon of catenation in carbon.
Answer:
Catenation is the ability of an element to form long chains, branched chains, and rings by bonding with atoms of the same element. Carbon shows exceptional catenation because carbon-carbon bonds are very strong and stable. This property allows carbon atoms to link together in numerous ways, forming a vast number of compounds. Carbon chains may be straight, branched, or cyclic. For example, propane contains a straight chain of carbon atoms, while cyclohexane contains a ring structure. Catenation is responsible for the enormous diversity of organic compounds found in nature. It also enables the formation of complex molecules such as proteins, carbohydrates, and synthetic polymers.
3. Why are carbon compounds poor conductors of electricity?
Answer:
Most carbon compounds are poor conductors of electricity because they are covalent compounds. In covalent compounds, electrons are shared between atoms and remain localized within the bonds. As a result, there are no free electrons or ions available to carry electric current. Unlike ionic compounds, carbon compounds do not dissociate into charged particles when dissolved in water. For example, methane, ethanol, and glucose do not conduct electricity. However, graphite is an exception because it contains free electrons that can move between layers of carbon atoms. Therefore, while most carbon compounds are non-conductors, graphite can conduct electricity due to its unique structure.
4. Describe the structure and properties of methane.
Answer:
Methane (CH₄) is the simplest hydrocarbon and belongs to the alkane family. It consists of one carbon atom bonded covalently to four hydrogen atoms. The molecule has a tetrahedral structure, making it highly stable. Methane is a colorless, odorless, and highly combustible gas. It is the main constituent of natural gas and biogas. On complete combustion, methane produces carbon dioxide, water, and a large amount of heat. Due to its high calorific value and clean burning nature, it is widely used as a fuel. Methane is also used as a raw material in the chemical industry for manufacturing various organic compounds.
5. What are saturated and unsaturated hydrocarbons? Give examples.
Answer:
Hydrocarbons are compounds containing only carbon and hydrogen atoms. Saturated hydrocarbons have only single bonds between carbon atoms and are called alkanes. Examples include methane (CH₄) and ethane (C₂H₆). Unsaturated hydrocarbons contain at least one double or triple bond between carbon atoms. Those with double bonds are called alkenes, such as ethene (C₂H₄), while those with triple bonds are called alkynes, such as ethyne (C₂H₂). Unsaturated hydrocarbons are generally more reactive than saturated hydrocarbons because multiple bonds can break easily during chemical reactions. This difference in bonding significantly affects their chemical properties and uses.
6. What is homologous series? Mention its characteristics.
Answer:
A homologous series is a group of organic compounds having the same functional group and similar chemical properties. Successive members differ by a CH₂ unit and show a gradual change in physical properties. For example, methane, ethane, propane, and butane belong to the alkane homologous series. The characteristics of a homologous series include a common functional group, similar chemical behavior, a difference of 14 atomic mass units between adjacent members, and a regular gradation in boiling and melting points. Homologous series help in the systematic study of organic compounds because understanding one member helps predict the properties of others within the same series.
7. What are functional groups? Why are they important?
Answer:
A functional group is an atom or group of atoms that determines the characteristic chemical properties of an organic compound. Functional groups replace one or more hydrogen atoms in a hydrocarbon chain. Common examples include alcohol (-OH), aldehyde (-CHO), carboxylic acid (-COOH), and ketone (>C=O). Functional groups are important because they influence the reactivity, physical properties, and uses of compounds. For instance, ethanol and ethanoic acid have different properties because they contain different functional groups. The presence of a specific functional group helps classify organic compounds and predict their behavior during chemical reactions.
8. Explain the process of combustion of carbon compounds.
Answer:
Combustion is a chemical reaction in which a carbon compound reacts with oxygen to produce carbon dioxide, water, and heat. For example, methane burns in oxygen according to the equation: CH₄ + 2O₂ → CO₂ + 2H₂O + Heat. Complete combustion occurs when sufficient oxygen is available and produces a clean blue flame. Incomplete combustion occurs when oxygen is limited, producing carbon monoxide or soot. Carbon compounds are widely used as fuels because they release a large amount of energy during combustion. The heat produced is utilized for cooking, heating, transportation, and industrial purposes. Efficient combustion minimizes pollution and improves fuel economy.
9. What is oxidation? Explain with an example.
Answer:
Oxidation is a chemical process in which a substance gains oxygen or loses hydrogen. In organic chemistry, oxidation often converts alcohols into acids using oxidizing agents such as alkaline potassium permanganate or acidified potassium dichromate. For example, ethanol is oxidized to ethanoic acid in the presence of an oxidizing agent. This reaction can be represented as: Ethanol + Oxygen → Ethanoic Acid + Water. Oxidation reactions are important in industrial chemistry and biological processes. They help manufacture useful products such as vinegar and various chemicals. Understanding oxidation is also essential for studying corrosion, respiration, and energy production in living organisms.
10. What are addition reactions? Give an example.
Answer:
Addition reactions are chemical reactions in which atoms or groups of atoms add across a double or triple bond of an unsaturated hydrocarbon. These reactions convert unsaturated compounds into saturated compounds. For example, ethene reacts with hydrogen in the presence of a nickel catalyst to form ethane. The reaction is represented as: C₂H₄ + H₂ → C₂H₆. This process is called hydrogenation. Addition reactions are characteristic of unsaturated hydrocarbons because their multiple bonds can easily break and accommodate new atoms. Such reactions are widely used in industries, including the conversion of vegetable oils into vanaspati ghee through hydrogenation.
11. What are substitution reactions? Explain with an example.
Answer:
Substitution reactions are reactions in which one atom or group of atoms in a molecule is replaced by another atom or group. Saturated hydrocarbons commonly undergo substitution reactions. For example, methane reacts with chlorine in the presence of sunlight to form chloromethane and hydrogen chloride. The reaction is: CH₄ + Cl₂ → CH₃Cl + HCl. In this reaction, one hydrogen atom of methane is replaced by a chlorine atom. Substitution reactions are important in organic chemistry because they help prepare a variety of useful compounds. They are widely used in the manufacture of pharmaceuticals, dyes, and industrial chemicals.
12. Explain the cleansing action of soap.
Answer:
Soap molecules contain two parts: a hydrophilic (water-attracting) head and a hydrophobic (water-repelling) tail. The hydrophobic tail attaches to grease and dirt, while the hydrophilic head remains in water. When soap is added to water, the molecules form structures called micelles. The oily dirt gets trapped inside these micelles. During rinsing, the micelles carrying dirt are washed away with water. This process effectively removes grease and impurities from surfaces. The cleansing action of soap works best in soft water because soap forms lather easily. Thus, the unique structure of soap molecules enables efficient cleaning through micelle formation.
13. Why are soaps ineffective in hard water?
Answer:
Hard water contains dissolved calcium and magnesium salts. When soap is added to hard water, it reacts with these salts to form insoluble precipitates called scum. This scum prevents soap from producing sufficient lather and reduces its cleansing efficiency. As a result, more soap is required for cleaning in hard water. The formation of scum also leaves deposits on clothes and utensils. Because of this limitation, detergents are preferred in hard water. Unlike soaps, detergents do not react with calcium and magnesium ions and therefore remain effective. Thus, the presence of mineral salts makes soaps less efficient in hard water.
14. Differentiate between soaps and detergents.
Answer:
Soaps are sodium or potassium salts of long-chain fatty acids, whereas detergents are synthetic cleansing agents made from petrochemicals. Soaps work effectively in soft water but form scum in hard water, reducing their cleaning ability. Detergents, on the other hand, remain effective in both hard and soft water because they do not react with calcium and magnesium ions. Soaps are biodegradable and environmentally friendly, while some detergents may cause water pollution if they are non-biodegradable. Both soaps and detergents lower the surface tension of water and remove dirt through micelle formation, but detergents are generally more versatile for household cleaning.
15. What is ethanol? Mention its uses.
Answer:
Ethanol (C₂H₅OH) is an alcohol containing the hydroxyl (-OH) functional group. It is a colorless liquid with a characteristic odor and is highly soluble in water. Ethanol is produced by the fermentation of sugars and starches. It is widely used as a solvent in medicines, perfumes, and cosmetics. Ethanol is also used as a fuel and fuel additive because it burns cleanly and produces less pollution. In laboratories, it serves as an important chemical reagent. Additionally, ethanol is used in the manufacture of alcoholic beverages. Its versatility makes it one of the most important organic compounds in everyday life.
16. What is ethanoic acid? State its properties and uses.
Answer:
Ethanoic acid (CH₃COOH), commonly known as acetic acid, is a carboxylic acid with a sour taste and pungent smell. A dilute solution of ethanoic acid is called vinegar. Pure ethanoic acid freezes at about 16.6°C and is known as glacial acetic acid. It reacts with bases to form salts and water and reacts with carbonates to release carbon dioxide. Ethanoic acid is widely used in food preservation, flavoring, and the manufacture of chemicals, dyes, and plastics. It is also used in laboratories and industries. Its acidic nature and preservative properties make it highly useful.
17. What is hydrogenation of vegetable oils?
Answer:
Hydrogenation is the process of adding hydrogen to unsaturated vegetable oils in the presence of a catalyst such as nickel. During this process, double bonds present in the oil molecules are converted into single bonds, making the oil more saturated. This conversion changes liquid oils into semi-solid or solid fats such as vanaspati ghee. Hydrogenation increases the shelf life and stability of oils. It is widely used in the food industry for producing margarine and other processed foods. However, excessive consumption of hydrogenated fats may have adverse health effects due to the presence of trans fats formed during processing.
18. Why does carbon form a large number of compounds?
Answer:
Carbon forms a large number of compounds because of its tetravalency and catenation properties. Tetravalency allows carbon to form four stable covalent bonds with other atoms, while catenation enables it to bond with other carbon atoms to form long chains, branches, and rings. Carbon can also form single, double, and triple bonds with itself and other elements such as hydrogen, oxygen, nitrogen, and chlorine. These bonding possibilities create an enormous variety of structures and compounds. As a result, carbon forms the basis of organic chemistry and is present in countless natural and synthetic substances essential for life and industry.
19. What is the difference between complete and incomplete combustion?
Answer:
Complete combustion occurs when a fuel burns in the presence of sufficient oxygen. It produces carbon dioxide, water, and a large amount of energy while giving a clean blue flame. Incomplete combustion occurs when oxygen is insufficient. In this case, carbon monoxide, soot, and less energy are produced. Incomplete combustion results in a yellow, smoky flame and causes air pollution. Carbon monoxide is particularly dangerous because it is poisonous and can interfere with oxygen transport in the body. Therefore, complete combustion is preferred as it is more efficient, safer, and environmentally friendly than incomplete combustion.
20. Explain the unique properties of carbon that make it important.
Answer:
Carbon is unique because of its tetravalency, catenation, and ability to form strong covalent bonds. Tetravalency allows it to bond with many elements, while catenation enables the formation of long chains and complex structures. Carbon can also form single, double, and triple bonds, increasing the variety of compounds it can create. These properties make carbon the foundation of organic chemistry. All living organisms contain carbon compounds such as proteins, carbohydrates, fats, and nucleic acids. Carbon compounds are also used in fuels, medicines, plastics, textiles, and numerous industrial products, making carbon one of the most important elements on Earth.