NCERT Solutions For Class 10 Science Chapter 4 Carbon And Its Compounds Definitions
1. Carbon:
Carbon is tetravalent and easily forms a large variety of compounds on combination with carbon and other atoms by sharing ofelectrons. It exhibits the phenomenon of allotropy, i.e., exists in two or more forms in the same state
For example: Graphite and diamond.
2. Covalent bond:
When two atoms achieve stability by sharing an electron pair, each contributes one electron to the electron pair. In this way, the atoms can be regarded as having acquired a noble gas configuration.
3. Covalent compounds:
Some atoms share electrons among themselves or with other atoms to attain a stable octet configuration. Bonds formed are covalent bonds and the compounds so formed are covalent compounds. Depending on the number of shared pair ofelectrons, there is a single covalent bond, a double covalent bond, or a triple covalent bond.
These compounds have low melting points and boiling points. They do not dissolve in water, do not conduct electricity, and are directional.
4. Catenation: Catenation is the property of carbon to get attached to other carbon atoms. Up to 60 carbon atoms are found to get attached in the shape of a ball (Fullerenes).
5. Allotropy:
When an element possesses two or more different forms in the same state, they are called allotropes and the phenomenon is known as allotropy. For example, diamond and graphite are the two allotropes of carbon.
6. Fullerenes: Fullerenes are allotropes of carbon.
For example: Buckminsterfullerene is an allotrope of carbon with 60 carbon atoms arranged in the shape of a football.
7. Hydrocarbons: Hydrocarbon Compounds containing only carbon and hydrogen are called hydrocarbons.
8. Saturated hydrocarbons: Compounds of carbon and hydrogen attached in chains, branches, or rings, by single covalent bonds
For example: Alkanes (CnH2n+2): methane (CH4), ethane (C2H5), propane (C3H8), and butane (C4H10).
9. Unsaturated hydrocarbons:
Unsaturated hydrocarbons are compounds of carbon and hydrogen with either a carbon-carbon double bond (C ≡ C) or a carbon-carbon triple bond (C = C) in their molecules.
For example: Alkenes (CnH2), ethene (C2H4), propene (C3H6), butene (C4Hg). Alkynes (CnH32n-2): ethyne (C2H2), propyne (C3H4) and butyne (C4H6).
10. Functional groups:
Groups that determine the chemical nature of an organic compound. In a hydrocarbon, one or more hydrogen atoms are replaced by another element or group. These confer specific properties to the compound regardless of the length and nature chain and hence are called functional groups.
11. Alkyl radical: The group formed by removing the hydrogen atom from an alkane molecule is called an alkyl radical or group.
12. Functional group: A group that determines the chemical nature of a compound is called a functional group.
13. A homologous: A homologous series is a group or family of compounds that contain the same functional group but have different chain lengths. In their molecular formulae, two successive homologs differ by one carbon and two hydrogen atoms.
14. Isomerism: In carbon compounds, different compounds are formed with different structures but with the same molecular formula.
These compounds are called isomers and the phenomenon is called isomerism.
For example: Butane(C4H10)can be written as:
15. Nomenclature of carbon compounds:
- Identify the number of carbon atoms in the main chain. For example, methane, ethane, propane, and butane for one, two, three, four carbon atoms, etc.
- The name of the functional group can be given either a prefix or a suffix (see table below).
- If the functional group is used as a suffix, the e- of hydrocarbon is substituted by a suffix.
- The carbon atoms are numbered from the side of the functional group.
16. Fatty acid:
Carboxylic acids containing long chains of carbon atoms are fatty acids. They are used as mild acids in foods, cold drinks, drugs, and perfumes and are chief constituents of washing soap.
17. Chemical properties of carbon compounds:
Carbon and its compounds burn in oxygen with liberation of heat and light. Saturated hydrocarbons burn with a blue flame and unsaturated with a yellow flame.
18. Addition reactions: Unsaturated hydrocarbons add up hydrogen or halogens
⇒ \(\underset{\text { Ethene }}{\mathrm{CH}_2}=\mathrm{CH}_2+\mathrm{Br}_2 \longrightarrow \underset{\text { 1, 2-Dibromoethane }}{\mathrm{CH}_2 \mathrm{Br}-\mathrm{CH}_2 \mathrm{Br}}\)
19. Substitution reactions: In the presence of light, halogens are substituted with hydrogen in saturated hydrocarbons
20. Ethanol: Ethanol is produced on a large scale by the fermentation of sugar or starch with enzymes (invertase and zymase) present in the yeast.
- It reacts vigorously with sodium with the evolution of H2
- It can be dehydrated at 443 K with excess concentrated sulphuric acid.
21. Esterification: Reaction in which alcohol combines with carboxylic acid in the presence of concentrated sulphuric acid.
It gives ethanal (acetaldehyde) in the presence of mild oxidizing agents like cupric oxide (Cu2O) and acetic acid in the presence of acidified potassium dichromate.
21. Enzymes: Enzymes are catalysts, which bring about biochemical changes.
For example: Lactose enzyme turns milk-sugar, lactose into lactic acid.
22. Denatured alcohol: To make alcohol unfit for drinking, it is mixed with poisonous substances like methanol, pyridine, and blue dyes.
23. Ethanoic acid gets neutralized by bases like sodium hydroxide, and sodium carbonate to give sodium acetate salt.
24. Soaps:
Soaps are sodium or potassium salts of chain carboxylic acid molecules, in which two ends have different properties, one is hydrophilic (dissolves in water), while the other end is hydrophobic (dissolves in hydrocarbons). When cleansing clothes, the hydrophobic part attaches to dirt (oily) and the hydrophilic part attaches to water which on washing pulls away the hydrophobic part along with dirt.
25. Detergents: Detergents are similar to soaps buthydrocarbonpart here is obtained from p
NCERT Solutions For Class 10 Science Chapter 4 Carbon And Its Compounds Short Question And Answers
Question 1. What would be the electron dot structure of carbon dioxide which has the formula CO2 The electron-dot formula of CO2 is given as:
Answer:
Question 2. What would be the electron dot structure ofa molecule of sulfur which is made up of eight atoms of sulfur?
Answer:
The electron-dot structure of sulfur is made of eight atoms of sulfur in a ring as shown.
Question 3. How many structural isomers can you draw for pentane?
Answer:
Three structural isomers can be drawn for pentane:
Question 4. What are the properties of carbon which lead to the huge number of carbon compounds we see around us?
Answer:
Two properties of carbon which lead to the huge number of carbon compounds are:
- Tetravalency
- Catenation
Question 5. What will be the formula and electron dot structure of cyclopentane?
Answer:
The formula of cyclopentane will be C5H10 Its electron-dot structure is written as
Question 6. Draw the structure for the following compounds
- Ethanoic acid
- Bromopentane
- Butanone
- Hexanal
Answer:
Question 7. How can ethanol and ethanoic acid be differentiated based on their physical and chemical properties?
Question 8. Explain the formation of sum when hard water is treated with soap.
Answer:
Hard water contains soluble salts of sulfates and chlorides of calcium and magnesium. When soap is used to clean using hard water, calcium and magnesium present in hard water produce an insoluble substance called scum that remains after washing the cloth with water.
Question 9. What is hydrogenation? What is its industrial application?
Answer:
The addition of hydrogen to an unsaturated hydrocarbon in the presence of catalysts such as palladium or nickel to give saturated hydrocarbons is called hydrogenation. The process is used to make healthy vegetable oils in industry.
Question 10. What is the difference between two consecutive members in a homologous series of alkanes in terms of:
- Molecular mass.
- Number of atoms of elements.
Answer:
Difference in: Molecular mass = 14u Number of atoms = CH2
Question 11. State the valency ofeach carbon atom in
- An alkane and
- An alkyne.
Answer:
- The valency of a carbon atom in an alkane is four.
- The valency of carbon atoms in an alkyne is four
Question 12. Write the molecular formula of benzene and state the number of double bonds in its structure.
Answer:
- The molecular formula of benzene is C6H6.
- It contains three double bonds in its structure.
Question 13. Which of these has the minimum number of carbon atoms? Answer:
- Unsaturated hydrocarbons with double bonds.
- Compound ‘C’ has a minimum number of carbon atoms.
Question 14. Why are alkanes regarded as saturated hydrocarbons while alkenes and alkynes as unsaturated hydrocarbons?
Answer:
Alkanes contain C—C bonds in the molecules and a normal tetrahedral bond angle of 109°. 28′ is not disturbed. On the other hand, both alkenes and alkynes have multiple bonds (>C = C< or – C ≡ C – bond). The tetrahedral bond angle is no longer present. These are under strain and therefore, unsaturated.
Question 15. A compound ‘X has the molecular formula C3H6O with the structural formula CH3CH2CHO. Give its IUPAC name. Can another compound have the same molecular formula? Give the structure and IUPAC name of that compound also.
Answer:
The IUPAC name of X’ is: Propanal. Another compound Y can also have the same molecular formula but a different structural formula. It is propanone.
The compounds X and Y are related to each other as functional isomers.
Question 16. What happens when- (Give equations also).
- Methane reacts with excess chlorine in the presence of diffused sunlight.
- Ethanol undergoes combustion.
Answer:
Question 17. List four characteristics of homologous series.
Answer:
- Same functional group.
- Similar chemical properties.
- Successive member differ by- CH2unit/14u./
- Gradation in physical properties.
Question 18.
1. Draw the structures of the following compounds
- Ethanoic acid
- Butanone
2. Why is the conversion of ethanol to ethanoic acid considered an oxidation reaction?
Answer:
1.
2. When ethanol (C2H5OH) changes to ethanoic acid (CH3COOH):
- There is a decrease in the number of hydrogen atoms by two.
- There is an increase in the number of oxygen atoms by one.
2. Therefore, the conversion represents an oxidation reaction.
Question 19.
- What are esters? How are they formed?
- Write two uses of esters.
Answer:
1. Esters are the group of organic compounds that contain the functional group(—COOR) called the ester group. The value of R may change as -CH3, -C2H5, -C3H7, etc.
A few examples of esters are:
Esters are formed as a result of a chemical reaction called esterification.
2. Uses of esters
- Esters have a pleasant smell. These are used as flavoring agents and also in perfumes.
- Esters of glycerol known as triglycerides, are used in the manufacture of soaps.
- This reaction is called the saponification reaction.
Question 20. State the meaning of the functional group in a carbon compound. Write the functional group present in Ethanol and Ethanoic acid and also draw their structures.
Answer:
An atom or group of atoms that determine the chemical properties of organic compounds is called a functional group. They confer specific properties to the compound, regardless of the length and nature of the carbon chain
Question 21.
- Distinguish between saturated hydrocarbon and unsaturated hydrocarbon with the help of the combustion process.
- Write the chemical names of C4H10 and C2H2 and draw their structures.
Answer:
Question 22. Why vegetable oils are considered healthy as compared to vegetable ghee? How are vegetable oils converted into vegetable ghee? Name and define the process involved. Answer:
- Vegetable oils contain unsaturated fatty acids while vegetable ghee contains saturated fatty acids which are said to be harmful to health.
- Vegetable oils are converted into vegetable ghee by adding hydrogen in the presence of catalysts palladium (Pd) or nickel (Ni).
- The process involved in this is an addition reaction (hydrogenation).It is defined as the addition of hydrogen atoms in carbon atoms of an organic compound.
Question 23. Give the electron dot structure and structural formula of the first member of the alkene and alkyne families Answer:
Question 24. How will you convert ethene into ethanol? Give the chemical reaction involved.
Answer:
Ethene is converted into ethanol by passing its vapors through water in the presence of sulphuric acid. This reaction is called the hydration of ethane
Question 25. What happens when:
- Ethanol is burnt in the air and
- It is oxidized? Write the corresponding reactions involved.
Answer: 1. Ethanol produces carbon dioxide, water, and energy when it is burnt in the air CH3CH2OH + 3O2 → 2CO2 + 3H2O + Energy 2. Ethanol is oxidized into ethanoic acid
Question 26. Why are detergents preferred over soaps for washing clothes in hard water? Explain
Answer:
Detergents are preferred over soaps for washing clothes in hard water because:
- They do not form insoluble precipitates with calcium and magnesium ions in hard water.
- They give more lather in hard water.
- They have strong cleansing action.
Question 27. What happens when ethyl alcohol and acetic acid react with each other in the presence of cone H2SO4
Answer:
When C2H5OH reacts with CH3COOH, i.e., ethanol with ethanoic acid, a sweet-smelling ester is formed.
Question 28. State two characteristic features of carbon which when put together give rise to a large number of carbon compounds.
Answer:
The size of the carbon atom is very small (Atomic radius bonds = 77 pm) The strength of the C-C bond is quite high (355 kJ mol-1) Therefore, any number of carbon atoms can be linked by covalent bonds. This self-linking property is called catenation.
Question 29. Why are some compounds saturated while others are unsaturated?
Answer:
The compounds in which all the four valences of carbon are satisfied by hydrogen atoms are saturated in nature while the compounds in which the required number of hydrogen atoms are not available are unsaturated.
For example:
Question 30. Write the name and molecular formula ofan organic compound having its name suffixed with ‘-ol’ and having two carbon atoms in the molecule. With the help of a balanced chemical equation indicate what happens when it is heated with an excess of one. H2SO4
Answer:
The compound is ethanol and its molecular formula is C2H5OH. Upon heating with an excess of the cone. H2SO4 loses a molecule of H2O and forms ethene. The reaction is known as the dehydration reaction.
Question 31. Explain with the help of chemical equations, the following properties of carbon.
- Combustion
- Oxidation
Answer:
1. Combustion:
C2H5OH + 3O2→ 2CO2 + 3H2O
2. Oxidation
Question 32. Name an element, other than carbon which exhibits the property of catenation upto seven or eight atoms’? Are these compounds stable?
Answer:
The element is silicon (Si). It also belongs to group 14. It also shows catenation but its size is more than that of carbon, and the Si-Si bond is comparatively weak. Though upto seven to eight atoms can be linked to form different compounds, the stability of these compounds decreases gradually.
Question 33. Write the structure of
- Hexanal
- But-2-ene
Answer:
Question 34. How will you name the following compounds?
Answer:
- Ethanal
- Ethanol
- Methanal
- Chloroethane
Question 35. The molecular formula C2H6O can represent an aldehyde as well as a ketone. Write their structures and name them. How are they related to each other?
Answer:
The aldehyde and ketone with formula C2H6O are propanal and propanone. Having the same molecular formula, these are isomers. As the functional groups are different, these are regarded as functional isomers.
For example:
Question 36. Write the name and general formula of a chain of hydrocarbons in which an addition reaction with hydrogen can take place. Stating the essential conditions required for an addition reaction to occur write the chemical equation giving the name of the reactants and the product of such a reaction.
Answer:
1. Alkanes. General formula: CnH2n+2
2. Conditions: Presence of catalysts such as Palladium or Nickel.
3. Equations:
Question 37. Draw the electron-dot structure for ethyne. A mixture ofethyne and oxygen is burnt for welding. In your opinion, why cannot we use a mixture of ethyne and air for this purpose?
Answer: In pure oxygen, ethyne undergoes complete combustion, and a high temperature suitable for welding is attained.
Whereas air contains less percentage/amount of oxygen which results in incomplete combustion of ethyne and the temperature required for welding is not attained.
Question 38. What is meant by a homologous series of carbon compounds? Classify the following carbon compounds into two homologous series and name them.
- C3H4
- C3H6
- C4H6
- C4H8
- C5H8
- C5H10
Answer: A group of organic compounds having the same functional group and similar structures in which the successive members differ by CH2 group. C3H4, C4H6, C5H8 : Alkyne C3H6,C4H8, C5H10 , Alkenes
Question 39. Why does micelle formation take place when soap is added to water? Will a micelle be formed in all types of solvents? Justify your answer.
Answer:
When soap is added to water, micelle formation takes place. This is because the hydrocarbon chains of soap molecules are hydrophobic while the ionic ends are hydrophilic and hence soluble in water. Micelle will not form in all types of solvents. It will form in such type of solvent where soap is insoluble in that particular solvent.
Question 40. Carbon does not from ionic compounds. Why?
Answer:
To form ionic bonds carbon either should lose 4e+ or gain 4e- to form C4+ or C-4-ion. Losing 4e+ needs a large amount of energy and by gaining 4e-, it will have 10e-. It is difficult for 6 protons to hold 10e–. So C4+ and C4- is not possible.
Question 41. What are soaps? Why do they form scum with hard water?
Answer:
- Soaps are sodium or potassium salts of higher fatty acids.
- The soluble Ca and Mg salts present in hard water react with soap to give insoluble salts to produce scum.
Question 42. Write the names ofthe following compounds
Answer:
- Pentanoic acid
- Butyne
- Heptanal
- Pentanol
Question 43. Identify and name the functional groups present in the following compounds
.
Answer:
Question 44. A compound X is formed by the reaction ofa carboxylic acid and alcohol in the presence ofa few drops of H2SO4. The alcohol on oxidation with alkaline KMnO4 followed by acidification gives the same carboxylic acid as used in this reaction. Give the names and structures of
- Carboxylic acid
- Alcohol and
- The compound X.
Also, write the reaction.
Answer:
- Carboxylic acid is ethanoic acid
- Alcohol is ethanol
- X is ethyl ethanoate
Question 45. A gas is evolved when ethanol reacts with sodium. Name the gas that evolved and also write the balanced chemical equation ofthe reaction involved.
Answer:
The gas that evolved is hydrogen.
The reaction is as follows:
2C2H5OH + 2Na → 2C2H5ONa + H2↑
Question 46. Carbon, a Group (14) element in the Periodic Table, is known to form compounds with many elements. Write an example of a compound formed with
- Chlorine (Group 17 of Periodic Table)
- Oxygen (Group 16 of the periodic Table)
Answer: Carbon tetrachloride (CCl4) Carbon dioxide (CO2)
Question 47. In electron dot structure, the valence shell electrons are represented by crosses or dots.
- The atomic number of chlorine is 17. Write its electronic configuration.
- Draw the electron dot structure of the chlorine molecule.
Answer:
1. K =2 ,L = 8 , M = 7
2. Chlorine
Question 48. Match the reactions given in Column (A) with the names given in column (B)
Answer: 1- D,2- A,3- B,4-C
Question 49. A compound having molecular formula C2H6O is a saturated hydrocarbon and is a very good solvent. How can you convert it into unsaturated hydrocarbon?IdentifyXand shows its conversion with the help of the equation.
Answer:
‘X’is CH3—CH2OH (ethanol). It can be made unsaturated by heating it with a cone. H2SO4 which is a dehydrating agent that removes water from it, thereby forming ethene.
Question 50. Ethene is formed when ethanol at 443 K is heated with excess ofconcentrated sulphuric acid. What is the role of sulphuric acid in this reaction? Write the balanced chemical equation of this reaction.
Answer:
Sulphuric acid acts as a dehydrating agent.
NCERT Solutions For Class 10 Science Chapter 4 Carbon And Its Compounds Multiple Choice Questions
Question 1. Carbon exists in the atmosphere in the form of
- Carbon monoxide only
- Carbon monoxide in traces and carbon dioxide
- Carbon dioxide only
- Coal
Answer: 3. Carbon dioxide only
Question 2. Which of the following statements is usually correct for carbon compounds?
1. These are good conductors of electricity
2. These are poor conductors of electricity
3. These have strong forces of attraction between their molecules
4. These do not have strong forces of attraction between their molecules
- 1 and 3
- 2 and 3
- 1 and 4
- 2 and 4
Answer: 4. 2 and 4
Question 3. A molecule of ammonia (NH3) has
- Only single bonds
- only double bonds
- Only triple bonds
- Two double bonds and one single bond
Answer: 1. Only single bonds
Question 4. Buckminsterfullerene is an allotropic form of
- Phosphorus
- Sulfur
- Carbon
- Tin
Answer: 3. Carbon
Question 5. Which of the following are the correct structural isomers of butane?
Answer: 1
Question 6.
In the above-given reaction, alkaline KmnO4 acts as
- Reducing agent
- Oxidizing agent
- Catalyst
- Dehydrating agent
Answer: 2. Oxidizing agent
Question 7. Oils treated with hydrogen in the presence of palladium or nickel catalyst form fats. This is an example of
- Addition reaction
- Substitution reaction
- Displacement reaction
- Oxidation reaction
Answer: 1. Addition reaction
Question 8. In which ofthe following compounds, —OH is the functional group?
- Butanone
- Butanol
- Butanoic acid
- Butanal
Answer: 2. Butanol
Question 9. The soap molecule has a
- A hydrophilic head and a hydrophobic tail
- A hydrophobic head and a hydrophilic tail
- A hydrophobic head and a hydrophobic tail
- A hydrophilic head and a hydrophilic tail
Answer: 1. A hydrophilic head and a hydrophobic tail
Question 10. Which ofthe following is the correct representation of the electron dot structure of nitrogen?
Answer: 4
Question 11. The structural formula of ethyne is
Answer: 1
Question 12. Identify the unsaturated compounds from the following
1. Propane
2. Propene
3. Propyne
4. Chloropropane
- 1 and 2
- 2 and 4
- 3 and 4
- 2 and 3
Answer: 4. 2 and 3
Question 13. Chlorine reacts with saturated hydrocarbons at room temperature in the
- Absence of sunlight
- Presence of sunlight
- Presence of water
- Presence of hydrochloric acid
Answer: 2. Presence of sunlight
Question 14. In the soap micelles
- The ionic end of soap is on the surface ofthe cluster while the carbon chain is in the interior ofthe cluster
- The ionic end of soap is in the interior of the cluster and the carbon chain is out of the cluster
- Both the ionic end and carbon chain are in the interior ofthe cluster
- Both the ionic end and carbon chain are on the exterior ofthe cluster
Answer: 1. The ionic end of soap is on the surface ofthe cluster while the carbon chain is in the interior ofthe cluster
Question 15. Pentane has the molecular formula C5H12. It has
- 5 covalent bonds
- 12 covalent bonds
- 16 covalent bonds
- 17 covalent bonds
Answer: 3. 16 covalent bonds
Question 16. The structural formula of benzene is
Answer: 3
Question 17. Ethanol reacts with sodium and forms two products. These are
- Sodium ethanoate and hydrogen
- Sodium ethanoate and oxygen
- Sodium ethoxide and hydrogen
- Sodium ethoxide and oxygen
Answer: 3. Sodium ethoxide and hydrogen
Question 18. The correct structural formula of butanoic acid is
Answer: 4
Question 19. Vinegar is a solution of
- 50%- 60% acetic acid in alcohol
- 5%- 8% acetic acid in alcohol
- 5%- 8% acetic acid in water
- 50%- 60% acetic acid in water
Answer: 3. 5%- 8% acetic acid in water
Question 20. Mineral acids are stronger acids than carboxylic acids because
1. Mineral acids are completely ionised
2. Carboxylic acids are completely ionised
3. Mineral acids are partially ionised
4. Carboxylic acids are partially ionised
- 1 and 4
- 2 and 3
- 1 and 2
- 3 and 4
Answer: 1. 1 and 4
Question 21. Carbon forms four covalent bonds by sharing its four valence electrons with four univalent atoms,
Example: Hydrogen. After the formation of four bonds, carbon attains the electronic configuration of
- Helium
- Neon
- Argon
- Krypton
Answer: 2. Neon
Question 22. The correct electron dot structure ofa water molecule is
Answer: 3
Question 23. Which of the following is not a straight-chain hydrocarbon?
Answer: 4
Question 24. Which among the following are unsaturated hydrocarbons?
- 1 and 3
- 2 and 3
- 2 and 4
- 3 and 4
Answer: 3. 2 and 4
Question 25. Which ofthe following does not belong to the same homologous series?
- CH
- C2H6
- C2H6
- C4H8
Answer: 4. C4H8
Question 26. The name ofthe compound CH3—CH2—CHO is
- Propanal
- Propane
- Ethanol
- Ethanal
Answer: 1. Propanal
Question 27. The heteroatoms present in CH3—CH3—O—CH2—CH2 Cl are
1. Oxygen
2. Carbon
3. Hydrogen
4. Chlorine
- 1 and 2
- 2 and 3
- 3 and 4
- 1 and 4
Answer: 1. 1 and 2
Question 28. Which of the following represents the saponification reaction?
Answer: 4
Question 29. The first member of the alkyne homologous series is
- Ethyne
- Ethene
- Propyne
- Methane
Answer: 1. Ethyne
Question 30. Ethane, with the molecular formula C2H6 has
- 6 covalent bonds
- 7 covalent bonds
- 8 covalent bonds
- 9 covalent bonds
Answer: 2. 7 covalent bonds
Question 31. Butanone is, a four-carbon the functional compound
- Carboxylic acid.
- Aldehyde
- Ketone
- Alcohol.
Answer: 3. Ketone
Question 32. While cooking, if the bottom of the vessel is getting blackened on the outside, it means that
- The food is not cooked completely.
- The fuel is not burning completely.
- The fuel is wet.
- The fuel is burning completely.
Answer: 2. The fuel is not burning completely.