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Science [5 marks Questions]

Carbon and its Compounds · CBSE STD 10 (CBSE - English Medium). 15 questions.

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Question 15 Marks
An organic compound $A$ having the molecular formula $C _3 H _8 O$ is a liquid at room temperature. The organic liquid $A$ reacts with sodium metal to evolve a gas which burns causing a little explosion. When the organic liquid A heated with concentrated sulphuric acid at $170^{\circ} C$, it forms a compound B which decolourizes bromine water. The compound B adds on one molecule of hydrogen in the presence of Ni as catalyst to forms compound C which gives substitution reactions with chlorine.
a. What is compound A ?
b. What is compound B ?
c. What type of reaction occurs when A is converted into B ?
d. What is compound C?
e. What type of reaction takes place when $B$ is converted into $C$ ?
Answer
a. A is propanal, $CH _3- CH - CH OH$
b. B is propene, $CH CH = CH$
c. Dehydration reaction.
d. C is propane, $CHCH - CH$
e. Addition reaction.
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Question 25 Marks
Describe the structure of graphite with the help of a labelled diagram.
Answer
The structure of graphite is very different from that of diamond. A graphite crystal consists of layers of carbon atoms or sheets of carbon atoms. Each carbon atom in a graphite layer is joined to other three carbon atoms by strong covalent bonds to form flat hexagonal rings. The various layers of carbon atoms in graphite are held together by weak Van der Waals forces. Due to this sheet like structure, graphite is a comparatively soft substance.
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Question 35 Marks
You are given the following molecular formulae of some hydrocarbons:
$C _5 H _8 ; C _7 H _{14} ; C _6 H _6 ; C _5 H _{10} ; C _7 H _{12} ; C _6 H _{12}$
a. Which formula represent cyclohexane as well as hexene?
b. Which formula represents benzene?
c. Which three formulae represent open chain unsaturated hydrocarbons having double bonds?
d. Which two formulae represent unsaturated hydrocarbons having triple bonds?
e. Which three formulae can represent cyclic hydrocarbons?
Answer
a. $C _6 H _{12}$ is the molecular formula which represents cyclohexane as well as hexene.
b. $C _6 H _6$ represents is the molecular formula of benzene.
c. $C _7 H _{14}, C _5 H _{10}, C _6 H _{12}$ are the molecular formulae for the open chain unsaturated hydrocarbons having double bonds.
d. $C _5 H _8$ and $C _7 H _{12}$ are the molecular formulae for the unsaturated hydrocarbons comprising triple bonds.
e. $C _7 H _{14}, C _5 H _{10}, C _6 H _{12}$ are the three molecular formulae which represent cyclic hydrocarbons.
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Question 45 Marks
Two organic compounds A and B have the same molecular formula $C_6H_{12}$. Write the names and structural formulae:
  1. If A is a cyclic compound.
  2. If B is an open chain compound.
  3. Which compound contains single bonds as well as a double bond?
  4. Which compound contains only single bonds?
Answer
  1. If A is a cyclic compound, then $C_6H_{12}$ is the molecular formula of the compound named cyclohexane.
  1. If B is an open chain compound, then $C_6H_{12}$is the molecular formula of the compound named hexene.
  1. Hexene (B) is an unsaturated hydrocarbon comprising both single-bonded and double-bonded carbon atoms.
  2. Cyclohexane is an alkane which contains only single-bonded carbon atoms.
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Question 55 Marks
Describe the structure of diamond. Draw a simple diagram to show the arrangement of carbon atoms in diamond.
Answer
A diamond crystal is a giant molecule of carbon atoms. Each carbon atom in the diamond crystal is linked to four other carbon atoms by strong covalent bonds. The four surrounding carbon atoms are at the four vertices of a regular tetrahedron. This rigid structure of diamond makes it a very hard substance.
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Question 65 Marks
The solid element A exhibits the property of catenation. It is also present in the form of a gas B in the air which is utilised by plants in photosynthesis. An allotrope C of this element is used in glass cutters.
  1. What is element A?
  2. What is the gas B?
  3. Name the allotrope C.
  4. State another use of allotrope C (other than in glass cutters).
  5. Name another allotrope of element A which exists as spherical molecules.
  6. Name a yet another allotrope of element A which conduct electricity.
Answer
  1. Carbon is the element A which exhibits the property of catenation.
  2. Carbon dioxide is the gas B which is utilised by the plants for photosynthesis.
  3. Diamond is the allotrope C of carbon which is used in glass cutters.
  4. Diamond is used in making jewellery.
  5. Buckminsterfullerene is an allotrope of carbon which exists as spherical molecules.
  6. Graphite is another allotrope of carbon which conducts electricity.
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Question 75 Marks
A colourless organic liquid $X$ of molecular formula $C _2 H _4 O _2$ turns blue litmus to red. Another colourless organic liquid Y of molecular formula $C _3 H _6 O$ has no action on any litmus but it is used as a nail polish remover. A yet another colourless organic liquid Z of molecular formula $C _2 H _6 O$ has also no action on litmus but it is used in tincture of iodine.
a. Name the liquid X . To which homologous series does it belong? Give the name of another member of this homologous series.
b. Name the liquid Y. To which homologous series does it belong? Write the name of another member of this homologous series.
c. Can you name an organic compound having the same molecular formula as liquid $Y$ but which belongs to a different homologous series? What is this homologous series?
d. Name the liquid Z. To which homologous series does it belong? Write the name of another member of this homologous series.
Answer
  1. The liquid 'X' is ethanoic acid which turns the blue litmus red. It belongs to the homologous series of carboxylic acid. Methanoic acid is the other member of this homologous series.
  2. The liquid 'Y' is propanone which is colourless, and is widely used as a nail polish remover. It belongs to the homologous series of ketones. Butanone is the name of the other member of this homologous series.
  3. Propanal is an organic compound having the same molecular formula as of the liquid 'Y' (Propanone), but the former belongs to the homologous series of aldehydes.
  4. The liquid 'Z' is ethanol which has no action on the litmus, and is used in the tincture of iodine. It belongs to the homologous series of alcohols. Methanol is the other member of this homologous series.
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Question 85 Marks
An organic compound having the molecular formula $C_3H_6O$ can exist in the form of two isomers A and B having different functional groups. The isomer A is a liquid which is used as a solvent for nail polish. The isomer B is also a liquid. An aqueous solution of one of the lower homologues of B is used for preserving biological specimens in the laboratory.
  1. What is compound A?
  2. Write the electron-dot structure of A.
  3. What is compound B?
  4. Write the electron-dot structure of B.
  5. Name the lower homologue of compound B which is used in preserving biological specimens.
Answer
  1. The isomer A is propanone (acetone).
  2. The electron-dot structure of propanone is:
  3. The isomer B is propanal.
  4. The electron-dot structure of propanal is:
  1. Methanol or formaldehyde is the lower homologue of compound B, which is used to preserve biological specimens.
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Question 95 Marks
Consider the following organic compounds:
$CH _3 OH , C _2 H _5 OH , CH _3 COCH _3, CH _3 COOH , C _2 H _5 COOH , C _4 H _9 COOC _2 H _5, CH _4, C _2 H _6, CH _3 CHO , HCHO$
Out of these compounds:
a. Which compound is most likely to be sweet-smelling?
b. Which compound on treatment with conc. $H _2 SO _4$ at $170^{\circ} C$ forms an alkene?
c. Which compound on repeated chlorination forms chloroform?
d. Which compound is added to alcohol to denature it?
e. Which compound is a constituent of vinegar?
f. Which compound is used to sterilise wounds and syringes?
Answer
a. $C _4 H _9 COOC _2 H _5$ : Ester.
b. $C _2 H _5 OH$ : Alcohol forms ethane, $C _2 H$
c. $CH _4$ : Methane.
d. $CH _3 OH$ : Methanol.
e. $CH _3 COOH$ : Acetic acid.
f. $C _2 H _5 OH$ : Ethanol.
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Question 105 Marks
A four carbons atoms containing neutral organic compound $X$ reacts with sodium metal to evolve a gas which burns with 'pop' sound. Another four carbon atoms containing carbon compound reacts with sodium hydrogen carbonate to evolve a gas which turns lime water milky. When compounds $X$ and $Y$ are heated together in the presence of a little of concentrated sulphuric acid, then a new compound $Z$ is formed.
a. What is compound $X$ ? Also write its formula.
b. What is compound Y? Also write its formula.
c. What is compound Z? Also write its formula.
d. What type of smell is given by compound Z?
e. What is the general name of compounds like $Z$ ?
f. What is the general name of the reaction which which takes place between $X$ and $Y$ to form $Z$ ?
Answer
a. The compound $X$ is Butanol, and its formula is $C _4 H _9 OH$.
b. The compound $Y$ is butanoic acid, and its formula is $C _3 H _7 COOH$.
c. The compound $Z$ is butyl butanoate, and its formula is $C _3 H _7 COOC _4 H$.
d. The type of smell produced by the compound is a sweet, fruity fragrance.
e. Ester is the general name given to the compound formed by the reaction of X and Y .
f. Esterification is the general name of the reaction which takes place between Butanol and butanoic acid to produce butyl butanoate.
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Question 115 Marks
An organic compound A (molecular formula $C _2 H _4 O _2$ ) reacts with Na metal to form a compound B and evolves a gas which burns with a pop sound. Compound A on treatment with an alcohol C in the presence of a little of concentrated sulphuric acid forms a sweet-smelling compound D (molecular formula $C _3 H _6 O _2$ ). Compound D on treatment with NaOH solution gives back B and C. Identify A, B, C and D.
Answer
a. A is ethanoic acid, $CH _3 COOH$
b. B is sodium ethanoate, $CH _3 COONa$
c. C is methanol, $CH _3 OH$
d. D is methyl ethanoate, $CH _3 COOCH _3$​​​​​​​
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Question 125 Marks
When ethanoic acid reacts with sodium hydrogencarbonate, then a salt X is formed and a gas Y is evolved.
Name the salt X and gas Y. Describe an activity with the help of a labelled diagram of the apparatus used to prove that the evolved gas is the one which you have named. Also write the chemical equation of the reaction involved.
Answer
Salt X is sodium ethanoate, $CH _3 COONa$; Gas Y is carbon dioxide, $CO _2$.

Activity: Take a boiling tube and put about 0.5g of sodium carbonate in it. Add 2ml of dilute ethanoic acid to the boiling tube (through a thistle funnel). We will observe that brisk effervescence of carbon dioxide gas is produced. Let us pass this gas through lime water taken in a test tube. We will find that lime water turns milky. Only carbon dioxide gas can turn lime water milky. So, this experiment proves that when ethanoic acid reacts with sodium carbonate, then carbon dioxide gas is evolved.



$\text{CH}_3\text{COOH}+\text{NaHCO}_3\rightarrow\text{CH}_3\text{COONa}+\text{CO}_2\uparrow+\text{H}_2\text{O} $
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Question 135 Marks
You are given an organic compound having the molecular formula $C _3 H _8$. Give the name and formula of the compound formed:
a. When one H atom of $C _3 H _8$ is replaced by a Cl atom.
b. When one H atom of $C _3 H _8$ is replaced by a OH group.
c. When one H atom of $C _3 H _8$ is replaced by a CHO group.
d. When one H atom of $C _3 H _8$ is replaced by a COOH group.
e. When two H atoms joined to the middle carbon atom of $C _3 H _8$ are replaced by one O atom.
Answer
a. Chloropropane- $CH _3- CH _2- CH _2- Cl$.
b. Propanal- $CH _3- CH _2- CH _2- OH$.
c. Butanal- $CH _3- CH _2- CH _2- CHO$.
d. Butanoic acid- $CH _3- CH _2- CH _2- COOH$.
e. Propanone- $CH _3- CO - CH _3$.
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Question 145 Marks
Explain the cleansing action of soap. Draw diagrams to illustrate your answer.
Answer
Cleaning action of soap has been explained with the help of the image below:
Soaps are molecules in which the two ends have differing properties, one is hydrophilic, that is it dissolves in water, while the other end is hydrophobic, that is it dissolves in hydrocarbons. When soap is at the surface of water, the hydrophobic ‘tail’ of soap will not be soluble in water and the soap will align along the surface of water with the ionic end in water and the hydrocarbon ‘tail’ protruding out of water.

Inside water, these molecules have a unique orientation that keeps the hydrocarbon portion inside the water. This is achieved by forming clusters of molecules in which the hydrophobic tails are in the interior of the cluster and the ionic ends are on the surface of the cluster. This formation is called a micelle. When a dirty cloth is put in water containing dissolved soap, then soap in the form of a micelle is able to clean. The hydrocarbon ends of the soap attach to the oily dirt particles and entrap them at the centre of the micelle. the ionic ends in the micelles remain attached to water. When the dirty cloth is agitated in soap solution, the oily dirt particles entrapped by soap micelles get dispersed in water and the cloth gets cleaned.
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Question 155 Marks
An element E exists in three allotropic forms A, B and C. In allotrope A, the atoms of element E are joined to form spherical molecules. In allotrope B, each atom of element E is surrounded by three other E atoms to form a sheet like structure. In allotrope C, each atom of element E is surrounded by four other E atoms to form a rigid structure.
  1. Name the element E.
  2. What is allotrope A.
  3. What is allotrope B?
  4. What is allotrope C?
  5. Which allotrope is used in making jewellery?
  6. Which allotrope is used in making anode of a dry cell?
Answer
  1. Carbon is the element 'E' which exists in three allotropic forms A, B and C.
  2. Buckminsterfullerene is the allotrope, A, of carbon in which the atoms are joined to form spherical molecules.
  3. Graphite is the allotrope, B, of carbon which is surrounded by three other carbon atoms to form a sheet like structure.
  4. Diamond is the allotrope, C, of carbon in which each atom is surrounded by four other carbon atoms to form a rigid structure.
  5. The allotrope C (Diamond) is used to make jewellery.
  6. The allotrope B (Graphite) is used in making anode of a dry cell.
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