CBSE Class X

Question 11:

What change will you observe if you test soap with litmus paper (red and blue)?

Answer:

Since soap is basic in nature, it will turn red litmus blue. However, the colour of blue

litmus will remain blue.

Question 10:

Explain the formation of scum when hard water is treated with soap.

Answer:

Soap does not work properly when the water is hard. A soap is a sodium or potassium

salt of long chain fatty acids. Hard water contains salts of calcium and magnesium.

When soap is added to hard water, calcium and magnesium ions present in water

displace sodium or potassium ions from the soap molecules forming an insoluble

substance called scum. A lot of soap is wasted in the process.

Question 9:

Why are carbon and its compounds used as fuels for most applications?

Answer:

Most of the carbon compounds give a lot of heat and light when burnt in air.

Saturated hydrocarbons burn with a clean flame and no smoke is produced. The

carbon compounds, used as a fuel, have high calorific values. Therefore, carbon

and its compounds are used as fuels for most applications.

Question 8:

In the electrolytic refining of a metal M, what would you take as the anode, the

cathode and the electrolyte?

Answer:

In the electrolytic refining of a metal M:

Anode → Impure metal M

Cathode → Thin strip of pure metal M

Electrolyte → Solution of salt of the metal M

Question 7:

How can ethanol and ethanoic acid be differentiated on the basis of their physical and

chemical properties?

Answer:

• Ethanol is a liquid at room temperature with a pleasant odour while ethanoic acid

   has vinegar-like smell. The melting point of ethanoic acid is 17°C. This is below  

   room temperature and hence, it freezes during winters.

• Ethanoic acid reacts with metal carbonates and metal hydrogencarbonates to form

   salt,water, and carbon dioxide gas while ethanol does not react with them.

   Metal Carbonates/Metal Hydrogencarbonates + Carboxylic acid]

                                                       ↓

                                       Salt + Water + Carbon dioxide

For example,

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂

Metal Carbonates/Metal Hydrogencarbonates + Alcohols

                                                ↓

                                        No reaction

For example,

CH₃CH₂OH + Na₂CO₃ → No reaction

Question 6:

What is a homologous series? Explain with an example.

Answer:

A homologous series is a series of carbon compounds that have different numbers of

carbon atoms but contain the same functional group.

For example, methane, ethane, propane, butane, etc. are all part of the alkane

homologous series. The general formula of this series is C_nH_2n+2.

Methane CH₄

Ethane CH₃CH₃

Propane CH₃CH₂CH₃

Butane CH₃CH₂CH₂CH₃

It can be noticed that there is a difference of −CH₂ unit between each successive

compound. 

Question 5:

Draw the electron dot structures for

(a) ethanoic acid.

(b) H₂S.

(c) propanone.

(d) F₂.

Answer:

(a) Ethanoic acid

 

(b) H₂S

 

(c) Propanone

 

(d) F₂

 

Question 4:

Explain the nature of the covalent bond using the bond formation in CH3Cl.

Answer:

Carbon can neither lose four of its electrons nor gain four electrons as both the

processes require extra amount of energy and would make the system unstable.

Therefore, it completes its octet by sharing its four electrons with other carbon

atoms or with atoms of other elements. The bonds that are formed by sharing

electrons are known as covalent bonds. In covalent bonding, both the atoms share

the valence electrons, i.e., the shared electrons belong to the valence shells of both

the atoms.

 

Here, carbon requires 4 electrons to complete its octet, while each hydrogen atom

requires one electron to complete its duplet. Also, chlorine requires an electron to

complete the octet. Therefore, all of these share the electrons and as a result,

carbon forms 3 bonds with hydrogen and one with chlorine.

Question 3:

While cooking, if the bottom of the vessel is getting blackened on the outside, it .

means that

(a) the food is not cooked completely.

(b) the fuel is not burning completely.

(c) the fuel is wet.

(d) the fuel is burning completely.

Answer:

(b) While cooking, if the bottom of the vessel is getting blackened on the outside,

then it means that the fuel is not burning completely.

Question 2:

Butanone is a four-carbon compound with the functional group

(a) carboxylic acid.

(b) aldehyde.

(c) ketone.

(d) alcohol.

Answer:

(c) The functional group of butanone is ketone.

Question 1:

Ethane, with the molecular formula C2H6 has

(a) 6 covalent bonds.

(b) 7 covalent bonds.

(c) 8 covalent bonds.

(d) 9 covalent bonds.

Answer:

(b) Ethane has 7 covalent bonds.

Question 2:

People use a variety of methods to wash clothes. Usually after adding the soap, they

‘beat’ the clothes on a stone, or beat it with a paddle, scrub with a brush or the

mixture is agitated in a washing machine. Why is agitation necessary to get clean

clothes?

Answer:

A soap molecule has two parts namely hydrophobic and hydrophilic. With the help of

these, it attaches to the grease or dirt particle and forms a cluster called micelle.

These micelles remain suspended as a colloid. To remove these micelles (entrapping

the dirt), it is necessary to agitate clothes.

Question 1:

Would you be able to check if water is hard by using a detergent?

Answer:

Detergents are ammonium or sulphonate salts of long chain carboxylic acids. Unlike

soap, they do not react with calcium and magnesium ions present in hard water to

form scum. They give a good amount of lather irrespective of whether the water is

hard or means that detergents can be used in both soft and hard water. Therefore,

it cannot be used to check whether the water is hard or not.

Question 2:

What are oxidising agents?

Answer:

Some substances such as alkaline potassium permanganate and acidified potassium

dichromate are capable of adding oxygen to others. These are known as oxidising

agents.

Question 1:

How would you distinguish experimentally between an alcohol and a carboxylic acid?

Answer:

We can distinguish between an alcohol and a carboxylic acid on the basis of their

reaction with carbonates and hydrogen carbonates. Acid reacts with carbonate and

hydrogen carbonate to evolve CO₂ gas that turns lime water milky.

Metal Carbonate/Metal Hydrogencarbonate + Carboxylic acid

                                                      ↓

                                Salt + Water + Carbon dioxide

Alcohols, on the other hand, do not react with carbonates and hydrogen carbonates.

Question 2:

A mixture of oxygen and ethyne is burnt for welding. Can you tell why a mixture of

ethyne and air is not used?

Answer:

2HC ≡ CH + 5O₂ → 4CO₂ + 2H₂O + Heat

When ethyne is burnt in air, it gives a sooty flame. This is due to incomplete

combustion caused by limited supply of air. However, if ethyne is burnt with oxygen,

it gives a clean flame with temperature 3000°C because of complete combustion.

This oxy-acetylene flame is used for welding. It is not possible to attain such a high

temperature without mixing oxygen. This is the reason why a mixture of ethyne and

air is not used.

Question 1:

Why is the conversion of ethanol to ethanoic acid an oxidation reaction?

Answer:

 

Since the conversion of ethanol to ethanoic acid involves the addition of oxygen to

ethanol, it is an oxidation reaction.

Question 1:

Why do ionic compounds have high melting points?

Answer:

Ionic compounds have strong electrostatic forces of attraction between the ions.

Therefore, it requires a lot of energy to overcome these forces. That is why ionic

compounds have high melting points.

Question 4:

Draw the structures for the following compounds.

(i) Ethanoic acid    (ii) Bromopentane*

(iii) Butanone        (iv) Hexanal

*Are structural isomers possible for bromopentane?

Answer:

(ii) There are many structural isomers possible for bromopentane. Among them, the

     structures of three isomers are given.

Question 3:

What will be the formula and electron dot structure of cyclopentane?

Answer:

The formula for cyclopentane is C₅H₁₀. Its electron dot structure is given below.