CBSE EXAMINATION PAPER-2025

Carbohydrates are polyhydroxy aldehydes or ketones that represent enormous structural diversity in terms of the arrangement of atoms in space, resulting in hundreds of stereoisomers. Although the chemical properties of most stereoisomers may not be very different, their metabolic rate and utilization in biological systems is significantly different and known to influence the overall carbohydrate metabolism. Structural variants, which arise due to a different arrangement of atoms in three-dimensional space are known as stereoisomers. The number of stereoisomers can be theoretically estimated by using the formula 2n, where ‘n’ is the number of stereocenters or asymmetric (chiral) carbon atoms in a molecule. Out of these stereoisomers, there are some structures, which are mirror images of each other, and they are referred to as enantiomers.

Answer the following questions :

Werner’s coordination theory in 1893 was the first attempt to explain the bonding in coordination complexes. It must be remembered that this theory was put forward before the electron had been discovered by J.J. Thomson in 1897, and before the electronic theory of valency. Werner did not have any of the modern instrumental techniques and all his studies were made using simple experimental techniques. Werner was able to explain the nature of bonding in complexes and he concluded that in complexes, the metal shows two different sorts of valency : primary and secondary. Primary valences are normally ionisable whereas secondary valences are non ionisable.

Answer the following questions :

The role of a catalyst is to change:

Which of the following molecules is chiral in nature?

CH₃CH₂OH can be converted to CH₃CHO by:

The IUPAC name for CH₃-CH₂-N(CH₃)-CH₂-CH₂-CH₃ is:

The treatment of ethyl bromide with alcoholic silver nitrite gives:

Which of the following aqueous solutions will have the highest freezing point?

In which of the following groups are both ions coloured in aqueous solution?

I. Cu⁺ II. Ti⁴⁺ III. Co²⁺ IV. Fe²⁺

[Atomic number : Cu = 29, Ti = 22, Co = 27, Fe = 26]

CH₃CH₂CHO and CH₃CH₂COOH can be distinguished by:

While doing qualitative analysis in chemistry lab, Abhishek added yellow coloured potassium chromate solution into a test tube. He was surprised to see the colour of the solution changing immediately to orange. He realised that the test tube was not clean and contained a few drops of some liquid. Which of the following substances will be the most likely liquid to be present in the test tube before adding potassium chromate solution?

Assertion (A) : For measuring resistance of an ionic solution an AC source is used.

Reason (R) : Concentration of ionic solution will change if DC source is used.

Assertion (A) : Henry’s law constant (KH) decreases with increase in temperature.

Reason (R) : As the temperature increases, solubility of gases in liquids decreases.

Assertion (A) : The solubility of aldehydes and ketones in water decreases with increase in size of the alkyl group.

Reason (R) : Aldehydes and ketones have dipole-dipole interaction.

Assertion (A) : The boiling points of alkyl halides decrease in the order RI > RBr > RCl > RF.

Reason (R) : The van der Waals forces of attraction decrease in the order RI > RBr > RCl > RF

Calculate the elevation of boiling point of a solution when 3 g of CaCl₂ (Molar mass = 111 g mol⁻¹) was dissolved in 260 g of water, assuming that CaCl₂ undergoes complete dissociation. (Kb for water = 0·52 K kg mol⁻¹)

Liquids ‘X’ and ‘Y’ form an ideal solution. The vapour pressure of pure ‘X’ and pure ‘Y’ are 120 mm Hg and 160 mm Hg respectively. Calculate the vapour pressure of the solution containing equal moles of ‘X’ and ‘Y’.

Explain the mechanism of acid catalysed hydration of ethene.

Shweta mixed two liquids A and B of 10 mL each. After mixing, the volume of the solution was found to be 20·2 mL.

(i) Why was there a volume change after mixing the liquids ?

(ii) Will there be an increase or decrease of temperature after mixing ?

(iii) Give one example for this type of solution.

(i) How does sprinkling of salt help in clearing the snow covered roads in hilly areas ?

(ii) What happens when red blood cells are kept in 0·5% (mass/vol) NaCl solution ? Justify your answer.

(iii) Write an application of reverse osmosis.

An organic compound ‘A’ (molecular formula C₈H₈O) gives 2,4-DNP test. It does not give Tollen’s test, but gives a yellow precipitate ‘B’ with NaOH and I₂. On drastic oxidation, it gives a carboxylic acid ‘C’ with formula C₇H₆O₂. Identify ‘A’, ‘B’, ‘C’ and write the reactions involved.