Solutions & Colloids — Chemistry

Q.1

WBCS Prelims 2023

Identify the substance which can dissolve in water:

A. Glucose

B. Urea

C. Acetone

D. All of the above

Explanation

Why Correct: Glucose, urea, and acetone all dissolve in water due to their polar nature and ability to form hydrogen bonds with water molecules.
Distractor Analysis: Glucose is a polar sugar soluble in water. Urea is a polar organic compound highly soluble in water. Acetone is a polar organic solvent completely miscible with water.
Takeaway: Polar substances and those capable of hydrogen bonding typically dissolve well in water, while nonpolar substances like oils and fats do not.

Answer or skip previous question to unlock.

Q.2

WBCS Prelims 2015

How many grams of NaOH are needed to make 100 ml of a 0.5 M solution of NaOH?

A. 2

B. 20

C. 4

D. 1

Explanation

Core Formula/Logic: Molarity (M) = moles of solute / volume of solution in liters. Mass = moles × molar mass.
Step-by-Step Solution: 1. Calculate molar mass of NaOH: Na (23) + O (16) + H (1) = 40 g/mol.
2. Convert volume to liters: 100 ml = 0.1 L.
3. Calculate moles needed: Molarity × Volume = 0.5 mol/L × 0.1 L = 0.05 mol.
4. Calculate mass: 0.05 mol × 40 g/mol = 2.0 g.
Common Pitfall: Forgetting to convert ml to liters gives 0.5 × 100 = 50 moles, then 50 × 40 = 2000 g (not an option). Using 100 ml directly as liters gives 0.5 × 1 = 0.5 moles, then 0.5 × 40 = 20 g (option B).
Shortcut/Takeaway: For 0.5 M solution, mass in grams = (Molarity × Volume in L × Molar mass). Quick check: 0.5 × 0.1 × 40 = 2 g. Always convert volume to liters first.

Answer or skip previous question to unlock.

Q.3

WBCS Prelims 2012

How many grams of dibasic acid (mol wt 200) should be present in 100 ml of its aqueous solution to give 0.1(N) strength ?

A. 2 gm

B. 1gm

C. 10 gm

D. 20 gm

Explanation

Core Formula/Logic: Normality = (Mass × n) / (Molar mass × Volume in liters), where n is basicity (2 for dibasic acid).
Step-by-Step Solution: 1. Convert volume: 100 ml = 0.1 L.
2. Rearrange formula: Mass = (Normality × Molar mass × Volume) / n.
3. Substitute values: Mass = (0.1 × 200 × 0.1) / 2.
4. Calculate: (2) / 2 = 1 gram.
Common Pitfall: Forgetting to divide by basicity gives 2 grams (option A), while using volume in ml without conversion yields 20 grams (option D).
Shortcut/Takeaway: For dibasic acids, equivalent weight = Molar mass/2 = 100 g/equiv. Then mass = Normality × Equivalent weight × Volume(L) = 0.1 × 100 × 0.1 = 1 gram.

Answer or skip previous question to unlock.

Q.4

WBCS Prelims 2009

Normal saline is an aqueous solution of sodium chloride of strength

A. 0.84 % (0.9%)

B. 1.00 normal

C. 1.00 molar

D. 1 %

Explanation

Why Correct: Normal saline contains 0.9% w/v sodium chloride, which is isotonic with human blood plasma and matches physiological osmotic pressure.
Distractor Analysis: 1.00 normal refers to a solution containing one equivalent per liter, not the clinical standard for saline. 1.00 molar means 58.44 g/L of NaCl, which is hypertonic and can damage cells. 1% w/v is slightly hypertonic compared to the standard 0.9% concentration.
Takeaway: Other common intravenous solutions include Ringer's lactate (contains sodium, potassium, calcium, and lactate) and dextrose solutions (5% glucose), each designed for specific fluid and electrolyte replacement needs.

Answer or skip previous question to unlock.

Q.5

WBCS Prelims 2007

Milk is an example of

A. emulsion

B. gel

C. foam

D. sol

Explanation

Why Correct: Milk is an oil-in-water emulsion where fat globules disperse in water, stabilized by casein proteins that prevent separation.
Distractor Analysis: Gels are semi-solid networks like jelly where liquid disperses in solid. Foams are gas bubbles in liquid like whipped cream. Sols are solid particles in liquid that settle over time, like muddy water.
Takeaway: Butter is a water-in-oil emulsion, the inverse of milk's oil-in-water structure.

Answer or skip previous question to unlock.

Q.6

WBCS Prelims 2002

Colloidal particles do not settle down because

A. Their specific gravity is very low

B. They are present in far smaller quantities than that of the dispersion medium

C. they carry electrical charge

D. The become solvated

Explanation

Why Correct: Colloidal particles carry similar electrical charges that create repulsive forces, preventing aggregation and settling.
Distractor Analysis: Specific gravity affects sedimentation rate but doesn't prevent settling entirely. Quantity ratio affects concentration but not stability. Solvation forms a protective layer but the primary stabilization mechanism is electrostatic repulsion.
Takeaway: The Tyndall effect distinguishes colloidal solutions from true solutions by scattering light through suspended particles.

Sign in to Papersetters