Board-style MCQs and reasoning questions in the BIEK / Sindh Board pattern. Tap an option to check yourself instantly. Solved questions are at the bottom.
Multiple-choice questions
Solved reasoning & self-assessment (past papers)
Why does water have a higher boiling point than HF, even though F is more electronegative?
Each water molecule can form up to 4 hydrogen bonds (2 O–H donors + 2 lone pairs), forming a 3-D network.
HF forms only 1–2 per molecule (one H, but 3 lone pairs limit the chain). So overall H-bonding is stronger in water → water boils higher (100 °C vs 19.5 °C).
Heat to vaporise 36 g of water at 100 °C (ΔH_vap = 40.7 kJ/mol)
moles = 36 / 18 = 2 mol
Q = n × ΔH_vap = 2 × 40.7 = 81.4 kJ
Why does evaporation cause cooling?
The molecules that escape are the fastest (highest energy) ones. Removing them lowers the average kinetic energy — and therefore the temperature — of the molecules left behind → the liquid cools.
Why does ice float on water?
In ice, hydrogen bonds lock the molecules into an open hexagonal cage with large empty spaces, so ice is less dense than liquid water → ice floats.
Why does food cook slowly on a high mountain?
At high altitude the atmospheric pressure is low, so the vapour pressure equals it at a lower temperature → water boils below 100 °C → the water is not hot enough to cook quickly.