All Free Chemistry MCQs with Answers

501. Pick the crystalline solid:

• A. Cement
• B. Ceramics
• C. Concrete
• D. Copper

Explanation: Copper is a metal whose atoms sit in a regular repeating lattice, so it is crystalline and melts at one sharp temperature. Cement, ceramics and concrete are amorphous or mixed materials with only short range order, so they soften over a range rather than melting cleanly. A sharp melting point is the practical test for crystallinity.

502. Number of unpaired electrons in boron in ground state is/are:

• A. 1
• B. 2
• C. 3
• D. 4

Explanation: Boron has atomic number 5, so its configuration is 1s2 2s2 2p1 and the single 2p electron is the only unpaired one. The 1s and 2s electrons are paired within their orbitals. Boron's three valence electrons allow three bonds, but only one of them is unpaired in the ground state, which is the distinction being tested.

503. Chlorination of benzene in the presence of iron (III) chloride involves mechanism of:

• A. Electrophilic addition
• B. Electrophilic substitution
• C. Free radical substitution
• D. Free radical halogenation

Explanation: Iron three chloride acts as a halogen carrier, polarising the chlorine molecule to generate an electrophile that attacks the electron rich ring, and a hydrogen is then lost so the aromatic system is preserved. Substitution rather than addition is what keeps the delocalised sextet intact. In ultraviolet light and without the catalyst, benzene undergoes free radical addition instead, which is why the conditions matter.

504. The element with smallest value of ionization energy is

• A. Li
• B. Al
• C. Ca
• D. Ba

Explanation: Ionisation energy falls down a group as the outer electron lies further from the nucleus and is better shielded, and barium is the lowest of these four in the periodic table. Lithium is small and holds its outer electron tightly by comparison. Across a period the trend runs the other way, which is why aluminium is not the answer despite being a metal.

505. Carboxylic acid reacts with ammonia to form ammonium salts which on heating produces

• A. Carbonates
• B. Alkane
• C. Ester
• D. Amide

Explanation: The ammonium carboxylate loses a molecule of water on strong heating and the nitrogen ends up bonded directly to the carbonyl carbon, giving an amide. An ester would require an alcohol rather than ammonia, which is the standard confusion here. The amide link formed in this way is the same one that joins amino acids in a protein.