Enzymes are biological catalysts that are: MCQ with Answer and Explanation

Explanation:
Zn + 2HCl → ZnCl₂ + H₂↑. Zinc displaces hydrogen from acid because it is more reactive than hydrogen in the reactivity series. Hydrogen gas burns with a pop sound when a burning splint is brought near the mouth of the test tube. Metals above hydrogen in the series react with acids to evolve H₂; metals below (Cu, Ag) do not. Nitric acid is an oxidizer, so may not yield H₂.

Explanation:
Graphite, an allotrope of carbon, is a good conductor of electricity. This is because each carbon atom is bonded to three others, leaving one free valence electron per atom that is delocalized and can move freely between the layers to conduct electricity, unlike diamond which is an insulator.

Explanation:
Across a period, the number of electron shells remains the same, but the nuclear charge (protons) increases. The added electrons enter the same valence shell, so the effective nuclear attraction on the electrons increases, pulling them closer and shrinking the atomic radius. Down a group, radius increases due to addition of shells. Electron-electron repulsion alone does not account for the trend. Atomic mass generally increases across a period.