The oil drop experiment to determine the charge of an electron was performed by: MCQ with Answer and Explanation

Explanation:
Melting of wax is a physical change because it involves only a change in state from solid to liquid without altering the chemical composition. The wax remains the same hydrocarbon mixture. In contrast, rusting of iron (Fe₂O₃·xH₂O formation), burning of magnesium (MgO formation), and digestion of food (enzymatic breakdown into simpler molecules) are chemical changes where new substances are formed. Physical changes are reversible, while most chemical changes are irreversible. Exam tip: Remember, changes of state (melting, boiling, condensation) are physical changes unless a chemical reaction accompanies them.

Explanation:
In diamond, each carbon atom is sp³ hybridized and forms four strong covalent bonds, utilizing all four valence electrons. No free electrons are available for conduction. Graphite has free delocalized electrons per carbon, making it a conductor.

Explanation:
Soap solution is basic, pH > 8.3, so it turns phenolphthalein pink. Lemon juice (citric acid) and vinegar (acetic acid) are acidic, pH < 7, phenolphthalein remains colorless. Hydrochloric acid is strongly acidic, colorless. Phenolphthalein indicator is colorless in acidic and neutral, pink in alkaline solutions. It is used in titrations.