Reactivity increases down Group 1

Steps:

• Identify Group 1 trend: reactivity of alkali metals increases down the group due to larger atomic size and lower ionization energy.
• Locate elements: potassium (atomic number 19) is above rubidium (atomic number 37).
• Compare properties: rubidium, being lower, is more reactive than potassium.
• Eliminate distractors: check melting points and water reaction products against trends.

Why D is correct:

• Reactivity increases down Group 1 as ionization energy decreases, making it easier for rubidium to lose its outer electron than potassium.

Why the others are wrong:

• A: Melting points decrease down Group 1 due to weaker metallic bonding in larger atoms, so rubidium's is lower.
• B: Reaction with water yields hydroxide ions, forming a basic solution, not acidic.
• C: Reaction produces hydrogen gas, not oxygen.

Final answer: D