Question

1. which of the following observations is the best evidence that unknown liquid x is composed of only one pure substance?

a. liquid x forms a solid when added to hydrochloric acid.
b. liquid x boils away completely at a single temperature
c. liquid x appears clear and uniform throughout.
d. liquid x can be easily dissolved in water.

Explanation:

To determine the best evidence that liquid X is a pure substance, we analyze each option:

• Option A: Reacting to form a solid in HCl shows a chemical reaction, not purity (mixtures can also react).
• Option C: A uniform appearance (homogeneous) can be a mixture (e.g., a solution like salt water is uniform but a mixture).
• Option D: Solubility in water is a property of both pure substances and mixtures (e.g., sugar water is a mixture and soluble).
• Option B: Pure substances have a definite boiling point (boil completely at one temperature). Mixtures (like solutions) boil over a range of temperatures. So boiling at a single temperature is strong evidence of purity.

Answer:

B. Liquid X boils away completely at a single temperature