Pure substances and mixtures Transitional Reasoning Practice

Questions

1. 1.

   A chemistry student investigates three clear, colorless liquid samples, X, Y, and Z, at standard atmospheric pressure. Sample X boils at a constant temperature of 78.4 °C until it completely vaporizes, leaving behind no residue. Sample Y begins boiling at 99 °C, but its boiling temperature steadily climbs to 105 °C during vaporization, ultimately leaving a white crystalline solid at the bottom of the vessel. Sample Z has a sharp, invariant boiling point of 100.0 °C and cannot be separated into simpler components by physical means, but when subjected to an electric current (electrolysis), it chemically decomposes into two distinct elemental gases in a fixed 2:1 volume ratio. Based on these observations, which of the following evaluations is correct?

   • A) Sample X must be an elemental substance because pure compounds never maintain a fixed boiling point during phase change.
   • B) Sample Y is a homogeneous pure substance because the solid solute was completely dissolved throughout the liquid prior to heating.
   • C) Sample Y is a mixture whose boiling temperature increases as solvent evaporates, while Sample Z is a chemical compound.
   • D) Samples X and Z are both classified as mixtures because all liquid materials are composed of multiple distinct phases.

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