The table above summarizes data given to a student to evaluate the type of change that took place when substance X was mixed with water.

Question

Substance appearance, boiling point, Density
Water	Clear, colorless liquid	100.0°C	1.00g/mL
X	White, crystalline solid	1935.5°C	2.15g/mL
Mixture of water +X	Clear, colorless liquid	101.6°C	1.08g/mL

The table above summarizes data given to a student to evaluate the type of change that took place when substance X was mixed with water. The student claimed that the data did not provide enough evidence to determine whether a chemical or physical change took place and that additional tests were needed. Which of the following identifies the best way to gather evidence to support the type of change that occurred when water and Xwere mixed?

A. Measuring the melting point of the mixture of water and X

B. Adding another substance to the mixture of water and X to see whether a solid forms

C Measuring and comparing the masses of the water,  X, and the mixture of water and X

D Measuring the electrical conductivities of X and the mixture of water and X

Answer 1

Hi, Jan,

None of the options provide conclusive proof that a chemical change occurred, in my opinion. But the question reads "Which of the following identifies the best way to gather evidence to support the type of change that occurred . . .". So let's approach it by trying to decide what it might be, before choosing what else we could use to bolster our thinking.

We have a few facts:

1. X has physical properties consistent with those of a salt, such as high melting temperature and crystalline, white appearance.
2. When added to water, water's boiling point rises, suggesting dissolution (water soluble) of an ionic compound.
3. X is compatible with water, but we don't know whether it is simply dispersed (emulsion) or dissolved. Since the solution is clear, it would appear to be dissolved.

Our hypothesis at this point is that X is an inorganic salt. Inorganic, since it has a high melting point. But we'd like more evidence. Option D allows us to check the conductivity of the samples. YES, I say. Gimme a conductivity meter and I'll use that to bolster my hypothesis that X is an inorganic salt. It would dissolve, provide a clear solution with water, and elevate water's boiling point.

I would normally choose "Read the label on the bottle of X," but that option was left off the list. Darn.

Bob