Detecting Water: Understanding the Color Change Test

This question explores a common chemical test used to detect the presence of water. The key information provided is that a scientist starts with a white substance, adds a sample to it, and observes a color change to blue. We need to identify which chemical substance would exhibit this specific behavior upon contact with water.

The Principle of Water Detection via Color Change

Certain chemical compounds have the ability to absorb water molecules from their surroundings. These substances are often referred to as desiccants or drying agents. Some of these compounds also exhibit a distinct color change when they become hydrated (i.e., when they bind with water molecules). This color change serves as a visual indicator that water is present in the sample being tested.

Analyzing the Observation: White to Blue Transition

The scientist's observation is crucial:

• The initial substance used for the test was white.
• Upon adding the sample, the substance turned blue.

This implies that the substance reacted with water in the sample, transforming from its white, anhydrous (water-free) state into a blue, hydrated form.

Evaluating the Potential Substances

Let's examine each option to see which one fits the observed white-to-blue color change:

1. Hydrated copper(II) sulfate

Hydrated copper(II) sulfate, typically copper(II) sulfate pentahydrate ($CuSO_4 \cdot 5H_2O$), is known for its vibrant blue color. Since the substance in the test started as white, hydrated copper(II) sulfate is not the correct answer, as it would already be blue.

2. Anhydrous copper(II) sulfate

Anhydrous copper(II) sulfate ($CuSO_4$) is a white or slightly off-white crystalline powder. It is well-known for its strong affinity for water. When it absorbs water, it undergoes a chemical reaction to form copper(II) sulfate pentahydrate ($CuSO_4 \cdot 5H_2O$), which is distinctly blue.

The reaction can be represented as:

$$ CuSO_4 \text{ (white)} + 5H_2O \rightarrow CuSO_4 \cdot 5H_2O \text{ (blue)} $$

This reaction perfectly matches the observation described in the question: a white substance turning blue in the presence of water.

3. Anhydrous cobalt(II) chloride

Anhydrous cobalt(II) chloride ($CoCl_2$) is typically blue. When it absorbs water, it forms hydrated cobalt(II) chloride ($CoCl_2 \cdot 6H_2O$), which is pink. The common color change associated with cobalt(II) chloride as a water indicator is blue to pink, not white to blue. Therefore, this option does not fit the observation.

4. Anhydrous calcium chloride

Anhydrous calcium chloride ($CaCl_2$) is indeed a white solid and a powerful desiccant, meaning it effectively absorbs water. However, it does not produce a characteristic color change upon hydration. Its primary function is to remove water, but it doesn't signal its presence with a color change like copper(II) sulfate.

Conclusion: Identifying the Correct Substance

Based on the analysis, anhydrous copper(II) sulfate is the substance that starts as white and turns blue upon absorbing water. This specific color change is a classic indicator test for the presence of water.