Problem 23 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is the best scientific method to test presence of water in a liquid? (a) taste (b) smell (c) use of litmus paper (d) use of anhydrous copper sulphate.

Step-by-Step Solution

Answer

The best method is (d) use of anhydrous copper(II) sulfate.

Step 1: Understanding the Question

We need to determine which method is most scientifically appropriate for testing the presence of water in a liquid. We are considering the options: taste, smell, use of litmus paper, and use of anhydrous copper(II) sulfate.

Step 2: Analyzing Each Option

- **Taste (a):** This is not a scientific method as it is subjective and not specific to water. - **Smell (b):** Water is odorless, and using smell is not an effective scientific method. - **Use of litmus paper (c):** Litmus paper tests acidity or alkalinity, but doesn't specifically detect water. - **Use of anhydrous copper(II) sulfate (d):** This is a scientific method. Anhydrous copper(II) sulfate turns from white to blue in the presence of water by forming a hydrate.

Step 3: Evaluating the Best Option

Anhydrous copper(II) sulfate changing from white to blue is a qualitative change that directly indicates the presence of water through a chemical reaction, making it the most scientifically reliable method of those listed.

Step 4: Concluding the Analysis

Considering the nature of all options, using anhydrous copper(II) sulfate (option d) provides a clear and specific detection of water through a chemical reaction that is both specific and reliable.

Key Concepts

Chemistry Testing Methods

When it comes to identifying the presence of specific substances in a solution, scientists employ various chemistry testing methods. These methods involve observing chemical reactions or changes that occur when a substance is exposed to different conditions.
For detecting water, appropriate scientific methods should provide reliable and clear results. Without such reliability, results could be deemed inaccurate or lack specificity.
The methods include observing color changes, gas evolution, or precipitation reactions. All of these are aimed at identifying the presence of specific molecules or compounds. In the case of water, a method that involves a chemical change specific to water molecules as the target is preferred.

• Subjective methods like tasting or smelling are not considered reliable because they lack specificity and are not measurable.
• Visual indicators, like the color change in a chemical reaction, provide observable evidence of the process occurring.

Anhydrous Copper(II) Sulfate Reaction

Anhydrous copper(II) sulfate is a popular reagent used specifically for detecting the presence of water due to its unique reaction properties. When anhydrous, it appears as a white powder. However, the presence of water causes a noticeable chemical change.

Anhydrous copper(II) sulfate forms a hydrated compound, changing from its classic white color to a vibrant blue. This color transformation occurs because water molecules interact with the copper ions to form copper(II) sulfate pentahydrate.

• This process not only indicates the presence of water but also provides a visual verification, simplifying identification.
• The color change is reversible; upon removing water, the compound turns back to white, demonstrating its efficacy and reusability.

This reaction makes anhydrous copper(II) sulfate one of the most reliable substances for detecting water, as it provides both qualitative and tangible evidence.

Chemical Detection of Water

The chemical detection of water leverages the principles of chemistry to identify water in a solution. Unlike physical detection methods that may rely on impractical measures, chemical detection provides clear evidence via reactions.

In chemical reactions, specific reagents are used to identify water by forming new compounds or changing characteristics. Anhydrous copper(II) sulfate serves as an excellent example, where water facilitates the chemical transformation essential for detection.

• This method ensures that even small quantities of water can be detected, providing a critical tool for scientists and chemists.
• The resulting changes are not only visible but also confirm the precise presence of water without ambiguity.

By using chemical methods, one attains accuracy and specificity in results, which are indispensable in scientific inquiries and analysis.