Chemistry of the Environment - Water

Objective: Describe how to test for the purity of water using melting point and boiling point

Introduction

The purity of water is crucial for various applications, from drinking and agriculture to industrial processes. While visual inspection might give some indication, more precise methods are needed to assess its quality. Melting point and boiling point are physical properties that can be used to provide insights into the presence of impurities in water.

Testing for Purity using Melting Point

The melting point of a substance is the temperature at which it transitions from a solid to a liquid. Pure substances have a sharp melting point, meaning they melt over a narrow temperature range. Impurities disrupt the crystal lattice structure, leading to a broader melting range and a lower melting point.

Procedure

1. Obtain a small sample of the water to be tested.
2. Place the water in a small, clean capillary tube.
3. Slowly heat the capillary tube in a melting point apparatus.
4. Carefully observe the temperature at which the water begins to melt and the temperature at which it is completely melted.
5. Record the initial and final temperatures.

Analysis

A pure substance will have a sharp melting point, meaning the temperature change is small over a short temperature interval. The presence of impurities will cause the melting point to be lower and the melting range to be wider.

Testing for Purity using Boiling Point

The boiling point of a substance is the temperature at which it transitions from a liquid to a gas. Similar to melting point, pure substances have a specific boiling point. Impurities will raise the boiling point and broaden the boiling range.

Procedure

1. Obtain a sample of the water to be tested.
2. Place the water in a boiling tube.
3. Set up a boiling water bath or use a heating mantle.
4. Heat the water gently, observing the temperature.
5. Record the temperature at which the water starts to boil and the temperature at which it is completely boiling.

Analysis

Pure water boils at a specific temperature ($100^\circ C$ at standard atmospheric pressure). The presence of impurities will cause the boiling point to be higher than $100^\circ C$ and the boiling range to be wider.

Table Summarizing the Tests

Test	Principle	Expected Result for Pure Water	Effect of Impurities
Melting Point	Sharp melting point indicates purity.	Sharp melting point at $0^\circ C$ (for pure ice).	Lower melting point and wider melting range.
Boiling Point	Boiling at a specific temperature indicates purity.	Boils at $100^\circ C$ (at standard pressure).	Higher boiling point and wider boiling range.

Conclusion

By carefully observing the melting point and boiling point of water, we can gain an understanding of its purity. A sharp melting point and a boiling point close to the expected values indicate a high degree of purity, while a broader range suggests the presence of impurities.