Question

How would you confirm that a colorless liquid given to you is pure water?

Answer 1

Each pure substance has its characteristic melting point and boiling point. Melting points and boiling points are altered if any impurity is present in the substance.
Pure water is the only colorless liquid which does not leave any residue after evaporation.
Boiling point of water is 100 degree Celsius at atmospheric pressure
Melting point of water is 0 degree Celsius at atmospheric pressure

Complete solution step by step:
Water: Water is a colorless and tasteless liquid.
The molecules of water have extensive hydrogen bonds which results in high melting and boiling points.
Boiling point
Boiling point is a temperature at which the liquid is converted into a vapour at atmospheric pressure.
A temperature at which the vapour pressure of the liquid becomes equal to the atmospheric pressure of the liquid’s environment is called the boiling point of that liquid.
Each chemical substance has unique bond energy and structure and therefore each chemical substance has unique boiling point.
Melting Point
At atmospheric pressure, when solid changes to liquid state at particular temperature, this temperature is known as melting point.
It is a unique characteristic property of liquid.

If a colorless and tasteless liquid is given to us, we can measure its boiling point and melting point. If boiling point is 100 degree Celsius and melting point is 0 degree Celsius then it confirms that the liquid is pure water.

Note: Capillary tube is used to determine melting point.
When a substance just melts completely, that temperature indicates melting point.
Boiling point is a temperature at which the liquid is converted into a vapour at atmospheric pressure.