To confirm that a colourless liquid is pure water, you can perform the following tests:

1. Measure the boiling point of the liquid. Pure water boils at 100 degrees Celsius (°C) under standard atmospheric pressure (1 atmosphere). If the liquid boils exactly at 100°C, it indicates purity.
2. Measure the freezing point of the liquid. Pure water freezes at 0 degrees Celsius (°C) under standard atmospheric pressure. If the liquid freezes exactly at 0°C, it also indicates purity.
3. Conduct a pH test. Pure water is neutral with a pH of 7. Using litmus paper or a pH meter, if the liquid shows a neutral pH, it supports that it is pure water.
4. Look for the absence of suspended particles or sediments by observing the liquid in a transparent container and letting it stand to see if any sediment forms.

The boiling and freezing point tests are the most reliable physical tests for confirming pure water because pure substances have fixed boiling and freezing points. The pH test is a useful chemical test to confirm the neutrality of the liquid. Combining these tests can help ensure that the colourless liquid given is indeed pure water.