Regardless of what you are taking a sample of water for, it is important to determine whether or not that sample is pure or whether it has been mixed with some other materials. There are a number of different ways that you can test a sample of water to determine whether or not the sample is pure or mixed, but measuring the sample's pH balance is probably the cheapest and most simple test to perform.
Remove a small amount of the water you wish to sample from its source. The less water there is to test, the more accurate the reading will be.
Dip the pH strip into the isolated water sample you wish to test for. After a moment, the strip should change color.
Remove the strip from the sample and observe the color on the end of the strip. Refer to the color chart that came with your pH testing kit. Pure water has a pH level of 7. If the color on your test strip doesn't match the color for a neutral pH level of 7, your water sample isn't pure and contains foreign materials.