Determining if pH Increases as Standing Rainwater Evaporates

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Updated on Mar 02, 2010

The purpose is to determine if rainwater becomes more acidic as it evaporates and concentrates the contents that are left. Does less water make more acidity?

One environmental concern today centers on the damaging effects of acid rain. Acid rain is a term used to describe precipitation (rain, snow, hail, sleet, and fog) that has a low pH. The amount of alkalinity or acidity in a liquid is measured on a scale called the pH scale. The scale goes from zero to 14, with zero being the strongest acids, 7 neutral, and 14 the most alkaline.

Because the Earth's environment has natural sources of sulfur and nitrogen, it is normal for rainwater to be slightly on the acidic side, having a pH of 5. But when the pH drops to 4, scientists consider this acid rain.

Scientists theorize that acid rain is caused by chemicals in the atmosphere, including sulfur dioxide (produced by industries burning oil and coal) and nitrogen oxide (which comes from automobile exhaust). Winds can carry these airborne chemicals thousands of miles. Acid rain has been discovered in many areas around the world.

Acid rain attacks metal and stone structures, and over a period of time can damage them. Most importantly, it can fall into ponds, lakes, and other bodies of water, where it can make the water too acidic for the animal and plant life to survive.

Does the acid in rainwater become even more concentrated as the water evaporates, thus worsening the effects of acid rain?

Hypothesize that as rainwater evaporates, the concentration of acidity increases (the pH of the remaining water decreases). Or, hypothesize the opposite to be true.

  • 6- or 8-ounce jar
  • Large funnel
  • Rainy day
  • Large, shallow dinner dish
  • Sunny window
  • Litmus paper pH test kit with color comparator chart

Using a 6- or 8-ounce jar or a drinking glass, collect about 5 or 6 ounces of rainwater on a rainy day. Place a large funnel in the top to increase the area of rainwater collection. Do not go outside to collect rainwater when lightning is present!

If it doesn't rain enough to fill your jar, put a lid on the jar or cover it with a tight-sealing piece of plastic wrap to prevent evaporation. Then, the next time it rains, set out your collection jar and funnel again.

Indoors, place a large, shallow dinner dish in an undisturbed area that, during sunny days, receives a lot of warm sunlight. Pour as much rainwater into the dish as it will hold.

Using a pH test kit, determine the level of acidity on the pH scale. Once a day, test the pH level, and then write down the date and the pH number. Continue testing daily until all the water has evaporated.

Has the water become measurably more acidic as it evaporated?

You may want to enhance your project by also testing the pH of small areas in your neighborhood daily, where rainwater collects: mud puddles, birdbaths, swales, depressions at the base of downspouts, and similar places.

Write down the results of your experiment. Document all observations and data collected.

Come to a conclusion as to whether or not your hypothesis was correct.

Something More

Acid rain can destroy plant and animal life living in bodies of water. Start a log of daily pH measurements of nearby lakes and ponds. In your daily log, include information on rainfall that occurs (the date and the inches of rain).