Acids are an important group of chemicals. All acids contain chemicals that are combined with hydrogen in a specific way. Acids are present in aspirin, carbonated drinks, and your stomach.

Another group of chemicals is called bases. They contain oxygen and hydrogen that are combined with other chemicals. Chemicals containing bases are called alkaline. Examples of alkaline substances are deodorants, ammonia cleaners, and the mortar around bricks.

Liquids are sometimes measured in terms of acidity or alkalinity. A scale called the pH scale is used for this measurement. The scale ranges from zero to 14. Anything with a pH below 7 is considered acidic. The lower the pH, the more acidic it is. Substances above 7 are alkaline. The higher the pH, the more alkaline it is. Liquids that are very acidic or alkaline are dangerous to handle. A substance that is neither acidic or alkaline would have a pH of 7. Absolutely pure water would have a pH of 7.

Normal rainwater has a pH of around 5.6. This is because some carbon dioxide in the atmosphere combines with atmospheric water to form a weak acid. The term acid rain is used when rainwater becomes more acidic.

Acid precipitation occurs when water (rain, snow, sleet and fog) combines with pollutants containing sulfur or nitrogen. This may bring the pH down to between 4 and 5.

Many laws and regulations have been created in United States and other countries to cut down the amount of sulfur and nitrogen that is released into the air by human activity. This seems to already be helping to raise the pH of precipitation back to normal. An Illinois State Water Survey study has shown that the amount of sulfur in precipitation has decreased the last few years over Illinois and much of the Northeastern United States.