Tuesday

Raven draws a long line on the Bench paper and numbers it 0 to 14. "The pH scale," she says. "Seven is the middle."

Rocket grabs the pencil. "Vinegar goes on the low end, right? Low is acid."

"And rain?" Raven asks. "What do you notice? Rain is just water, so it should be at seven."

Nova projects a number onto the paper: 5.6. "Normal rain is slightly acidic," she says. "Something in the air dissolves into it. Can you guess what?"

"Carbon dioxide!" Rocket says. "The same gas from the bag."

Raven marks 5.6 on the line. "So the scale is a ruler for how acidic or basic water is."

"And every step is ten times," Nova adds. "Think about what that means for pH 4 and pH 6."

pH is a measure of how acidic or basic water is. The range goes from 0 to 14, with 7 being neutral.

A pH of less than 7 indicates acidity. A pH of greater than 7 indicates a base.

pH is reported in logarithmic units. Each number represents a ten-fold change in the acidity or basicness of the water.

Water with a pH of 5 is ten times more acidic than water with a pH of 6.

The letters pH stand for the power of the hydrogen ion. A Danish scientist named Sørensen proposed the scale.

Normal rainfall has a pH of about 5.6. It is slightly acidic because carbon dioxide gas from the atmosphere dissolves into it.

Carbon dioxide and water form a weak acid called carbonic acid.

Acid rain is different. It usually has a pH between 4.2 and 4.4, because other gases from burning fuels dissolve in the rain too.

Those numbers come from the U.S. Geological Survey and the Environmental Protection Agency.

Nice measuring. Tomorrow is Bench Lab: a red cabbage becomes your pH indicator.