Raven tapes a printed table beside the pH line on the Bench. Six liquids, each with a number.
"Stomach acid is near 3," Rocket reads. "Grapefruit juice too. What do you notice, Raven?"
"Acid rain sits above them at about 4.3," she says. "And normal rain at 5.6. Every one of these is below 7."
Nova hovers at the right end of the line. "And where do your green cups go?" she asks.
"Above seven," Rocket says. "Nothing on this list is a base. Our soap water is the only one up here."
Raven marks each one on the line with a small triangle. "Now we can put our cabbage cups between them."
"Vinegar made the juice as red as the grapefruit would," Rocket says. "So it belongs down there."
The table below collects pH values from three references. They are the U.S. Geological Survey Water Science School, the Environmental Protection Agency, and a university chemistry text.
Every value is for a watery liquid at about 25 degrees Celsius. Remember that lower means more acidic, and each step is ten times.
| Liquid | pH (about) | Acid, neutral or base? | Reference |
|---|---|---|---|
| Stomach acid | 2.9 | Acid | University chemistry text |
| Grapefruit juice | 2.9 to 3.3 | Acid | University chemistry text |
| Acid rain | 4.2 to 4.4 | Acid | Environmental Protection Agency |
| Normal rain | 5.6 | Acid (slightly) | U.S. Geological Survey |
| Water with air dissolved in it | 5.7 | Acid (slightly) | University chemistry text |
| Pure water | 7 | Neutral | U.S. Geological Survey |
Grapefruit juice contains carbon-based acids, with citric acid as one of the main ones. That is the same acid as in lemon juice.
Water that has been exposed to air holds dissolved carbon dioxide, which forms carbonic acid. That is why it is a little below 7.
No base is listed, because the references opened for this week give exact numbers only for acids and pure water.
| Statement | True or false? |
|---|---|
| Every liquid in the table below pH 7 is an acid. | ? |
| Normal rain is slightly acidic because of carbon dioxide. | ? |
| Grapefruit juice contains citric acid. | ? |
| Stomach acid is more acidic than acid rain. | ? |
| Pure water is an acid. | ? |
Your cabbage cups gave colors, not numbers. But you can place them roughly. Vinegar and lemon juice turned red, so they sit below 7, near the grapefruit juice.
Baking soda water and soap water turned green, so they sit above 7. Plain water sits at or near 7.
An indicator gives a range. A pH meter or pH paper gives a number. Both use the same scale.
Excellent reading. Tomorrow you will find acids and bases around the home and review the week.