Sour and slippery
Lemon juice tastes sour. Soap feels slippery. These are clues that lemon juice is an acid and soap is a base. Acids and bases are everywhere: in your kitchen, your body, the soil and the ocean.
Never taste or touch unknown chemicals. Strong acids and bases are corrosive and can burn skin and eyes.
Properties
| Property | Acids | Bases |
|---|---|---|
| Taste | Sour | Bitter |
| Feel | May sting | Slippery |
| pH | Below 7 | Above 7 |
| Ions in water | Release H⁺ (hydrogen ions) | Release OH⁻ (hydroxide ions) |
| Litmus paper | Turns blue litmus red | Turns red litmus blue |
| Reacts with metals | Many produce hydrogen gas | Usually not |
| Examples | Vinegar, citrus juice, stomach acid, soda pop | Baking soda, soap, ammonia, drain cleaner |
The pH scale
The pH scale runs from 0 to 14.
- pH below 7: acidic
- pH 7: neutral (pure water)
- pH above 7: basic (also called alkaline)
| pH | Example |
|---|---|
| 0–1 | Battery acid |
| 1.5–3.5 | Stomach acid |
| 2 | Lemon juice |
| 3 | Vinegar |
| 5.6 | Normal rain |
| 7 | Pure water |
| 7.4 | Human blood |
| 8.1 | Seawater |
| 9 | Baking soda solution |
| 11 | Household ammonia |
| 13–14 | Drain cleaner |
The pH scale is logarithmic: each step is a tenfold change. A solution with pH 3 is 10 times more acidic than pH 4, and 100 times more acidic than pH 5.
Indicators
An indicator changes colour depending on pH.
| Indicator | In acid | In base |
|---|---|---|
| Litmus paper | Red | Blue |
| Phenolphthalein | Colourless | Pink |
| Red cabbage juice | Red or pink | Green or yellow |
| Universal indicator | Red–orange | Blue–purple |
You can make your own indicator by boiling chopped red cabbage and straining the purple liquid.
Neutralization
When an acid reacts with a base, they cancel each other out. This is neutralization:
acid + base → salt + water
Example: HCl + NaOH → NaCl + H₂O
The H⁺ from the acid and the OH⁻ from the base join to form water.
Everyday neutralization
- Antacids (bases) neutralize excess stomach acid to relieve heartburn.
- Baking soda on a bee sting (acidic) may ease the pain.
- Lime (calcium carbonate) is added to acidic soil or lakes.
- Toothpaste is slightly basic to neutralize acids made by bacteria on teeth.
Acids, bases and the environment
Acid rain
Burning fossil fuels releases sulfur dioxide and nitrogen oxides. These react with water in the air to form acids, making acid rain (pH below about 5.6). Acid rain damages forests, harms fish in lakes and wears away stone buildings and statues. Pollution controls since the 1980s and 1990s have greatly reduced acid rain in Canada and the United States.
Ocean acidification
The ocean absorbs about a quarter of the CO₂ released by humans. CO₂ forms carbonic acid, lowering ocean pH. Since the Industrial Revolution, surface ocean pH has dropped from about 8.2 to about 8.1, which is about a 30% increase in acidity. This makes it harder for corals, clams and other animals to build shells.
Common mistakes
- Thinking all acids are dangerous. Many foods are acidic. Strength and concentration matter.
- Thinking pH 6 is only a little more acidic than pH 7. It’s 10 times more acidic.
- Thinking neutralization always makes table salt. "Salt" means any ionic compound formed.
Try it yourself
- Make red cabbage indicator. Test vinegar, lemon juice, baking soda water, soapy water and tap water. Record the colours and sort them from most acidic to most basic.
- If a lake has pH 5 and normal rain is pH 5.6, roughly how many times more acidic is the lake than pH 6?
- Write the word equation for vinegar (acetic acid) neutralizing baking soda.
Quick recap
- Acids have pH below 7 and release H⁺; bases have pH above 7 and release OH⁻.
- The pH scale is logarithmic; each step is a tenfold change.
- Indicators change colour to show pH.
- Neutralization: acid + base → salt + water.
- Acid rain and ocean acidification are environmental effects of acids.