Acids and bases in the world
Acid-base chemistry shapes the environment, the body, and industry, from acid rain to ocean chemistry to digestion.
Acid rain
Burning coal and fuels releases sulfur dioxide and nitrogen oxides, which form sulfuric and nitric acids in the atmosphere. Rain with pH below about 5.6 is called acid rain; it damages forests, acidifies lakes, and erodes limestone buildings. Scrubbers and cleaner fuels have greatly reduced it in many countries.
Ocean acidification
The ocean absorbs about a quarter of the carbon dioxide humans emit. Dissolved \(\text{CO}_2\) forms carbonic acid, lowering ocean pH; surface pH has fallen from about 8.2 to about 8.1 since pre-industrial times, roughly a 26% increase in \([\text{H}^+]\). This makes it harder for corals and shellfish to build calcium carbonate shells.
Digestion
The stomach secretes hydrochloric acid, reaching pH about 1.5-3.5, which activates digestive enzymes and kills microbes. The stomach lining is protected by mucus and bicarbonate. Antacids such as calcium carbonate neutralise excess acid; the small intestine then neutralises acid with bicarbonate from the pancreas.
Acids and bases in the home
Vinegar (acetic acid) dissolves limescale; baking soda (sodium bicarbonate) is a mild base that reacts with acids to produce \(\text{CO}_2\), making cakes rise. Drain cleaners often contain strong bases like sodium hydroxide that break down fats and hair. Mixing household chemicals can be dangerous: bleach and acids release toxic chlorine gas.
Industrial chemistry
Sulfuric acid is the world's most-produced industrial chemical, used to make fertilisers, refine metals, and manufacture batteries. Ammonia, a base made by the Haber process, feeds the fertiliser industry that supports a large share of the world's food production.
Soil chemistry
Soil pH affects which nutrients plants can absorb. Most crops prefer pH 6-7. Farmers add lime (calcium carbonate) to raise the pH of acidic soils. Hydrangea flowers are blue in acidic soil and pink in alkaline soil, a garden-scale indicator.
Worked example
Lake water drops from pH 7 to pH 5 because of acid rain. The hydrogen ion concentration rose from \(10^{-7}\) to \(10^{-5}\) M, a factor of 100. Many fish cannot reproduce below about pH 5, showing how a seemingly small pH change can matter greatly.
Safety
Strong acids and bases cause chemical burns; always add acid to water, not water to acid, to avoid violent boiling. Wear eye protection. In case of skin contact, rinse with plenty of water.
Key takeaways
Acid rain comes from sulfur and nitrogen oxides; ocean acidification from dissolved \(\text{CO}_2\). Stomach acid aids digestion and antacids neutralise it. Household and industrial chemistry relies on acids and bases. Because pH is logarithmic, small pH changes mean large concentration changes.
Flashcard
Rain below about pH 5.6
Check your understanding
Ocean acidification is caused mainly by: