What if there were no acids?

A lemon tastes sour. Vinegar tastes sour too. That sour taste comes from acids.

What if there were no acids anywhere?

Bite into a lemon and your face scrunches up. Sniff vinegar and it tingles your nose. Both are sour because they contain acids. The word acid can sound scary, but most acids we meet are mild and many are part of food and of our own bodies.

So what if there were no acids at all? No sour lemons, no vinegar for pickles, and, as we'll see, no acid in your stomach to help digest your lunch. In this lab we follow three clues: lemons and vinegar, stomach acid, and the pH scale.

Chemists noticed the properties of acids long before they understood them. OpenStax notes that by the 18th century acids were known to taste sour, to react with limestone releasing a gas (now known to be carbon dioxide), and to combine with alkalis to form neutral substances. They also change the colors of certain natural dyes.

This thought experiment imagines acids removed from the world. It follows three threads: the weak acids in everyday food, the strong acid our stomachs make, and the pH scale chemists use to measure acidity. Established chemistry and reasoned guesses are labeled separately.

Lemons, oranges and grapefruit have an acid called citric acid.

Vinegar has an acid called acetic acid. Both make food taste sour.

Citrus fruits such as lemons, oranges and grapefruit contain citric acid, and vinegar contains acetic acid. Both are weak acids, which means only a small share of their molecules break apart in water. Weak acids are partly responsible for the tangy taste of citrus fruit.

Vinegar is made by fermentation: first sugars become alcohol, then other microbes turn the alcohol into acetic acid. In the United States, the FDA's guidance says vinegar should contain at least 4 grams of acetic acid in every 100 milliliters.

OpenStax describes citrus fruits such as oranges, lemons and grapefruit as containing the weak acid citric acid, and food vinegars as containing the weak acid acetic acid, which provides their sour taste. A weak acid ionizes only partly in water, producing relatively few hydronium ions.

The FDA's compliance policy guide on vinegar defines cider vinegar as the product of alcoholic and then acetous fermentation of apple juice, and says vinegars should contain 4 grams of acetic acid per 100 mL. Rain is slightly acidic too: the U.S. Geological Survey gives normal rainfall a pH of about 5.6, because of carbon dioxide from the air.

Imagine all the acids are gone. Lemons would not taste sour.

Your tummy would have no acid to help break down food. That would be a big problem!

Now imagine there were no acids. Lemons and vinegar would lose their sour taste, and pickles could not be made the usual way. Those are the small changes.

The big changes would be inside us. Your stomach makes a strong acid to help digest food and to kill many of the germs you swallow. Without it, digesting protein would be much harder. And some of the most important molecules in your body have 'acid' in their names: amino acids build proteins, and DNA stands for deoxyribonucleic acid. A world with no acids at all might not be able to have life like ours.

Remove acids and the visible losses come first: no sour flavors, no vinegar, no acid rain, but also no naturally slightly acidic rain. Next comes digestion: the stomach's hydrochloric acid is needed to activate the protein-digesting enzyme pepsin, and it kills much of the bacteria ingested with food.

Deeper still, biochemistry is built on acid groups. The National Human Genome Research Institute describes amino acids as the building blocks of proteins and DNA, deoxyribonucleic acid, as the molecule that carries genetic information. A truly acid-free universe would therefore be one where life as we know it might be impossible; that conclusion is reasoned, not observed.

Scientists use numbers from 0 to 14 to measure acids. This is called pH.

Seven is in the middle. Lower numbers are more acidic.

Chemists measure acidity with the pH scale. The U.S. Geological Survey explains that it runs from 0 to 14, with 7 as neutral, like pure water. Numbers below 7 are acidic and numbers above 7 are basic.

Each step on the scale is a big jump: water with a pH of 5 is ten times more acidic than water with a pH of 6. The scale was introduced by the Danish biochemist S. P. L. Sørensen in 1909. Even pure water contains a tiny amount of hydrogen ions, which is why 'no acid' is harder to imagine than it sounds.

pH measures the relative amounts of free hydrogen ions (H⁺) and hydroxide ions (OH⁻); USGS describes the usual range as 0 to 14 with 7 neutral, and each unit as a tenfold change. The concept was introduced by S. P. L. Sørensen in two papers published in 1909.

Even pure water is not free of hydrogen ions: a tiny fraction of its molecules split into H⁺ and OH⁻. OpenStax's worked examples show how strong these effects are: a vinegar with a hydronium concentration of about 4 × 10⁻³ M has a pH of about 2.4. Because the scale is logarithmic, USGS notes that mine drainage at pH 2 is 100,000 times more acidic than neutral water.

Your stomach makes acid to help break down food.

A slippery layer of mucus keeps the acid from hurting your stomach.

Special cells in the lining of your stomach, called parietal cells, make hydrochloric acid. It makes the stomach very acidic, with a pH between about 1.5 and 3.5. The acid switches on an enzyme called pepsin that breaks down proteins, and it kills much of the bacteria you swallow with food.

Why doesn't the acid hurt your stomach? Cells lining the stomach make a protective coat of mucus, which is alkaline: the opposite of acidic. And when the acid gets too strong, the stomach slows down making it.

According to OpenStax Anatomy and Physiology, parietal cells in the gastric glands secrete hydrochloric acid, which is responsible for the high acidity (pH 1.5 to 3.5) of the stomach contents. HCl converts the inactive pepsinogen into the protein-digesting enzyme pepsin, kills much of the ingested bacteria and denatures proteins, making them easier to digest.

The stomach protects itself: surface mucus cells secrete a coat of alkaline mucus, and when pH drops too low, cells suspend HCl secretion and increase mucus production. A world without acids would remove this whole system, a reminder that acids are not simply dangerous chemicals but part of how bodies work.