Common Acids in Everyday Life

You probably touch, taste or smell acids dozens of times a day without noticing. The acids in your kitchen, bathroom, car and body do everything from giving fruit its zing to powering your phone to digesting your lunch. Some are weak and harmless (great in cooking). Others are dangerously strong (only for industry and labs). Here is a tour of the most important acids in everyday life and what each one does.

  • In your stomachHydrochloric acidpH 1.5, digests food
  • In lemonsCitric acidSour, found in all citrus fruit
  • In vinegarAcetic acidAround 5% acid, 95% water
  • In fizzy drinksCarbonic + phosphoricSource of the bubbles and tang
  • In yoghurtLactic acidMade by bacteria from milk
  • In ant stingsFormic acidNamed after ants (Latin formica)

Citric acid

Citric acid is found in lemons, limes, oranges, grapefruit and many other fruits. It is what makes citrus fruit taste so sharp. Pure citric acid is a white crystal powder; lemons contain about 5 per cent of it by weight.

It is also added to many processed foods and drinks as a flavouring and preservative (E330 on labels). It stops food going off by lowering the pH so bacteria struggle to grow. You will find it in jams, sweets, ice creams, fizzy drinks and even cleaning products.

Acetic acid (vinegar)

Acetic acid is the acid in vinegar. Vinegar is roughly 5 to 8 per cent acetic acid dissolved in water, made by letting bacteria turn the alcohol in wine, cider or other drinks into acid.

Vinegar has been used in cooking, food preservation and cleaning for thousands of years. You can use it to:

  • Pickle vegetables (the acid stops them rotting)
  • Make salad dressing
  • Clean windows and limescale
  • Calm a wasp sting (wasp venom is basic, so vinegar neutralises it)

Hydrochloric acid (in your stomach)

Hydrochloric acid (HCl) is a strong acid produced by special cells in your stomach lining. It keeps your stomach at a pH of about 1.5 to 3, more acidic than vinegar.

It serves three important roles:

  • Breaking down food into smaller pieces ready for digestion
  • Killing bacteria and viruses that come in with food
  • Activating digestive enzymes like pepsin, which start breaking down protein

Your stomach makes about 1.5 litres of acid every day. A thick layer of mucus stops the acid from eating through your stomach lining.

Fact If your stomach makes too much acid (often from stress, spicy food or some medicines) you might feel heartburn or indigestion as the acid sometimes splashes up into your food pipe. Indigestion tablets contain a base (often calcium carbonate or magnesium hydroxide) that neutralises some of the stomach acid and brings relief.

Carbonic acid (in fizzy drinks)

When carbon dioxide gas is dissolved in water under pressure, some of it reacts to form carbonic acid (H2CO3). This is what makes fizzy drinks, sparkling water and beer taste tangy and acidic. As soon as you open the can the pressure drops and most of the CO2 fizzes back out, which is why a flat fizzy drink tastes sweeter and less sharp than a fresh one.

Carbonic acid also forms in nature when CO2 from the air dissolves in rainwater, giving normal rain a pH of around 5.5 (slightly acidic). Over millions of years, this carbonic acid slowly dissolves limestone, carving out caves and gorges.

Sulfuric acid (in car batteries)

Sulfuric acid (H2SO4) is one of the most important industrial chemicals in the world. More sulfuric acid is made each year than any other man-made acid. It is used to:

  • Make car batteries (the liquid inside lead-acid batteries is dilute sulfuric acid)
  • Make fertilisers (over half of all the sulfuric acid made goes to fertiliser production)
  • Refine petrol and other fuels
  • Make detergents, dyes and many other chemicals

Concentrated sulfuric acid is extremely dangerous. It can dehydrate organic materials like wood and skin almost instantly, leaving them charred and black. It is strictly an industrial chemical, handled only by trained workers.

Did you know? A countries economic strength used to be measured by how much sulfuric acid it produced, because so many industries depend on it. The modern world still relies on around 270 million tonnes of sulfuric acid being made every year. Most of it ends up in fertilisers that grow the food the world eats.

Other acids you meet often

  • Lactic acid: forms in your muscles during hard exercise (the burning feeling), and in yoghurt as bacteria turn milk sugar into acid.
  • Formic acid: in ant stings (Latin formica = ant), nettle stings, and as a preservative in some animal feeds.
  • Tannic acid: gives the bitter "tang" to strong tea, red wine and unripe fruit.
  • Ascorbic acid (vitamin C): in oranges, peppers, broccoli and many other fruits and vegetables. Your body needs it to keep your skin, gums and immune system healthy.
  • Folic acid: a B vitamin found in leafy greens and beans, important for healthy red blood cells and developing babies.
  • Amino acids: the building blocks of proteins. Your body needs 20 different amino acids, 9 of which you must get from food.
  • Fatty acids: the building blocks of fats and oils, including the healthy omega-3 fatty acids in fish and seeds.
  • Phosphoric acid: in cola and many fizzy drinks, giving the sharp tang.
  • Salicylic acid: found in willow bark, used in skin treatments and as the basis for aspirin.
Try this Read the ingredient labels on five food or drink items in your kitchen. Look for any word ending in "-ic acid" (like citric acid, lactic acid, ascorbic acid, malic acid). You will probably find at least one in almost every processed food. Then look at the pH listed on a bottle of shampoo or skincare. Now you know what those numbers mean and what kind of chemistry your body has been in contact with all day.
Deeper dive: why does aspirin work?

Aspirin is one of the worlds most popular medicines and it is essentially an acid: acetylsalicylic acid. Its history is fascinating.

For thousands of years people knew that chewing willow bark or making tea from its leaves could ease pain and bring down fevers. Ancient Egyptian medical papyri from 1550 BC mention it, as did the Greek physician Hippocrates 2,400 years ago. They did not know why it worked, but it did.

In the 1820s, scientists managed to extract the active chemical from willow bark and called it salicin. In the body, salicin turns into salicylic acid, which is the real painkiller. But salicylic acid in its pure form irritated peoples stomachs, sometimes badly.

In 1897, German chemist Felix Hoffmann (working at the Bayer company) found a way to add an acetyl group to salicylic acid, making acetylsalicylic acid. The new chemical was easier on the stomach and just as good at relieving pain. Bayer named it Aspirin and started selling it in 1899. Within a few years it had become one of the most successful medicines ever made.

We now know aspirin works by blocking enzymes called cyclooxygenases, which would otherwise produce molecules that cause pain, fever and inflammation. Low doses of aspirin can also thin the blood, which is why doctors sometimes prescribe daily aspirin to reduce the risk of heart attacks and strokes. Not bad for a chemical first discovered in tree bark.

For more, see what is an acid and common bases in everyday life.