What Is a Base?

A base is a chemical that releases hydroxide ions (OH-) when dissolved in water, or that accepts hydrogen ions (H+) from an acid. Bases are the chemical opposite of acids. They taste bitter, feel slippery and turn special test papers blue. The word "alkali" means a base that dissolves in water. Bases are in soap, toothpaste, ammonia cleaners, bleach, baking soda and the antacid tablet you might take when your stomach feels sour.

  • What it isReleases OH- ions in waterOr accepts H+ from acids
  • TasteBitterSoap, coffee, cocoa powder
  • FeelSlipperyLike soap on wet skin
  • pH rangeAbove 7The higher, the stronger
  • Litmus testTurns blueRed paper changes colour
  • Strong household baseBleach (NaClO)Around pH 12

Bases and alkalis

The words base and alkali are often used interchangeably, but there is a small difference:

  • A base is any substance that can neutralise an acid.
  • An alkali is a base that dissolves in water.

So all alkalis are bases, but not all bases are alkalis. Many metal oxides (like rust on iron, or the white powder on a piece of magnesium) are bases but do not dissolve much in water.

How bases work

A base releases hydroxide ions (OH-) when it dissolves in water. The OH- ion is hungry for hydrogen. When it meets an H+ ion (from an acid), they grab each other and form a water molecule:

  • H+ + OH- -> H2O

That is why mixing an acid and a base produces water. The hydrogen and hydroxide ions cancel each other out, leaving behind plain water and a salt. This is called neutralisation.

What bases do

  • Taste bitter: try a tiny lick of coffee grounds or pure cocoa powder. Both are mildly basic.
  • Feel slippery: that soapy feeling between your fingers in the bath is the soap reacting with the natural oils on your skin.
  • Turn litmus paper blue: red litmus turns blue in a base.
  • Neutralise acids: they cancel acids out, producing water and a salt.
  • React with fats and oils: this is how soap is made (called saponification).
Fact The reason your skin feels squeaky-clean after soap is that the soap is breaking down the natural oils on your skin into soap molecules and removing them. Bases do this to fats whenever they touch them. Strong bases like drain cleaner can dissolve hair and grease in the same way, which is why they unblock drains so well.

How strong are bases?

Just like acids, bases come in strong and weak versions.

  • Strong bases: dissolve completely in water and release lots of OH- ions. Examples: sodium hydroxide (NaOH, in drain cleaner), potassium hydroxide (KOH).
  • Weak bases: dissolve only partly and release fewer OH- ions. Examples: ammonia (in some cleaners), baking soda, milk of magnesia (an indigestion medicine).

The pH scale measures how basic a solution is:

  • pH 8-9: mild bases (sea water, baking soda, eggs)
  • pH 10-12: stronger bases (milk of magnesia, ammonia, bleach)
  • pH 13-14: very strong bases (oven cleaner, drain cleaner)
Did you know? Strong bases can be more dangerous than strong acids. A drop of strong acid on your skin produces a sharp pain, telling you to wash it off immediately. A drop of strong base often feels strangely soapy and painless at first, while it is silently dissolving your skin. By the time it hurts, the damage may be severe. That is why labs treat strong bases with even more respect than strong acids.

Common bases around you

  • Soap: usually contains sodium hydroxide or potassium hydroxide.
  • Toothpaste: mildly basic to neutralise the acids left by sugary food and protect tooth enamel.
  • Baking soda: sodium hydrogen carbonate, used in cooking and as an indigestion remedy.
  • Antacid tablets: contain calcium carbonate or magnesium hydroxide, used to neutralise stomach acid.
  • Bleach: usually sodium hypochlorite, kills germs and removes stains.
  • Oven cleaner: strong sodium hydroxide solution that dissolves baked-on grease.
  • Drain cleaner: similar to oven cleaner, dissolves hair and food.
  • Ammonia cleaner: a weak base, cuts through grease and dirt.

Bases in your body

Many of the fluids in your body are slightly basic. Your blood is kept at a very steady pH of around 7.4 (just slightly basic) by clever chemistry called the buffer system. If your blood pH dropped to 7.0 (neutral) or rose to 7.8, you would be seriously ill. Your body works hard to keep it in that narrow safe range.

Your pancreas releases a basic juice into your small intestine to neutralise the acid coming down from your stomach, ready for the next stage of digestion.

Try this Test the bases in your home with red cabbage indicator (boil purple cabbage and use the juice). Dip a piece of red litmus paper or some red cabbage indicator into solutions of soap, baking soda, bicarbonate of soda, antacid tablets dissolved in water, and dilute bleach. Each one should make the indicator turn blue or green. Then add some lemon juice or vinegar and watch them flip back towards pink. You are watching acid-base chemistry in real time.
Deeper dive: the chemistry of soap

Humans have been making soap for at least 4,500 years. Ancient Babylonian clay tablets describe boiling fats with wood ash to make a cleansing paste, and the Egyptians, Greeks and Romans all used various soap-like substances.

The basic recipe has hardly changed. You take an animal fat or plant oil (these are long molecules made of fatty acids attached to glycerol) and mix them with a strong base, traditionally lye (sodium hydroxide) made from wood ash. The base attacks the fatty acid molecules and snips them off the glycerol backbone, producing two things: soap molecules (the sodium salts of fatty acids) and glycerol (a sweet syrupy liquid, useful in skincare).

What makes soap so good at cleaning is the shape of each soap molecule. One end is electrically attracted to water; the other end is attracted to oils and grease. When you wash a dirty dish, soap molecules surround droplets of grease with their oily ends pointing inward and their water-loving ends pointing outward. The grease droplets are now coated in a water-friendly shell and wash away with the rinse water. The same trick lets soap wash dirty oil from your skin, hair and clothes.

Different fats give different soaps. Olive oil gives a mild bar (the original Castile soap). Coconut oil makes a hard, bubbly bar. Beef tallow makes the slippery bars of older recipes. Modern detergents (washing-up liquid, washing powder) work on the same chemistry as soap but use synthetic molecules that work better in hard water.

For more, see what is an acid and neutralisation.