Acids and Bases

Acids and bases are two of the most important groups of chemicals. Acids are sour-tasting substances that release positive hydrogen ions (H+) when dissolved in water. Examples include lemon juice, vinegar and stomach acid. Bases (also called alkalis when they dissolve in water) are bitter-tasting, often slippery, and release negative hydroxide ions (OH-). Examples include baking soda, soap and bleach. The strength of an acid or base is measured on the pH scale, from 0 (very acidic) to 14 (very alkaline), with 7 being neutral (pure water).

  • pH scale0 to 140 = very acidic, 14 = very alkaline
  • NeutralpH 7Pure water
  • Strongest acidHydrofluoric, sulphuricEats through metal and glass
  • Strongest baseLye (sodium hydroxide)Used to make soap
  • Each pH step10x strongerLogarithmic scale
  • Acid + baseNeutralise each otherProducing salt and water

The pH scale

The pH scale measures how acidic or alkaline a substance is. It runs from 0 to 14, with 7 right in the middle being neutral. Lower numbers are more acidic; higher numbers are more alkaline. The scale is logarithmic: each whole number is 10 times stronger than the next.

  • pH 0 to 3: strong acid (battery acid, stomach acid, lemon juice).
  • pH 4 to 6: weak acid (coffee, milk, rain).
  • pH 7: neutral (pure water).
  • pH 8 to 10: weak base (sea water, baking soda).
  • pH 11 to 14: strong base (ammonia, bleach, drain cleaner).

Common acids in everyday life

Acids are everywhere around you.

  • Stomach acid: hydrochloric acid in your stomach, pH around 1.5. Strong enough to dissolve a steel nail in days.
  • Lemon juice and vinegar: weak acids (citric acid and acetic acid).
  • Cola drinks: phosphoric acid plus carbonic acid. pH around 2.5.
  • Rain: slightly acidic (pH 5 to 6) from dissolved carbon dioxide. Polluted rain can be more acidic.
  • Battery acid: concentrated sulphuric acid, pH below 1.

Common bases in everyday life

  • Baking soda: sodium bicarbonate, a weak base (pH around 9).
  • Soap: made by reacting fats with a strong base (lye). Most soaps end up slightly alkaline (pH 9 to 10).
  • Bleach: highly alkaline (pH around 13). Powerful cleaning agent.
  • Ammonia cleaners: alkaline (pH around 11). Used for windows and floors.
  • Toothpaste: slightly alkaline (pH 8 to 9). Neutralises mouth acids that cause tooth decay.
Fact Your stomach acid is so strong (pH about 1.5) that it could dissolve metal. The only reason it does not dissolve your stomach is that the stomach lining produces a thick layer of mucus that is constantly renewed. The mucus protects the stomach wall while the acid breaks down food. If the mucus layer fails (often because of certain bacteria called Helicobacter pylori), the acid can cause painful stomach ulcers.

Neutralisation

When an acid meets a base, they neutralise each other. The H+ ions from the acid combine with the OH- ions from the base to form water (H2O), and the leftover parts form a salt. For example, hydrochloric acid + sodium hydroxide gives water + sodium chloride (table salt). This is one of the most important reactions in chemistry, used in everything from medicines (antacids neutralise excess stomach acid) to food preservation.

Pick a topic below to dig deeper.

What Is an Acid?A chemical that gives away hydrogen ions when it dissolves in water. Acids often taste sour and turn blue litmus paper red.
What Is a Base?A chemical that accepts hydrogen ions. Bases often feel slippery and turn red litmus paper blue. Alkalis are bases that dissolve in water.
The pH ScaleA scale from 0 to 14 measuring how acidic or basic something is. 7 is neutral, lower is acidic, higher is alkaline.
NeutralisationWhat happens when you mix an acid and a base in the right amounts: they cancel each other out and produce a salt plus water.
Common Acids in Everyday LifeVinegar, lemon juice, fizzy drinks, and your stomach all contain acids. Most are weak enough to be safe to taste.
Common Bases in Everyday LifeSoap, baking soda, oven cleaner, and antacid tablets are all bases. Many household bases are strong and need careful handling.