The pH Scale

The pH scale is a number line from 0 to 14 that tells you how acidic or basic (alkaline) a solution is. Low numbers are acidic. High numbers are basic. Right in the middle, at pH 7, is neutral. Pure water sits at exactly 7. Lemon juice is around 2. Bleach is around 12. The pH scale is one of the most useful tools in chemistry because it lets you compare the acidity of any two liquids with a single number.

  • Range0 to 140 strongest acid, 14 strongest base
  • NeutralpH 7Pure water
  • AcidicBelow 7Lemon, vinegar, stomach acid
  • Basic (alkaline)Above 7Soap, bleach, drain cleaner
  • Each step10x strongerpH 4 is 10x more acidic than pH 5
  • Healthy blood pHAround 7.4Tightly controlled by the body

How the scale works

The pH scale measures the concentration of hydrogen ions (H+) in a solution. The more H+ ions, the more acidic. The fewer H+ ions, the more basic.

The "p" in pH stands for "power" or potential. The "H" stands for hydrogen. Together, pH means "power of hydrogen".

The trick to understanding the pH scale is that it is logarithmic: each step on the scale represents a 10-fold change. That means:

  • A solution with pH 5 is 10 times more acidic than one with pH 6.
  • A solution with pH 4 is 100 times more acidic than one with pH 6.
  • A solution with pH 1 is 1 million times more acidic than pH 7.

So even a small change in pH means a huge change in how acidic or basic something is.

The whole scale at a glance

  • pH 0: battery acid, very concentrated sulfuric acid
  • pH 1: stomach acid
  • pH 2: lemon juice, vinegar, cola
  • pH 3: orange juice, wine, beer
  • pH 4: tomato juice, apple juice, acid rain
  • pH 5: black coffee, rainwater
  • pH 6: urine, saliva, milk
  • pH 7: pure water (neutral)
  • pH 7.4: human blood
  • pH 8: sea water
  • pH 9: baking soda
  • pH 10: milk of magnesia
  • pH 11: ammonia cleaner
  • pH 12: bleach, soapy water
  • pH 13: oven cleaner
  • pH 14: drain cleaner
Fact Your stomach acid is around pH 1.5, which is more acidic than vinegar. Yet you do not feel it because a thick mucus layer protects your stomach lining. The acid is strong enough to dissolve steel razor blades over several days (yes, scientists have tested that). It is so strong because it needs to break down food and kill the bacteria that come in with your meals.

Measuring pH

Scientists measure pH in three main ways:

  • Litmus paper: a simple paper strip that turns red in acid and blue in base. Quick but only tells you "acid or base", not the exact number.
  • Universal indicator paper: a special paper that turns a range of colours, from red (acid) through green (neutral) to blue/purple (base). You compare the colour to a chart to read the pH to within 1 unit.
  • pH meter: an electronic probe that gives a precise pH reading on a screen. Used in labs, swimming pools, factories and farms.
Did you know? The acid in cola can dissolve a tooth in about 48 hours. The phosphoric and carbonic acids in fizzy drinks (along with the sugar that feeds tooth bacteria) are one of the main reasons dentists worry about how much fizzy drink children consume. Drinking through a straw keeps the acid away from your teeth a little, but the only real protection is brushing afterwards.

Why pH matters

Almost every living and chemical process is sensitive to pH:

  • Your blood: must stay at pH 7.35 to 7.45. Outside that, enzymes stop working and you become seriously ill.
  • Soil: different plants thrive at different pH levels. Blueberries like acidic soil (pH 4.5 to 5.5). Lavender prefers slightly basic soil (pH 7 to 8). Farmers test soil pH before deciding what to grow.
  • Swimming pools: pH is kept around 7.4. Too low and the water stings eyes and corrodes pipes. Too high and chlorine stops killing germs.
  • Aquariums: tropical fish often need a very specific pH. The wrong pH can kill them in days.
  • Brewing and baking: yeast works best at a specific pH. Brewers and bakers test and adjust the pH of their ingredients carefully.
  • Shampoo: usually pH 5 to 6 to match your hair and scalp. Wrong pH leaves hair frizzy or greasy.

How pH changes

You can change a solution pH by adding more acid or more base. Add lemon juice to milk and the pH drops (the milk curdles). Add baking soda to vinegar and the pH rises (a foaming reaction). When acids and bases meet, they cancel each other out in a neutralisation reaction, ending up closer to pH 7.

Try this Make rainbow pH testing at home. Boil red cabbage in water until the water turns deep purple. Strain and cool. Pour the purple liquid into several clear glasses. Add different substances to each: lemon juice, vinegar, baking soda, soap solution, fizzy water, milk, antacid tablet dissolved in water. The cabbage juice changes colour based on the pH of whatever you add: pink in strong acids, purple at neutral, blue/green in bases. Line them up and you have a rainbow that maps to the pH scale.
Deeper dive: ocean acidification and the disappearing reefs

The worlds oceans have an average pH of about 8.1, which makes them slightly basic. They have been at roughly this pH for millions of years. The animals and plants of the ocean (including corals, shellfish, plankton and many fish) are perfectly adapted to live at this pH.

In the last 200 years, however, ocean pH has been dropping. The cause is carbon dioxide. As humans burn fossil fuels and release CO2 into the atmosphere, the ocean absorbs a large fraction of it. When CO2 dissolves in water it forms carbonic acid, which lowers the oceans pH. Since 1750, the average ocean pH has dropped from about 8.2 to about 8.1: a change of only 0.1 units, but a logarithmic one, meaning the oceans are now 30 per cent more acidic than they were before the Industrial Revolution.

This is bad news for any sea creature that builds a shell or skeleton from calcium carbonate (such as corals, oysters, mussels, scallops, sea urchins and many tiny plankton). Calcium carbonate dissolves in acid. The lower the ocean pH, the harder it becomes for these animals to build and keep their shells.

Coral reefs are especially vulnerable. Acidification, combined with rising sea temperatures (which causes coral bleaching), is killing reefs around the world. Australia Great Barrier Reef has lost about half of its corals since 1995. Reefs across the Caribbean, Indian Ocean and Pacific have suffered similar losses.

This matters because coral reefs are some of the most important ecosystems on Earth. A quarter of all marine species depend on them at some point in their lives. The only long-term solution is reducing CO2 emissions and giving the oceans time to recover.

For more, see neutralisation and what is an acid.