Acids, Bases and the pH Scale Explained Simply
Lemon juice tastes sour, soapy water feels slippery, and drain cleaner dissolves the grease stuck in your pipes. Behind these very different observations lies one and the same question: how many hydrogen ions are swimming around in the liquid? The answer fits into a single number - the pH value. It tells you whether a substance is acidic, neutral or basic. And it has one property that surprises a lot of people: it does not count in ordinary steps, it counts in powers of ten.
In everyday life the scale runs from 0 to 14: anything below 7 is acidic, exactly 7 is neutral, anything above 7 is basic (alkaline). The values are guide numbers - coffee, cola and blood vary a little by brand and by person.
So what actually makes a substance acidic? The Danish chemist Johannes Brønsted found a surprisingly simple answer in 1923. An acid, on his definition, is a particle that can hand over a hydrogen ion. Such a hydrogen ion (H+) is nothing but a proton - a hydrogen atom missing its single electron. A base is the opposite: a particle that can take such a proton on. Acidic and basic are therefore not properties a substance owns on its own, but roles in a reaction: one gives, the other takes. Without a partner, nothing happens at all.
A lone proton does not stay lonely in water for even a second. It attaches itself to a water molecule straight away and forms a hydronium ion (H3O+). So when chemists talk about an acidic solution, what they strictly mean is a solution with an unusually large number of hydronium ions. Its counterpart is the hydroxide ion (OH-), which is typical of basic solutions. Dissolve hydrogen chloride in water and you get hydrochloric acid, and with it a surplus of hydronium ions. Dissolve sodium hydroxide and you get caustic soda with a surplus of hydroxide ions.
The astonishing part is that even pure water contains both kinds. Every now and then one water molecule hands a proton to another water molecule. This process is called autoprotolysis: two H2O turn into one H3O+ and one OH-. It happens extremely rarely, though. At 25 degrees Celsius there are only 0.0000001 moles per litre of each kind of ion, that is 10-7 mol/l. Because both are exactly equally common, pure water is neutral - and that single number is where the famous value of 7 comes from.
The pH value is nothing but a convenient way of writing that concentration down. It is the negative base-ten logarithm of the hydronium concentration: pH = -log c(H3O+). Instead of 0.0000001 mol/l you simply write 7; instead of 0.01 mol/l you write 2. The logarithm is also the reason for the most important property of the scale: one step on the pH scale means a factor of ten. A solution at pH 4 contains ten times as many hydronium ions as one at pH 5, and a hundred times as many as one at pH 6.
From pH 1 to pH 7 there are six steps. Each single step divides the number of hydronium ions by ten - taken together that is a factor of one million.
This is why sentences like "cola is twice as acidic as coffee" are simply wrong. Coffee sits at about pH 5, cola at about pH 2.5 - that is two and a half steps and therefore roughly 300 times as many hydronium ions. And stomach acid at pH 1 to 2 is not five or six times as acidic as pure water but about a hundred thousand to a million times. Once you have understood this point, you also understand why even tiny shifts in pH inside the body are dangerous: human blood sits very precisely at pH 7.4, and a lasting deviation of just a few tenths is life-threatening.
How do you measure a pH value in the first place? The simplest way is an indicator, a dye that changes colour depending on the pH. Litmus, for instance, turns red in acid and blue in base. A universal indicator is a mixture of several such dyes: as a test strip or a solution it shows different colours across the whole scale, which you compare against a colour chart - and that is exactly where the colour gradient in the diagram above comes from. If you need more precision you use a pH meter with a glass electrode. It measures a tiny electrical voltage and calculates the pH from it to two decimal places.
When an acid meets a base, the two cancel each other out. The hydronium ions and the hydroxide ions react with one another, and what comes out is perfectly ordinary water. What is left over are the other ions, and together they form a salt. The standard example is hydrochloric acid (HCl) reacting with caustic soda (NaOH): the products are table salt (NaCl) and water. Two corrosive liquids, mixed in the right proportion, turn into salt water. This reaction is called neutralisation. It is used in the lab to make acid residues harmless, in agriculture to lime acidic soils, and in the stomach whenever a heartburn tablet mops up excess stomach acid.
Everybody carries a demonstration of how powerfully small pH values act around in their mouth. Tooth enamel consists almost entirely of hydroxyapatite, a very hard calcium phosphate mineral. It is the hardest substance in the human body, but it has one weakness: it dissolves in acid. If the pH in the mouth drops below about 5.5 - the so-called critical pH - calcium and phosphate ions are pulled out of the enamel. Cola at pH 2.5 is far below that, lemon juice at pH 2 lower still. On top of that come the caries bacteria in dental plaque: they feed on sugar and release acids of their own. Saliva normally balances this out again within half an hour and deposits minerals back into the enamel. That is exactly why dentists recommend waiting a little after an acidic meal before brushing - otherwise you simply scrub the softened enamel away.
💬 Mia asks Grandpa Theo
Why does the pH scale stop at 0 and at 14 of all numbers?
Strictly speaking it does not stop there at all. The scale is a calculation, not a ruler with an end stop. Very concentrated acids can have negative pH values and very concentrated alkalis values above 14. It is just that such solutions practically never turn up in everyday life or in a school lab - which is why people settled on 0 to 14 as a handy range.
If stomach acid sits at pH 1, why does it not dissolve the stomach itself?
Because the stomach protects itself. Its inner wall is coated with a thick layer of mucus, and the cells release hydrogen carbonate into that mucus - a base that neutralises arriving acid immediately. Right at the cell wall the pH is therefore around 7, even though one millimetre further in it is 1 to 2. On top of that the stomach lining renews itself constantly. When that protection fails, you get what is called a stomach ulcer.
Does tap water have exactly pH 7?
No, only chemically pure water has exactly 7. Tap water contains dissolved minerals, above all hydrogen carbonate from limestone, and therefore usually sits between 7 and 8.5, so it is slightly basic. Distilled water left standing in the open goes the other way and turns slightly acidic over time: it takes up carbon dioxide from the air, that forms carbonic acid, and the pH slides down to about 5.5. A clean pH of 7 is more of an ideal than an everyday state.
The best known home-made indicator is sitting in the vegetable aisle. Red cabbage contains dyes from the anthocyanin family, and these dyes change colour according to pH. Chop a few cabbage leaves, boil them in water, let the brew cool and pour it off. Share the purple juice out between several glasses and add something different to each one, and you get a whole fan of colours: with lemon juice or vinegar it turns a strong red, with plain water it stays purple, with baking powder solution it goes blue to green, and with soapy water it tips into yellow-green. In exactly that order you can rebuild the pH scale from your own kitchen - and it even explains the German name for the vegetable, which is red cabbage in some regions and blue cabbage in others: cook it with vinegar or apple and it stays red, cook it in hard, slightly basic water and it turns blue.
Why water expands as it freezes is covered in the anomaly of water. How solid, liquid and gas hang together is explained by the three states of matter. And how cells get energy out of sugar while releasing carbon dioxide is shown in cellular respiration.
The sentence to remember for the test: acids give protons away, bases take them on, and the pH value counts those protons in powers of ten. A step on the scale is therefore never a small difference - it is always a factor of 10.
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