Explainer video · 9:09 min · Chemistry, Grades 8 to 10
Lemon juice has about pH 2, black coffee about pH 5. That sounds like a small difference. But if you calculate with these round values, the lemon juice has about 1,000 times as many hydronium ions per liter. You will learn what acidic, basic and neutral mean, why every pH step is a factor of ten, where everyday things from stomach acid to drain cleaner sit on the scale, how red cabbage juice and universal indicator show the pH value, and what happens during neutralization and dilution. The everyday values are typical values, and the calculations hold at 25 °C.
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Safety: Never mix cleaners. A chlorine bleach cleaner mixed with vinegar or descaler releases toxic chlorine gas, even when used one after the other. Mixed with ammonia cleaners, it forms toxic chloramines. Hydrochloric acid and sodium hydroxide solution belong only in a supervised school lab experiment with a teacher and safety goggles. Never try unknown substances or cleaners.
School model at 25 °C. The everyday values are typical values; type, brand and temperature shift them.
| Substance | pH value (typical) |
|---|---|
| Stomach acid | about 1 to 2 |
| Lemon juice | between 2 and 3 (calculation value in the example: 2) |
| Cola | between 2 and 3 |
| Orange juice | between 3 and 4 |
| Black coffee | about 5 (calculation value in the example: 5) |
| Skin (acid mantle) | about 5 to 5.5 |
| Fresh milk | just under 7 |
| Pure water (at 25 °C) | 7 |
| Blood (healthy) | 7.4, range 7.35 to 7.45 |
| Seawater, baking soda in water | about 8 |
| Soap solution | 9 to 10 |
| Ammonia cleaner | about 11 |
| Drain cleaner (sodium hydroxide solution) | 13 to 14 |
Typical values: depending on type, brand, temperature and condition, the values differ. The calculation example (lemon juice pH 2, coffee pH 5, about 1,000 times as many hydronium ions per liter) deliberately uses round values.
Trap 1: The scale is linear, so pH 4 is twice as acidic as pH 6. No: pH 4 has 100 times as many hydronium ions as pH 6, not twice as many. That is two steps, 10 · 10.
Trap 2: Diluting an acid eventually makes it basic. No, the value only gets closer to 7.
Trap 3: The one-step rise per tenfold dilution always holds. It holds for strong acids in a clearly acidic solution. With a weak acid like citric acid, the pH value often rises by less than one step.
Safety trap: Never mix cleaners and never taste them. Acids and alkaline solutions can be corrosive.
The pH value shows how acidic or basic an aqueous solution is. For dilute solutions, it is roughly the negative base-10 logarithm of the hydronium ion concentration in moles per liter. The more hydronium ions (H₃O⁺) there are in a liter, the smaller the pH number. Example: 0.001 mol/L is 10⁻³ mol/L, so the pH value is 3.
An acid releases hydrogen ions (protons) in water, which attach to water molecules: hydronium ions form. A base takes up hydrogen ions or supplies hydroxide ions (OH⁻) itself. A base dissolved in water is called an alkaline solution. In daily life, the pH scale runs from 0 to 14. At 25 °C, pH 7 is neutral: pure water has exactly as many hydronium ions as hydroxide ions. Below 7 it is acidic, above 7 it is basic.
Because the pH value is a base-10 logarithm. Every step down means ten times as many hydronium ions per liter, every step up only one tenth as many. That is why pH 4 is not twice as acidic as pH 6: it is two steps, so 10 · 10 = 100 times as many hydronium ions per liter.
Calculation example with round values: lemon juice pH 2, coffee pH 5. That is 3 steps, so 10 · 10 · 10 = 1,000. In this example, the lemon juice has about 1,000 times as many hydronium ions per liter as the coffee. The real values are typical values with a range: lemon juice is between pH 2 and 3, coffee is at about pH 5.
Red cabbage juice is a natural indicator: in acid it turns red, in neutral solution violet, in weakly basic solution blue to green, and in strongly basic solution yellow. This is only a rough estimate. Universal indicator changes color like a traffic light, from red through orange and yellow to green at pH 7, then blue and violet. You read the color off the color chart on the package. A pH meter measures more precisely. Never try unknown substances or cleaners.
In neutralization, hydronium ions and hydroxide ions react to form water: H₃O⁺ + OH⁻ → 2 H₂O. Example from the school lab at 25 °C: 10 mL of hydrochloric acid (0.1 mol/L, pH 1) and 10 mL of sodium hydroxide solution (0.1 mol/L, pH 13) give a table salt solution with pH 7. If the amounts do not match, the amounts of substance decide: if acid is left over, the mixture is acidic, and if alkaline solution is left over, it is basic. This is an experiment for chemistry class with a teacher and safety goggles, not for the kitchen. Cleaners are never mixed: a chlorine bleach cleaner with vinegar or descaler releases toxic chlorine gas.
No. One part hydrochloric acid with pH 1 plus 9 parts water makes 10 parts in total: per liter, only one tenth of the hydronium ions remain, and the pH value rises by one step to 2. This holds for strong acids like hydrochloric acid in a clearly acidic solution. A weak acid like citric acid releases only a part of its hydrogen ions, and when diluted, a larger share. That is why the pH value there often rises by less than one step. And an acid does not turn basic when diluted: the value only gets closer to 7.
Explainer video. Calculation examples use the school model at 25 °C, everyday values are typical values, and this is not advice on health or nutrition. Voice, music and images are AI-generated. Idea, concept and editing: human.
Sour lemon sits low, Soap sits up high. One step down the scale: Ten times the jump!
Lemon juice has a pH of about two. Black coffee has a pH of about five. Sounds like a small difference, right? But with these round numbers, a liter of lemon juice holds about one thousand times as many of the particles that make it acidic as a liter of coffee.
How can such a small number make such a big difference? We will find out at a table full of everyday things. The pH scale builds up as we go, piece by piece.
Three steps. First, what acidic and basic actually mean. Second, how the scale works and how you can calculate with it. Third, what happens during neutralization and dilution.
How can you tell whether something is acidic at all? By taste? You never taste unknown substances or cleaners. Chemists look at tiny particles in the water instead.
An acid releases hydrogen ions in water. These are also called protons. They attach to water molecules, and hydronium ions form, H₃O⁺ for short.
A base takes up hydrogen ions or supplies hydroxide ions itself, OH⁻ for short. So a basic solution contains more hydroxide ions than hydronium ions. A base dissolved in water is called an alkaline solution. And pure water? It has only a tiny number of ions, but exactly the same number of each. That is why it is neutral.
The pH value only looks at the hydronium ions. The more of them there are in a liter, the smaller the pH number. An alkaline solution has fewer hydronium ions than neutral water. That is why it sits at the top of the scale.
In daily life, the pH scale runs from zero to fourteen. At twenty-five degrees Celsius, seven is neutral. Below that it is acidic, above it basic. And now the trick: every step down means ten times as many hydronium ions, and every step up means only one tenth as many.
Let us calculate with the round numbers: lemon juice pH two, coffee pH five. That is three steps: ten times ten times ten, so one thousand. The lemon juice has one thousand times as many hydronium ions per liter as the coffee. And pH four versus pH six: is that twice as acidic?
No! pH four has a hundred times as many hydronium ions as pH six: two steps, ten times ten. One step sounds like a walk in the park, but every single time it is a factor of ten.
Here is how you can calculate it. For dilute solutions, the pH value is roughly the negative base-ten logarithm of the hydronium ion concentration in moles per liter. Sounds like a trip to the dentist. For round values there is a trick: if there is just a single one after the decimal point, you count the places up to it, counting the one itself. Zero point zero zero one moles per liter: three places, so pH three.
Now let us place some everyday things on the scale. The numbers are typical values. Type, brand and temperature shift them a little. Far left, stomach acid, at about pH one to two. Lemon juice and cola both sit between two and three, orange juice between three and four, coffee at about five.
Your body is on the scale too. The surface of your skin sits at about five to five and a half, so it is slightly acidic. This is called the acid mantle. Fresh milk is just below seven, and pure water is at seven. And healthy blood? Seven point four, so slightly basic, within a range of only seven point three five to seven point four five. Stricter than any school dress code.
On the basic side: seawater and baking soda in water are at about eight, soap solution at nine to ten, an ammonia cleaner at about eleven. Right at the end, drain cleaner with sodium hydroxide solution: thirteen to fourteen. And which side is more dangerous?
Both. Acidic and basic products can be corrosive, depending on the substance, the concentration and the exposure time. That is why cleaners stay in their bottles and are never tried out.
And where do these numbers come from? For example, from color. Red cabbage juice is a natural indicator: in acid it turns red, in neutral solution violet, in weakly basic solution blue to green, and in strongly basic solution yellow.
For more precision, use universal indicator. It changes color like a traffic light: from red through orange and yellow to green at seven, then blue and violet. You read the color off the color chart on the package. A pH meter measures even more precisely. Your tongue stays in your mouth.
What happens when an acid meets a base? Think of heartburn: stomach acid rises into the esophagus. Antacids from the pharmacy contain, for example, magnesium hydroxide. Its hydroxide ions grab hydronium ions. Together, one hydronium ion and one hydroxide ion make two water molecules. That is how the antacid neutralizes part of the stomach acid. Cola against heartburn would mean fighting acid with acid. Chemically speaking, a bold move.
That is called neutralization. Here is an example from the school lab: ten milliliters of hydrochloric acid at pH one and ten milliliters of sodium hydroxide solution at pH thirteen, both at zero point one moles per liter. At twenty-five degrees Celsius, a table salt solution with pH seven comes out. The hydronium and hydroxide ions react in matching amounts to form water, and sodium and chloride ions stay dissolved. This is an experiment for chemistry class, with a teacher and safety goggles, and definitely not for your kitchen.
What if the amounts do not match? The amounts of substance decide: leftover acid makes the mixture acidic, leftover base makes it basic.
One more math trick: dilution. In chemistry class, the water goes into the beaker first and then, slowly, the acid, because heat can be released. One part hydrochloric acid with pH one plus nine parts water makes ten parts in total. Per liter, only one tenth of the hydronium ions remain: pH two. If you dilute a strong acid to ten times its volume, the pH value rises by one. Will it eventually become basic?
No, the value only gets closer to seven. This one-step rise per tenfold dilution holds for strong acids like hydrochloric acid in a clearly acidic solution. A weak acid like citric acid releases only a part of its hydrogen ions, and when diluted, a larger share. That is why the pH value there often rises by less than one step.
The cheat sheet: the standard scale runs from zero to fourteen, and at twenty-five degrees Celsius, seven is neutral. Every step down: ten times as many hydronium ions, every step up: one tenth. With hydrochloric acid and sodium hydroxide solution, acid plus base gives water and table salt. And very important: cleaners stay in the bottle and are never mixed. Least of all a chlorine bleach cleaner with vinegar or descaler, not even one after the other: that produces toxic chlorine gas.
Lemon juice pH two, coffee pH five: with these round numbers, that is about one thousand times as many hydronium ions per liter. That was the trick: every pH step is a factor of ten. Now you can compare pH values and predict what happens during dilution and neutralization. Try it yourself: how many times more hydronium ions does pH two have than pH six?
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Seven is the middle ground, Lemon sits below, Soap sits above. One step down the scale: Ten times the jump!
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