School Chemistry — Grades 1 to 12 · Grades 1–12

18Acids, Bases and pH

On a kitchen shelf stand a lemon, a bottle of vinegar, a bar of soap and a box of baking soda; under the sink, a bottle of drain cleaner with a black-and-red warning label. Some of these taste sour, some feel slippery, some can burn the skin. One single scale, numbered from 0 to 14, sorts them all — and the same scale is used for swimming pools, garden soil, the blood and the oceans.

You already know

Ions are atoms or groups of atoms carrying a charge; a cation such as $\ce{Na+}$ is positive, an anion such as $\ce{Cl-}$ or $\ce{OH-}$ negative. Ionic solutions contain ions marked (aq), which move freely (Chapter 17).

Lemon, vinegar, soap and baking soda: acids and bases in the kitchen.

18.1 Acidic, basic and neutral solutions

Definition 18.1 (Acidic, basic and neutral solutions)

Every aqueous solution contains hydrogen ions $\ce{H+(aq)}$ and hydroxide ions $\ce{OH-(aq)}$ . A solution is an acidic solution when it contains more $\ce{H+}$ ions than $\ce{OH-}$ ions, a basic solution when it contains more $\ce{OH-}$ ions than $\ce{H+}$ ions, and a neutral solution when it contains as many of each.

Example 18.2 (Three solutions)

Hydrochloric acid is an acidic solution: it holds $\ce{H+(aq)}$ and $\ce{Cl-(aq)}$ ions, with very few $\ce{OH-}$ . A sodium hydroxide solution is basic: $\ce{Na+(aq)}$ and $\ce{OH-(aq)}$ , with very few $\ce{H+}$ . Pure water and salt water are neutral.

18.2 The pH scale

Definition 18.3 (pH)

The pH of an aqueous solution is a number, usually between 0 and 14, that tells how acidic or basic it is: at $25\,{}^{\circ}\mathrm{C}$ , a neutral solution has pH 7, an acidic solution a pH below 7, a basic solution a pH above 7. The more $\ce{H+}$ ions a solution holds, the lower its pH.

The pH scale, with the pH of some common solutions.

Definition 18.4 (pH paper and pH meter)

pH paper is a strip of paper soaked in a mixture of dyes that takes a different colour at each pH; a drop of the solution is put on it and the colour is compared with a chart printed on the box. A pH meter measures the pH with a glass probe dipped in the solution and shows it as a number, to one or two decimal places.

Method 18.5 (Measuring a pH with pH paper)

Put a short strip of pH paper on a clean, dry dish.
With a clean glass rod, touch the strip with a drop of the solution (never dip the strip in the bottle).
Compare the colour at once with the chart, and read the pH, usually to the nearest whole number.

Measuring a pH with a pH meter: the glass probe dips in the solution, which is gently stirred.

18.3 Diluting an acid

Proposition 18.6 (Diluting brings the pH closer to 7)

Adding water to an acidic solution spreads its $\ce{H+}$ ions through a larger volume: the pH goes up, towards 7, but never beyond. For an acid such as hydrochloric acid, diluting ten times raises the pH by about one unit: one volume of acid at pH 2 plus nine volumes of water gives a solution at pH 3. Diluting a basic solution likewise lowers its pH towards 7.

Diluting an acid ten times, then ten times again: the pH goes from 2 to 3 to 4 (each beaker is a tenth of the previous solution topped up with water).

In the lab — Diluting an acid

To dilute an acid, the teacher always pours the acid slowly into the water, never the water into the acid: the mixing gives out heat, and water poured onto a concentrated acid can boil at once and spit acid out of the container.

18.4 Everyday acids and bases

Proposition 18.7 (Acids and bases around us)

Acidic: lemon and lime juice, vinegar, wine, fizzy drinks, coffee, gastric juices in the stomach. Basic: sea water (slightly), baking soda dissolved in water, soap, milk of magnesia (a remedy for stomach acidity), household ammonia, bleach, drain cleaner (very strongly).

Example 18.8 (The pH of the ocean)

The average pH of the ocean is about 8.1: slightly basic. Since the industrial era began, carbon dioxide dissolving from the air has lowered the pH of surface waters by about 0.1: the oceans are slowly becoming less basic, which harms shellfish and corals.

Red cabbage juice, prepared by a teacher, changes colour with the pH: red in acids, violet near neutral, blue, green then yellow in more and more basic solutions.

18.5 Safety with corrosive products

Safety

Concentrated hydrochloric acid and sodium hydroxide (the base of drain cleaners) carry the corrosion pictogram GHS05: they cause severe burns to skin and eyes and attack metals. GHS07: irritant. Goggles and gloves; a splash is rinsed at once with plenty of water.

Remark 18.9 (Never mix cleaning products)

Acidic and basic products react together, sometimes violently, and some mixtures release toxic gases: bleach mixed with an acidic descaler gives off chlorine. Cleaning products are never mixed.

18.6 Exercises

Exercise 18.1 ★

Acidic, basic or neutral? A solution of pH 3, one of pH 7, one of pH 11.

Solution

Solution of Exercise 18.1.

pH 3: acidic. pH 7: neutral. pH 11: basic.

Exercise 18.2 ★

Which ions are more numerous in an acidic solution? In a basic one?

Solution

Solution of Exercise 18.2.

Acidic: $\ce{H+}$ ions. Basic: $\ce{OH-}$ ions.

Exercise 18.3 ★

Using the pH scale of this chapter, put in order from the most acidic to the most basic: coffee, sea water, lime juice, household ammonia, pure water.

Solution

Solution of Exercise 18.3.

Lime juice (2), coffee (5), pure water (7), sea water (8.1), household ammonia (12).

Exercise 18.4 ★

Describe how to measure the pH of a solution with pH paper.

Solution

Solution of Exercise 18.4.

A strip on a clean dry dish is touched with a drop of the solution carried by a clean glass rod; the colour is compared at once with the chart.

Exercise 18.5 ★★

A solution of pH 4 is diluted ten times with water. What is its new pH, about? And if it is diluted a hundred times?

Solution

Solution of Exercise 18.5.

About 5 after a tenfold dilution; about 6 after a hundredfold one.

Exercise 18.6 ★★

Can an acidic solution become basic by adding water? Explain.

Solution

Solution of Exercise 18.6.

No. Diluting brings the pH closer to 7, but never past it: water itself is neutral, and adding it cannot make $\ce{OH-}$ ions outnumber $\ce{H+}$ ions.

Exercise 18.7 ★★

Why must an acid be poured into water, and not the other way round?

Solution

Solution of Exercise 18.7.

Mixing gives out heat. Water poured onto a concentrated acid could boil at once and throw acid out; acid poured slowly into water is spread at once in a large volume.

Exercise 18.8 ★★

A bottle carries the GHS05 pictogram. What does it mean? Give two precautions.

Solution

Solution of Exercise 18.8.

Corrosive: burns skin and eyes, attacks metals. Wear goggles and gloves; rinse any splash with plenty of water.

Exercise 18.9 ★★

How many tenfold dilutions are needed to take an acid from pH 1 to pH 4? What total dilution is that?

Solution

Solution of Exercise 18.9.

Three dilutions (1 to 2, 2 to 3, 3 to 4): $10 \times 10 \times 10 = 1000$ times.

Exercise 18.10 ★★

Milk of magnesia is taken for stomach acidity. Using the pH scale, explain why.

Solution

Solution of Exercise 18.10.

Milk of magnesia is basic (pH about 10.5): it reacts with part of the excess acid of the stomach.

Exercise 18.11 ★★★

The ocean’s pH has fallen by about 0.1 since the industrial era. Is the ocean acidic today? In which direction is it moving, and why?

Solution

Solution of Exercise 18.11.

No: with a pH of 8.1 it is still slightly basic. It is moving towards lower pH, less basic, because carbon dioxide from the air dissolves in it.

Exercise 18.12 ★★★

A student measures the pH of the same vinegar with pH paper (3) and with a pH meter (2.6). Explain why the two results differ and which is more precise.

Solution

Solution of Exercise 18.12.

pH paper is read by eye against a colour chart, to about one unit; the pH meter gives a number to one or two decimals. The meter is more precise; the two agree within the precision of the paper.

18.7 Problem: The Acid Spill

Problem 18.1

Weekend problem — a litre of acid spilt in a laboratory sink, and why diluting it is not the answer

A bottle containing $1\,\mathrm{L}$ of dilute hydrochloric acid, of pH 2, breaks in a laboratory sink. The drain must not receive any solution of pH below 5. Take the rule of this chapter: for this acid, each tenfold dilution raises the pH by one unit.

Part I — The acid.

Is the solution acidic, neutral or basic? Which ions does it contain most?
Which pictogram should the bottle carry, and why?
The teacher measures the pH with a pH meter rather than pH paper. Give one advantage.
How many more $\ce{H+}$ ions are there in a litre at pH 2 than in a litre at pH 3 of the same acid?

Part II — Diluting.

What volume of solution is obtained by diluting the litre of acid ten times? What is its pH?
How many tenfold dilutions are needed to reach pH 5?
What total volume of solution would that make?
Could any amount of water make the solution basic? Explain.

Part III — A better way. The teacher instead adds, little by little, a solution of a base until the pH paper shows 7: the $\ce{H+}$ ions of the acid react with the $\ce{OH-}$ ions of the base.

Write the equation of the reaction between $\ce{H+}$ and $\ce{OH-}$ ions.
Why is this better than diluting?
Name an everyday basic product that could be used in a kitchen to clean up a small spill of vinegar.
Compute the volume of water that would have to be added to the litre of acid to bring it to pH 5.

Solution

Solution of Problem 18.1.

1. Acidic: it contains mostly $\ce{H+(aq)}$ and $\ce{Cl-(aq)}$ ions.

2. GHS05, corrosive: an acid attacks skin, eyes and metals.

3. It gives a precise number instead of a colour to compare by eye.

4. Ten times more: one pH unit corresponds to a tenfold dilution.

5. $10\,\mathrm{L}$ , of pH 3.

6. Three: pH 2 to 3, 3 to 4, 4 to 5.

7. $1 \times 10 \times 10 \times 10 = 1000\,\mathrm{L}$ .

8. No: diluting only brings the pH closer to 7.

9. $\ce{H+ + OH- -> H2O}$ .

10. The acid is destroyed, turned into water, instead of being spread through a huge volume; only a little base is needed and no thousand litres of water.

11. Baking soda dissolved in water (or soapy water).

12. $1000 - 1 = 999\,\mathrm{L}$ of water.

Terms defined in this chapter

See all 852 terms in the glossary