Acids, bases and pH: run a titration drop by drop

How to use this simulation

1. Pick the solution in the beaker: HCl, CH₃COOH, NaOH or pure water. For an acid or water the burette holds NaOH; for the base it holds HCl.
2. Press ▶: drops fall from the burette, the beaker changes colour and the titration curve is drawn on the right.
3. Drag "Volume added from the burette" to stop at any volume and read pH, pOH and [H₃O⁺].
4. Switch the indicator: phenolphthalein turns pink in basic solution, methyl orange is red in acid and yellow in base.
5. Keep "Zoom in on the ions" on to see red H₃O⁺ and blue OH⁻ dots: two dots appear or vanish per pH unit, because the scale is logarithmic.
6. Change a concentration or volume and the beaker refills. Read the equivalence volume and check it in your notebook with M₁V₁ = M₂V₂.

Lemons, soap and heartburn are all the same story

Squeeze a lemon onto your tongue and it tastes sour. Vinegar is sour, tamarind is sour, and so is a fizzy drink once the sugar is gone. Now pick up a bar of soap: it feels slippery, and if you have ever tasted it by accident it was bitter. The sour things form one family and the slippery, bitter things another. Chemists call them acids and bases.

After a large, spicy meal some people get a burning feeling in the chest. A doctor hands them an antacid. Why does that work? The stomach has made too much hydrochloric acid, and the antacid is a mild base that cancels the extra acid out. An acid and a base destroying each other's properties is called neutralisation, and it is the main character of this page.

But how do you measure "how sour" or "how basic" something is? You cannot taste lab acid. So scientists invented a single number for it, the pH, which runs roughly from 0 to 14 and tells you at a glance how acidic or basic a solution is. In the simulation below you will watch that number change with every drop.

From zero: what acids, bases and alkalis are

The simplest definition comes from the Swedish chemist Svante Arrhenius. An acid is a substance that releases hydrogen ions (H⁺) when it dissolves in water, and a base releases hydroxide ions (OH⁻). HCl splits into H⁺ and Cl⁻, so it is an acid; NaOH splits into Na⁺ and OH⁻, so it is a base.

A small truth worth knowing: a bare H⁺ (a single proton) never floats around alone in water. It immediately grabs a water molecule to become the hydronium ion, H₃O⁺. So when one book writes H⁺ and another writes H₃O⁺, they mean the same thing.

Later, Brønsted and Lowry gave a wider definition: an acid is a proton (H⁺) donor and a base is a proton acceptor. Under this idea ammonia, NH₃, is a base, because it takes a proton from water to become NH₄⁺, even though it contains no OH at all.

Now the difference between a base and an alkali, which exams love. A base is any substance that reacts with an acid to give a salt and water: CuO, CaO and NaOH are all bases. Only the bases that dissolve in water, such as NaOH, KOH and Ca(OH)₂, are called alkalis. So every alkali is a base, but not every base is an alkali: copper oxide is a base that will not dissolve.

• Properties of acids: sour taste, turn blue litmus red, react with reactive metals such as zinc and magnesium to give hydrogen gas, react with carbonates to give CO₂, and conduct electricity in solution.
• Properties of alkalis: bitter taste, soapy feel, turn red litmus blue and phenolphthalein pink, and react with acids to give a salt and water.
• A salt: the compound formed when the H⁺ of an acid is replaced by a metal ion or NH₄⁺, such as NaCl, KNO₃ or CaSO₄.

The key words at a glance

Most "define" questions come straight from this table, so read it once carefully.

What is pH, and what does it tell you?

Even pure water breaks up a tiny bit on its own: H₂O + H₂O ⇌ H₃O⁺ + OH⁻. At 25 °C a litre of pure water holds only 1.0 × 10⁻⁷ mol of H₃O⁺ and exactly the same amount of OH⁻. The two are equal, so water is neutral.

Numbers that small are painful to work with. In 1909 the Danish chemist Søren Sørensen had a neat idea: take the logarithm of the concentration and flip its sign. Then 1.0 × 10⁻⁷ becomes a friendly 7. That number is the pH. The "p" stands for power, and the "H" for hydrogen.

So what does pH indicate? It tells you the concentration of hydrogen (hydronium) ions. The lower the pH, the more H₃O⁺ and the more acidic the solution. The higher the pH, the less H₃O⁺, the more OH⁻, and the more basic the solution.

Here is the single most important fact: the pH scale is logarithmic. Drop the pH by 1 and H₃O⁺ goes up ten times; drop it by 2 and it goes up a hundred times. Vinegar at pH 3 is not twice as acidic as milk at pH 6; it is 1,000 times more acidic. That is why the zoom circle in the simulation adds the same number of dots for every pH step: the dots are drawn on a log scale.

The pH scale from 0 to 14, with everyday examples

The coloured strip along the bottom of the simulation is the pH scale: red on the left for strongly acidic, green in the middle for neutral, and blue to violet on the right for strongly basic. Universal indicator changes through exactly these colours, so the strip is painted with them.

Here is where familiar things sit. Natural products vary a little, so treat these values as approximate.

Strong and weak acids: not all sour is equal

Put HCl in water and nearly every molecule splits into ions. That makes it a strong acid, and the arithmetic is simple: in 0.01 M HCl, [H₃O⁺] is simply 0.01 M.

Acetic acid, CH₃COOH, the acid in vinegar, behaves differently: only a few molecules split and most stay whole. That is a weak acid. How willing it is to split is measured by its dissociation constant Ka; for acetic acid Ka = 1.8 × 10⁻⁵, so pKa = 4.74.

The result? At the same 0.1 M, HCl has a pH of 1.00 while acetic acid has a pH of 2.88. Only 1.33% of its molecules split! Switch the beaker from HCl to CH₃COOH in the simulation and watch the starting pH rise, and the start of the curve flatten into a "shoulder" instead of staying low.

Strong is not the same as concentrated, and examiners test this often. Strong describes how completely an acid splits; concentrated describes how much of it is in each litre. Very dilute HCl is still a strong acid, and very concentrated acetic acid is still a weak one.

• Strong acids: HCl, HNO₃, H₂SO₄ (first step).
• Weak acids: CH₃COOH (vinegar), H₂CO₃ (fizzy drinks), citric acid (lemons), tartaric acid (tamarind).
• Strong bases: NaOH, KOH.
• Weak bases: NH₄OH (ammonia solution), Mg(OH)₂ (found in antacids).

Indicators: the solution tells you its pH in colour

An indicator is a substance whose colour depends on the pH of the solution it is in. Most indicators are themselves weak acids or weak bases, with one colour when they hold an H⁺ and another when they have given it away.

Each indicator changes colour over its own pH range. In a titration you choose an indicator whose range falls inside the steep jump at the equivalence point.

What is a neutralisation reaction?

When an acid and a base react to form a salt and water, the reaction is called neutralisation. Afterwards the acidity of the acid and the basicity of the base have both been "neutralised", that is, cancelled.

The real action happens between ions. Na⁺ and Cl⁻ just float around watching (they are called spectator ions); only H₃O⁺ and OH⁻ do anything, joining to make water. In the zoom circle, red dots vanishing as blue dots appear is exactly this.

Neutralisation is exothermic. A strong acid and a strong base release about 57 kJ of heat for every mole of water formed, which is why the beaker feels a little warm. It is not a redox reaction either: no element changes its oxidation number, since H stays +1 and O stays −2.

Titration: how chemists find an unknown concentration

Say you have some NaOH solution but do not know its concentration. You pipette a fixed volume into a conical flask, add a few drops of indicator, and run in HCl of known concentration from a burette, drop by drop. The moment the indicator changes colour for good (the end point), you read the burette. That whole procedure is a titration.

At the equivalence point the moles of acid equal the moles of base (for a one-to-one reaction). Moles = concentration × volume, so M₁V₁ = M₂V₂, and with three of the values known you can find the fourth.

The simulation starts with 25 mL of 0.1 M HCl being titrated with 0.1 M NaOH, so the equivalence point is at 25 mL. The starting pH is 1.00. Notice that the pH barely moves over the first 20 mL, yet just before and after equivalence adding only 0.2 mL (about four drops!) sends the pH from 3.70 to 10.30. That jump of about 6.6 pH units is the heart of a titration, and it is why you add the last few millilitres one drop at a time.

Six experiments to try in the simulation

Doing beats reading. Try each experiment below, and before you press anything, predict what will happen.

Worked problems, step by step

Every answer below is calculated with the same equations the simulation runs (at 25 °C). Try each one yourself first.

Mistakes everyone makes

Teachers see the same slips on paper after paper. Here they are, so you can skip them.

• Calling pH 3 "twice as acidic" as pH 6. It is a thousand times, because the scale is logarithmic.
• Writing −log[OH⁻] for a base and calling it the pH. That is the pOH; pH = 14 − pOH.
• Mixing up "strong" and "concentrated". Dilute HCl is still a strong acid.
• Calling every base an alkali. Only bases that dissolve in water are alkalis.
• Assuming the solution at equivalence is always neutral. For a weak acid against a strong base it is above 7.
• Forgetting the factor n for a diprotic acid such as H₂SO₄ when using M₁V₁ = M₂V₂.
• Mixing mL and L. Keep both sides in the same unit, but use litres when you calculate moles.

pH and neutralisation in real life

Your stomach makes hydrochloric acid (pH 1–3) to digest food. Too much of it causes acidity or heartburn. Antacids contain mild bases such as Mg(OH)₂ or Al(OH)₃ that neutralise the excess.

Soil pH decides how well crops grow; most do best between pH 6 and 7.5. Farmers add lime (CaO or CaCO₃) to soil that is too acidic, and organic matter or gypsum to soil that is too basic.

Sulfur dioxide and nitrogen oxides from factories and vehicles dissolve in rainwater to make acid rain, with a pH below 5.6. It damages plants, fish and marble buildings.

After you eat, bacteria in your mouth produce acids that wear away tooth enamel. Toothpaste is mildly basic, so brushing neutralises that acid.

A bee sting is acidic, so a mild base such as baking soda soothes it. A wasp sting is basic, so a mild acid such as vinegar is suggested instead. An ant bite injects formic acid.

Your blood is held between pH 7.35 and 7.45 by buffer systems; moving outside that narrow band is a medical emergency.

Exam corner: what school boards ask

Acids, bases, salts and pH appear in every school science syllabus, from Grade 7 general science to Grade 10 chemistry and Grade 11 equilibrium. Questions come in a few predictable shapes.

• Definitions: what is pH, what is neutralisation, what is an indicator? Give the one-line textbook definition.
• Explain: why every alkali is a base but not every base is an alkali; why neutralisation is not a redox reaction; what "logarithmic" means for the pH scale.
• Calculate: the pH of a given solution, or a concentration from titration data with M₁V₁ = M₂V₂. Show units and the log step clearly.
• Analyse: whether a mixture of two solutions ends up acidic or basic, by comparing moles; or which indicator suits a titration, with a reason.
• Multiple choice: the sum of pH and pOH, indicator colours, the formula of a salt, which substance is not an alkali.

One-page revision

Read just this list the night before the exam.

• Acids give H₃O⁺ in water, alkalis give OH⁻; an alkali is a base that dissolves in water.
• pH = −log[H₃O⁺]; pOH = −log[OH⁻]; at 25 °C pH + pOH = 14.
• pH < 7 acidic, = 7 neutral, > 7 basic; each unit is a factor of 10.
• Strong acids dissociate almost completely, weak acids only slightly.
• Neutralisation: acid + base → salt + water; net ionic H₃O⁺ + OH⁻ → 2H₂O; exothermic.
• In a titration M₁V₁ = M₂V₂ at the equivalence point, where the pH jumps steeply.
• Phenolphthalein: colourless in acid, pink in base. Methyl orange: red in acid, yellow in base.

Frequently asked questions