Strong and weak acids sim

What the sim shows

Both acids have the same concentration, but hydrochloric acid is strong: it is completely ionised (HCl → H⁺ + Cl⁻). Ethanoic acid is weak: only about 4 molecules in 1000 ionise (CH₃COOH ⇌ H⁺ + CH₃COO⁻). With fewer H⁺ ions it has a higher pH (pH = −log[H⁺]), conducts less and fizzes more slowly with magnesium. Strong and weak are not the same as concentrated and dilute: 0.10 mol/dm³ hydrochloric acid still conducts far better than 1.0 mol/dm³ ethanoic acid. The weak acid's [H⁺] comes from Ka = [H⁺]²/(c − [H⁺]), close to √(Ka × c), with Ka = 1.8 × 10⁻⁵ mol/dm³. For a strong acid each tenfold dilution raises the pH by 1. In the magnesium test each tube holds 5.0 cm³ of 1.0 mol/dm³ acid and a 10 cm (0.10 g) strip of magnesium, which is more than enough: the acid runs out first, so both give 0.0025 mol, or 60 cm³, of hydrogen at room temperature and pressure (Mg + 2H⁺ → Mg²⁺ + H₂). The weak acid still gives all its hydrogen because, as H⁺ ions are used up, more ethanoic acid molecules ionise. How fast each acid reacts is this figure's estimate of a school result: ethanoic acid fizzes about ten times more slowly at first, not the 240 times its H⁺ concentration would suggest. Many mark schemes say the lamp is "dim" with ethanoic acid. With this 6 V, 0.30 A filament lamp the 9.6 mA current is far too small to make it glow, so here it stays off, but the ammeter still shows the small current (a more sensitive bulb or an LED would glow dimly). The current comes from I = V ÷ (R lamp + R solution), with the lamp taken as a fixed 20 Ω and ionic conductivities from a data book, ignoring the fall in conductivity per ion in concentrated acid, so the currents are estimates in the right order. Each reading carries a small scatter fixed for that solution (the same solution always gives the same reading) and the instrument's resolution: pH to 0.1, syringe to 1 cm³. Real pH meters read a little above 0.0 in 1.0 mol/dm³ hydrochloric acid. The particle view leaves out water and shows ethanoic acid's ions far more often than they really occur. Time only moves when you press a button: each 'Read volume' is 10 s of reaction.

• pH probe: drag it from beaker to beaker
• Hydrochloric acid (strong) on the graph
• Ethanoic acid (weak) on the graph

Use it in your lessons

• Try it here: anyone can unlock 2 figures free, and a free account unlocks up to 5. Runs on a laptop, a Chromebook or a phone; nothing to install.
• Present it and share student links with the teacher plan, from US$5 a month, billed yearly.
• Put it on your own website or LMS for US$9, paid once, or every sim from US$29 a month.

Free guide: 12 practicals as sims, and how to run one in a lesson →

Need a version for your exact set-up? Have one made: US$49 a figure in an order of 5.