Chemical Mixing Simulation — Mix Acids & Bases

The Chemical Mixing & pH Testing Virtual Lab lets you select two chemicals from a library of 14 acids and bases, mix them at adjustable volumes, watch each beaker being poured and stirred (with CO₂ fizzing where a carbonate meets an acid), and then test the resulting solution with litmus paper. The simulator calculates pH rigorously using equilibrium chemistry (strong/weak acid-base neutralization, Henderson-Hasselbalch for buffers, polyprotic acid handling) and displays the result on a full pH color scale.

Designed for high school chemistry students, this virtual lab provides safe, unlimited experimentation with dangerous chemicals like concentrated sulfuric acid and sodium hydroxide that would require careful supervision in a real laboratory. Four modes cover hands-on simulation, concept exploration, practice problems, and quizzes.

The lab opens as a 3D bench you can walk round and operate (section 2b) on a computer or tablet; on a phone it opens in 2D, and the switch still offers 3D. The 2D switch next to Reset, below the bench, shows the classic flat bench, and the page remembers your choice. Both views run the same experiment: a mix started in one carries on in the other.

The simulator opens in Simulate mode with Chemical A set to Hydrochloric Acid (HCl, 0.1 M) and Chemical B set to Sodium Hydroxide (NaOH, 0.1 M). Both volumes default to 100 mL. The canvas shows a lab bench with a 250 mL beaker of each reagent (labelled on lab tape), a 500 mL mixing beaker in the middle, and a universal indicator colour chart pinned to the wall.

To begin: select your chemicals from the dropdowns, adjust volumes if desired, then click Mix. A gloved hand pours beaker A, then beaker B, into the mixing beaker, and a glass rod stirs it. After mixing completes, click Test pH: a strip of indicator paper is dipped with tweezers and held up so you can see the wetted end change colour. The readout cards below show the chemical equation, reaction product, pH value, and classification (strong acid / weak acid / neutral / weak base / strong base).

Not sure what to do? Press ▶ Auto Test, the first button under the bench. It runs the whole experiment for the chemicals, volumes and paper you have chosen, and captions every step:

1. Pour A into the mixing beaker. With universal indicator in, an acid turns it red.
2. Pour B. Watch the colour where it lands, and the thermometer.
3. Stir with the glass rod until the colour is even.
4. Observe: the heat, any gas or precipitate.
5. Test with the indicator paper and read the result against the chart.

In the 3D lab each step is filmed from its own camera angle, and the pour and colour change run slower so they can be followed. The whole demonstration takes about half a minute; in 2D it takes 15 to 20 seconds. A bench that has already been used is rinsed first. Press ■ Stop Auto Test, or do anything yourself (tap the bench, change a chemical, volume or paper), to stop it and carry on by hand. If you take the camera during a 3D run, the captions carry on and the camera stays with you. Key U starts it on the 3D bench.

In the 3D lab every step is done on the apparatus itself, with a click or a tap. The step chip at the top left always says what to do next.

• Choose the reagents: tap a bottle on the reagent shelf (acids on the bottom row, bases on the board above; the Shelf button looks at it). A chooser asks whether to fill beaker A or beaker B, and the stock bottle is carried over and poured in. The dropdowns below the bench still work too.
• Pour A: tap beaker A, or drag it toward the mixing beaker. It tips about its spout, and only pours once the liquid reaches the spout. Let go and it finishes the pour.
• Pour B: tap beaker B, or drag it across. B always goes in after A.
• Stir: tap the glass rod, or drag it round the mixing beaker. Where an acid meets a carbonate, CO₂ bubbles appear, and only then.
• Test: tap the tweezers, or drag them down into the mixture. The strip is lifted out and held beside the indicator chart, and a plate gives the reading.
• Paper: tap a vial (universal, red litmus, blue litmus) to change the paper, even after a result. Rinse: tap the sink to reset the bench.
• Tapping the mixing beaker always does the next step, so a whole experiment can be run by tapping it four times.
• Indicator dropper: tap it to add universal indicator. Thermometer: tap it for the reading. Bunsen burner: after mixing, tap it for a flame test.

Looking around: drag the background (one finger) to orbit. Right-drag, Shift-drag or two fingers pans. Pinch or Ctrl + scroll zooms toward your fingers or the pointer. A plain mouse wheel zooms only after you have clicked the bench; until then it scrolls the page. Two fingers twisting turn round the bench, and a double tap flies to an object. The camera bar picks Auto (the camera follows the experiment from a new angle at each step), Bench, Close-up, Chart or Shelf. Labels names the apparatus, and + / − / ⟲ zoom and reset the view.

Keys (click the bench first): Enter or Space next step, A and B pour, S stir, T test, R rinse, 1 2 3 paper, arrow keys orbit, + / − zoom, 0 reset view.

The Mix button below the bench still runs the whole mix in one go. After a step done by hand it reads Continue and finishes the rest. Export PNG saves the 3D view while it is showing. On a phone the 3D lab works, but it is easier to explore on a larger screen.

Every mix is checked for what a student would really see. The Observations card (both views) and the 3D lab's live plate list the results. These solutions are dilute (0.1 M), so most of the change is invisible. That is real chemistry, not a missing animation.

• Temperature: a digital thermometer stands in the mixing beaker. Neutralisation gives out 55.8 kJ per mole of water, so HCl + NaOH at 100 mL each warms by only 0.7 °C. Acid + sodium hydrogencarbonate cools slightly, because releasing CO₂ takes in heat.
• Gas: CO₂ fizzes only when an acid frees it from a carbonate or hydrogencarbonate.
• Precipitate: calcium hydroxide (limewater) gives a milky white solid with sodium carbonate or hydrogencarbonate (CaCO₃), and with a little phosphoric acid (calcium phosphate). With carbonic acid it turns milky and then clears again when the acid is in excess. That is the classic limewater test. Calcium sulfate stays dissolved at these concentrations. Solubility is worked out with activity coefficients (Davies equation), not raw concentrations.
• Colour: three drops of universal indicator are in the mixing beaker from the start, because neutralisation is otherwise invisible. Pour in an acid and it turns red. As the base arrives, each stream of it lands as a plume of its own colour, and the stirring blends it in until the whole solution reaches the colour of the final pH. Click Indicator in to leave it out; then only the test strip shows colour.
• Particle view (3D): the Inside the beaker panel shows the ions at the same moment. H₃O⁺ and OH⁻ meet and become H₂O, each flash is one more water molecule, and Na⁺ and Cl⁻ are left as spectator ions. A weak acid shows mostly whole molecules with a trace of H₃O⁺. It is off until you press Particles in the camera bar.
• Smell: left-over ammonia is reported as a smell.
• Flame test (3D): after mixing, tap the Bunsen burner. A nichrome loop picks up a drop of the mixture and is held in the flame. Sodium gives an intense yellow-orange, potassium lilac and calcium brick red. Sodium's yellow hides the others, which is why chemists look through cobalt-blue glass. Ammonium and acid-only mixtures leave the flame blue.

What you will not see: fumes, smoke, steam or flames from the mix itself. White NH₄Cl smoke needs concentrated ammonia and hydrochloric acid vapours, and 0.1 M solutions neither boil nor burn.

Everything on the bench is drawn to one scale. The beakers are real Griffin sizes, and the liquid height is the volume divided by each beaker’s own cross-section, so the level that leaves a source is exactly the level that arrives in the mixing beaker. A beaker only pours once its surface reaches the spout, which is why it has to tip further as it empties. All fourteen reagents are colourless solutions, as they are in a real lab; the faint tints only tell A from B. Universal indicator in the mixing beaker (on by default, section 2c) shows the pH of the whole solution as it mixes, and the indicator paper shows it again in the test. Neutralising 0.1 M solutions warms the mixture by well under 1 °C, so nothing steams. Bubbles appear only when CO₂ is actually released: an acid mixed with sodium hydrogencarbonate, or with sodium carbonate once the acid is in excess. Less acid than that only turns carbonate into hydrogencarbonate, which gives no gas.

Use the Paper selector — in the top bar, and again beside the Test button — to switch between Universal indicator paper (full color gradient), Red litmus (stays red in acid, turns blue in base), and Blue litmus (turns red in acid, stays blue in base). You can switch paper on a finished result to re-read the same mixture with a different indicator.

After the test, a Litmus reading card shows the paper before and after dipping, side by side. This matters for the cases where nothing changes: red litmus in an acid stays red, and that is the result — it rules out a base. The Litmus Result readout states the same conclusion in words.

Litmus is not a pH-7 switch. Azolitmin is red below about pH 4.5 and blue above about pH 8.3; between those the paper is purple and litmus genuinely cannot separate a weak acid from a weak base. The simulator reports “Cannot tell” in that band — use Universal paper, which reads the whole scale and gives a number. The chain instruction system guides you step by step: Select Chemicals → Adjust Volumes → Click Mix → Test with your chosen paper → Read Result.

Switch to Explore mode to browse concept cards across four categories: Acids, Bases, Reactions, and Applications. The Acids category explains strong vs weak acids, dissociation constants (Ka), and pH calculation for both types. Bases covers strong hydroxides, weak bases like ammonia, and the Kb equilibrium.

The Reactions category covers neutralization, buffer solutions, and the Henderson-Hasselbalch equation with worked examples showing step-by-step calculations. Applications includes real-world uses: antacids neutralizing stomach acid, buffer systems in blood, water treatment with Ca(OH)₂, and industrial processes using these reactions. Each card includes a formula, explanation, and worked example problem.

Practice mode generates randomized problems covering: calculating pH of pure acids/bases, finding pH after mixing specific volumes, determining volumes needed for complete neutralization, and identifying excess reagent in a mixture. Enter your answer and click Check for immediate feedback (5% tolerance for rounding), or use Show Solution for a full step-by-step walkthrough showing every calculation.

Quiz mode presents five multiple-choice questions per session covering conceptual and numerical problems. Topics include identifying strong vs weak acids/bases, predicting reaction products, ranking chemicals by pH, calculating mixed-solution pH, and understanding litmus paper color changes. After completing the quiz, review your performance with detailed feedback on each question.

Acids available: Hydrochloric Acid (HCl, 0.1 M), Sulfuric Acid (H₂SO₄, 0.1 M, diprotic), Nitric Acid (HNO₃, 0.1 M), Acetic Acid (CH₃COOH, 0.1 M, Ka = 1.8×10⁻⁵), Citric Acid (C₆H₈O₇, 0.1 M, triprotic), Phosphoric Acid (H₃PO₄, 0.1 M, triprotic), Carbonic Acid (H₂CO₃, 0.04 M, diprotic).

Bases available: Sodium Hydroxide (NaOH, 0.1 M), Potassium Hydroxide (KOH, 0.1 M), Calcium Hydroxide (Ca(OH)₂, 0.02 M, diprotic), Ammonia (NH₃, 0.1 M, Kb = 1.8×10⁻⁵), Sodium Bicarbonate (NaHCO₃, 0.1 M, amphoteric), Sodium Carbonate (Na₂CO₃, 0.1 M), Magnesium Hydroxide (Mg(OH)₂, saturated solution, 1.1×10⁻⁴ M).

Key equations: Strong acid pH = −log[H⁺]; Weak acid pH = −log(√(Ka×C)); Buffer pH = pKa + log([A⁻]/[HA]); Neutralization: H⁺ + OH⁻ → H₂O. Gas evolution occurs when acids react with carbonates/bicarbonates (CO₂↑).

• At equal concentrations and equal volumes, strong acid + strong base = pH 7 (complete neutralization). Try HCl + NaOH at 100 mL each.
• Weak acid + strong base creates a buffer solution (partially neutralized). Try CH₃COOH + NaOH at 100 mL + 50 mL to see pH ≈ 4.74.
• Watch for CO₂ gas: mix an acid with NaHCO₃ to see it fizz. With Na₂CO₃, you need more acid than carbonate, because the first H⁺ only makes HCO₃⁻. Try 100 mL HCl with 50 mL Na₂CO₃, then with 100 mL. Carbonic acid is too weak to release CO₂ at all.
• Diprotic acids (H₂SO₄) have twice the moles of H⁺ per mole of acid. You need twice the base volume to fully neutralize them.
• In a real lab, always add acid to water, never the reverse. With concentrated sulfuric acid, adding water to acid causes violent boiling.
• Pair this simulator with the Litmus Paper Test Simulator for deeper pH indicator exploration.