General Chemistry · Aqueous Reactions · PRECIPITATION REACTION

Precipitation Reaction —
Ions Combining

When you mix two solutions, their ions meet in one beaker. What happens depends on which ion pairs meet. The amount formed is not an input: it emerges from the particles meeting. First predict, then run it and observe, and finally check yourself.

Key summary: (1) In a solution of an electrolyte, the ions are separated into cations and anions that move freely. (2) When two solutions are mixed, new ion pairs meet, and different pairs can behave differently. (3) The number of precipitate particles is not set by you; it is counted from the pairs that bind.

Learning objectives After this activity you can …
explain that the amount of precipitate is not an input but emerges from binding.
predict the difference between an insoluble pair (Ag⁺·Cl⁻) and spectator ions (Na⁺·NO₃⁻).
identify the condition for precipitation (an insoluble pair meets).
Progress 0% · Predict → Observe → Check → Complete
Level Basic Standard Advanced

Beaker — the two solutions mixed

FREE IONS · BINDING · PRECIPITATE
Precipitate particles (bound pairs · measured every 2 s)—
Entry check Before you start — prerequisites and key terms

An electrolyte (e.g. NaCl, AgNO₃) is dissolved in water. In what form does it mostly exist in the water?

As cations and anions that move freely As whole lumps of molecules resting on the bottom It turns into a gas and escapes
Key terms

Electrolyte · ionization: a substance that dissolves in water and separates into ions. · Spectator ion: an ion that does not take part in the reaction and keeps moving freely. · Precipitate: an insoluble solid that forms when a pair with very low solubility binds and settles out.

Predict Think first, before you observe

What happens when Ag⁺ and Cl⁻ are mixed in the same beaker?

A solid precipitate forms They keep moving around freely A gas is given off
ConfidenceMedium
The explanation opens after you lock in a prediction

—

dilute · fewconcentrated · many
Ag⁺·Cl⁻ (reacting)Na⁺·NO₃⁻ (spectator)
Show bonds
Cation (Ag⁺ / Na⁺) Anion (Cl⁻ / NO₃⁻) Precipitate particle (bound, fixed pair) Spectator (keeps moving freely)

The explanation opens after you lock in a prediction.

Precipitation reaction · net ionic equation

Ions in solution are spread out and move freely. When two solutions are mixed and a pair with very low solubility meets, the ions bind and settle as a solid precipitate. Ions that do not take part and keep moving are called spectator ions. The same kind of reaction happens in soil and irrigation water, for example when calcium ions meet phosphate from fertilizer.

Ag⁺(aq) + Cl⁻(aq) → AgCl(s)↓
Net ionic equation · Na⁺ and NO₃⁻ are spectators (they cancel on both sides)

Emergence · the amount of precipitate comes from the number of bindings

We never set the amount of precipitate. Freely moving Ag⁺ and Cl⁻ come close and bind; the number of these bindings adds up to the number of precipitate particles. With only spectators (Na⁺·NO₃⁻) there is no insoluble pair, so the precipitate is 0.

Precipitate count = Σ (bindings of insoluble pairs)
Reacting pair → precipitate grows | spectators only → precipitate 0
Check Formative check — pass all items to complete
Q1A beaker contains only Na⁺ and NO₃⁻. What about a precipitate?
A lot of precipitate forms No precipitate forms (both are spectators) It escapes as a gas
Hint

NaNO₃ is a salt that dissolves well in water. A precipitate needs an insoluble pair.

Q2 · Hands-on taskAfter predicting, use the controls to reach the target precipitate count, then press [Check].
Measured precipitate now: — · target: — (hint: pair toggles and concentration slider)
Q3How many precipitate particles form? (enter a number)
Hint

One Ag⁺ and one Cl⁻ pair up to make one precipitate particle. Spectator ions do not form pairs.

Q4 · Fill in the blankFill in the blank: When an ionic compound with very low solubility forms, a [_____] forms.
Hint

An insoluble solid that settles out of the solution.

Reflect Reflect — metacognition · completion

My prediction: Not predicted yet

In one sentence, explain when a precipitate forms:

Going further (optional)

Switch off the reacting pair and keep only the spectators (Na⁺·NO₃⁻): check that the precipitate is 0. Switch both pairs on: only Ag⁺·Cl⁻ bind, and Na⁺·NO₃⁻ keep moving freely to the end. This is what "spectator ion" means.

COMPLETE · Completed — all checks passed.

Worksheet — Precipitation reaction: ions combining (black and white)

Name ________ Class ________ Date ________

Symbol key (no color needed): ● cation = filled circle ○ anion = open circle ■ precipitate = square (hatched)

1. Predict — What happens when Ag⁺ and Cl⁻ are mixed? (precipitate forms / no change / gas given off)

2. Observe — Write the number of precipitate particles for the reacting pair and for spectators only.

Ag⁺·Cl⁻ (reacting): precipitate = ____ · Na⁺·NO₃⁻ (spectators only): precipitate = ____

3. Conclusion — In one line each: the condition for precipitation, and the meaning of a spectator ion.

4. Net ionic equation — Ag⁺(aq) + ____⁻(aq) → ________(s)↓

5. Fill in the blank — When an ionic compound with very low solubility forms, a [_____] forms.

QR / link: the address given by your teacher

Teacher notes — open the lesson plan

Lesson plan · 5E phase and time

  • Explore · Explain stage. 10–15 min after the introduction.
  • Works as a whole-class demo (projector, ?mode=teacher) or as individual work on one device per student.

Demo in 3 steps

  • (1) Spectators only (Na⁺·NO₃⁻) → precipitate 0 (free motion only).
  • (2) Reacting pair ON (Ag⁺·Cl⁻) → observe binding and growing precipitate.
  • (3) Both pairs ON → only Ag⁺·Cl⁻ precipitate; Na⁺·NO₃⁻ stay spectators to the end.

Answer key (by level)

  • Prediction: a solid precipitate forms. Q1: (b). Q4 blank: precipitate.
  • Basic: Q2 reacting pair only, precipitate ≥ 6 · Q3 0 / Standard: Q2 both pairs on, ≥ 8 · Q3 min(10, 6) = 6 / Advanced: Q2 both pairs on, ≥ 12 · Q3 min(12, 9) = 9
  • Q2 counts only if the student changes the controls after predicting. Common Q3 errors: adding spectators or leftover ions, or giving the larger ion count.

3 key questions

  • "We never set the number of precipitate particles, yet a number appears. Where does it come from?"
  • "Why do Ag⁺ and Cl⁻ bind, but Na⁺ and NO₃⁻ do not?"
  • "Does mixing two solutions always give a precipitate?"

Common misconceptions

  • MC-precip-01: "Mixing always gives a precipitate" → only an insoluble pair precipitates (NaNO₃ is soluble).
  • MC-precip-02: "Spectator ions also react" → they are onlookers that cancel on both sides and do not appear in the net ionic equation.

Prerequisites

  • Electrolytes and ionization (ions in aqueous solution); solubility rules (soluble / insoluble salts).
University general chemistry · Aqueous reactions General Chemistry Engine E1 particle / MD Verified: physics · learning design · teacher (3-way, 2026-06-17)
Teacher mode — class controls