Predicting Products & the Activity Series
Learn to predict products of single- and double-replacement reactions using the activity series, ionic charges, and solubility rules — including when no reaction happens.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on Predicting Products & the Activity Series, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
You already know how to balance an equation and how to sort a reaction into a type. Now comes the harder skill: being handed only the reactants and figuring out what actually comes out. Predicting products is where chemistry stops being bookkeeping and starts being reasoning — you have to decide which atoms swap, what charges the new compounds need, and whether the reaction happens at all.
Two tools do almost all the work. The activity series tells you whether a free element is reactive enough to shove another element out of its compound, and the solubility rules tell you whether two dissolved ionic compounds will produce a solid when mixed. Both tools also let you say the two words students most often forget: no reaction. This lesson walks through single replacement, double replacement, correct formula writing from ionic charges, and the specific places students go wrong.
Two tools do almost all the work. The activity series tells you whether a free element is reactive enough to shove another element out of its compound, and the solubility rules tell you whether two dissolved ionic compounds will produce a solid when mixed. Both tools also let you say the two words students most often forget: no reaction. This lesson walks through single replacement, double replacement, correct formula writing from ionic charges, and the specific places students go wrong.
Single Replacement and the Activity Series
A single-replacement reaction has the pattern element plus compound gives new element plus new compound. A more reactive element takes the place of a less reactive one. The activity series is simply a ranked list of metals (and separately, halogens) from most reactive to least reactive.
Metals, most reactive at the top: Li, K, Ba, Ca, Na, Mg, Al, Zn, Cr, Fe, Cd, Co, Ni, Sn, Pb, (H), Cu, Ag, Hg, Pt, Au. Halogens: F₂ > Cl₂ > Br₂ > I₂.
The rule is one sentence: a free element replaces an element below it on the list, and does not replace an element above it. Hydrogen is placed in the metal list on purpose — metals above H displace hydrogen from acids (and the most active ones from water), while copper, silver, and gold do not react with hydrochloric acid at all.
One detail decides who swaps with whom. A free metal replaces the metal (the cation) in the compound. A free nonmetal — usually a halogen — replaces the nonmetal (the anion).
Zn + CuSO₄ → ZnSO₄ + Cu happens, because zinc sits above copper.
Cu + ZnSO₄ → no reaction, because copper sits below zinc.
Cl₂ + 2 NaBr → 2 NaCl + Br₂ happens, because chlorine outranks bromine.
I₂ + NaCl → no reaction.
Where students go wrong: they swap partners automatically without checking the series, so every single-replacement problem "works." Many of the problems you will be assigned are designed to come out as no reaction. Always locate both elements on the list before writing anything.
Also remember that the free element must be written in its natural form: the diatomic elements H₂, N₂, O₂, F₂, Cl₂, Br₂, and I₂ are never written as single atoms.
Metals, most reactive at the top: Li, K, Ba, Ca, Na, Mg, Al, Zn, Cr, Fe, Cd, Co, Ni, Sn, Pb, (H), Cu, Ag, Hg, Pt, Au. Halogens: F₂ > Cl₂ > Br₂ > I₂.
The rule is one sentence: a free element replaces an element below it on the list, and does not replace an element above it. Hydrogen is placed in the metal list on purpose — metals above H displace hydrogen from acids (and the most active ones from water), while copper, silver, and gold do not react with hydrochloric acid at all.
One detail decides who swaps with whom. A free metal replaces the metal (the cation) in the compound. A free nonmetal — usually a halogen — replaces the nonmetal (the anion).
Zn + CuSO₄ → ZnSO₄ + Cu happens, because zinc sits above copper.
Cu + ZnSO₄ → no reaction, because copper sits below zinc.
Cl₂ + 2 NaBr → 2 NaCl + Br₂ happens, because chlorine outranks bromine.
I₂ + NaCl → no reaction.
Where students go wrong: they swap partners automatically without checking the series, so every single-replacement problem "works." Many of the problems you will be assigned are designed to come out as no reaction. Always locate both elements on the list before writing anything.
Also remember that the free element must be written in its natural form: the diatomic elements H₂, N₂, O₂, F₂, Cl₂, Br₂, and I₂ are never written as single atoms.
Building Correct Formulas from Ionic Charges
Predicting the right products is worthless if the formulas are wrong, and this is the single most common error in the whole unit. You cannot carry a subscript over from the reactant side. You must rebuild each product from the charges of its ions.
Common charges: Group 1 metals are 1+, Group 2 are 2+, aluminum is 3+, zinc is 2+, silver is 1+. Group 17 nonmetals are 1−, oxygen and sulfur are 2−, nitrogen is 3−. Transition metals with variable charge show their charge in Roman numerals, as in iron(III) or copper(II). Polyatomic ions keep their identity and their charge through the reaction: nitrate NO₃⁻, sulfate SO₄²⁻, carbonate CO₃²⁻, phosphate PO₄³⁻, hydroxide OH⁻, ammonium NH₄⁺.
Charges are written as a superscript on the symbol, and a charge of one is shown by the sign alone with no numeral: K⁺ means 1+, and Cl⁻ means 1−. Read a bare sign as magnitude 1 whenever you balance charges.
Combine cation and anion so the total charge is zero, then reduce the subscripts to lowest terms.
Parentheses matter: they go around a polyatomic ion only when you need more than one of it. Ca(NO₃)₂ means one calcium and two nitrates; CaNO₃₂ means nothing.
A useful check: after you write the products, count each element on both sides. If a product formula is wrong, no amount of coefficient-juggling will balance the equation. If an equation "refuses" to balance, suspect a bad formula before you suspect your arithmetic. Only after every formula is correct do you add coefficients.
Common charges: Group 1 metals are 1+, Group 2 are 2+, aluminum is 3+, zinc is 2+, silver is 1+. Group 17 nonmetals are 1−, oxygen and sulfur are 2−, nitrogen is 3−. Transition metals with variable charge show their charge in Roman numerals, as in iron(III) or copper(II). Polyatomic ions keep their identity and their charge through the reaction: nitrate NO₃⁻, sulfate SO₄²⁻, carbonate CO₃²⁻, phosphate PO₄³⁻, hydroxide OH⁻, ammonium NH₄⁺.
Charges are written as a superscript on the symbol, and a charge of one is shown by the sign alone with no numeral: K⁺ means 1+, and Cl⁻ means 1−. Read a bare sign as magnitude 1 whenever you balance charges.
Combine cation and anion so the total charge is zero, then reduce the subscripts to lowest terms.
| Ions | Balanced formula | Common error |
|---|---|---|
| Al³⁺ and Cl⁻ | AlCl₃ | AlCl₂ copied from CuCl₂ |
| Ca²⁺ and NO₃⁻ | Ca(NO₃)₂ | CaNO₃₂ without parentheses |
| Mg²⁺ and O²⁻ | MgO | Mg₂O₂, not reduced |
| Fe³⁺ and SO₄²⁻ | Fe₂(SO₄)₃ | FeSO₄, wrong iron charge |
A useful check: after you write the products, count each element on both sides. If a product formula is wrong, no amount of coefficient-juggling will balance the equation. If an equation "refuses" to balance, suspect a bad formula before you suspect your arithmetic. Only after every formula is correct do you add coefficients.
Double Replacement and the Solubility Rules
A double-replacement reaction has the pattern compound plus compound gives two new compounds: the cations trade anions. Write cation A with anion B, and cation B with anion A, each built from charges as described above.
But swapping partners is not enough. A double-replacement reaction only truly occurs if one of the products leaves the solution — that is, if it is an insoluble solid (a precipitate), a gas, or water (a molecular compound). If both products are soluble ionic compounds, all the ions simply stay dissolved and drifting, and the correct answer is no reaction.
Solubility rules worth memorizing:
So AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq) is a real reaction — silver chloride is insoluble. But NaCl(aq) + KNO₃(aq) gives NaNO₃ and KCl, both soluble, so nothing happens.
Two special cases are worth recognizing. An acid plus a base gives water plus a salt — always a reaction, because water forms. And any carbonate plus an acid gives carbonic acid, which immediately falls apart: H₂CO₃ → H₂O + CO₂ gas. That is why vinegar fizzes on baking soda.
Where students go wrong: they cross the ions like an X and copy subscripts along with them. Break the reactants into ions first, note the charges, then rebuild.
But swapping partners is not enough. A double-replacement reaction only truly occurs if one of the products leaves the solution — that is, if it is an insoluble solid (a precipitate), a gas, or water (a molecular compound). If both products are soluble ionic compounds, all the ions simply stay dissolved and drifting, and the correct answer is no reaction.
Solubility rules worth memorizing:
| Rule | Compounds | Exceptions |
|---|---|---|
| Always soluble | Group 1 salts, ammonium, nitrates, acetates | none you need |
| Soluble | chlorides, bromides, iodides | Ag, Pb, Hg compounds |
| Soluble | sulfates | Ba, Pb, Ca, Sr sulfates |
| Insoluble | carbonates, phosphates, sulfides | Group 1 and ammonium |
| Insoluble | hydroxides | Group 1, plus Ba and Ca partly |
Two special cases are worth recognizing. An acid plus a base gives water plus a salt — always a reaction, because water forms. And any carbonate plus an acid gives carbonic acid, which immediately falls apart: H₂CO₃ → H₂O + CO₂ gas. That is why vinegar fizzes on baking soda.
Where students go wrong: they cross the ions like an X and copy subscripts along with them. Break the reactants into ions first, note the charges, then rebuild.
A Reliable Procedure and the "No Reaction" Habit
Use the same routine every time so the decision points do not get skipped.
First, classify the reaction from the reactants. Element plus compound means single replacement. Two compounds means double replacement.
Second, apply the correct test. Single replacement: is the free element above the element it would replace on the activity series? Double replacement: does the swap produce a precipitate, a gas, or water?
Third, if the test fails, write NR (no reaction) and stop. Do not write products anyway.
Fourth, if the test passes, identify the ions and their charges and build each product formula so charges cancel.
Fifth, balance with coefficients only, and add state symbols: (aq) for dissolved, (s) for the precipitate, (g) for gas, (l) for water.
The habit that separates a confident answer from a guess is treating "no reaction" as a legitimate, expected outcome rather than a sign you did something wrong. Chemists spend a lot of time knowing what will not happen — that is why gold jewelry survives contact with acid and copper pipes survive contact with water. In the next lesson on oxidation and reduction, the activity series returns with a deeper meaning: elements high on the list lose electrons easily, which is exactly what makes them able to displace the ones below.
First, classify the reaction from the reactants. Element plus compound means single replacement. Two compounds means double replacement.
Second, apply the correct test. Single replacement: is the free element above the element it would replace on the activity series? Double replacement: does the swap produce a precipitate, a gas, or water?
Third, if the test fails, write NR (no reaction) and stop. Do not write products anyway.
Fourth, if the test passes, identify the ions and their charges and build each product formula so charges cancel.
Fifth, balance with coefficients only, and add state symbols: (aq) for dissolved, (s) for the precipitate, (g) for gas, (l) for water.
| Situation | Prediction |
|---|---|
| Mg + HCl | Reaction; Mg is above H, gives MgCl₂ + H₂ gas |
| Ag + HCl | No reaction; Ag is below H |
| Br₂ + KI | Reaction; Br is above I, gives KBr + I₂ |
| Br₂ + KCl | No reaction; Br is below Cl |
| Pb(NO₃)₂ + KI | Reaction; PbI₂ precipitates |
| NaCl + KNO₃ | No reaction; all products soluble |
Reactions with Water, Acids, and Metal Oxides
A few single-replacement situations appear so often that they are worth knowing as patterns rather than rederiving each time.
Active metals plus water. The metals at the very top of the series — lithium, potassium, calcium, sodium — react with liquid water to produce a metal hydroxide plus hydrogen gas. For example, 2 Na + 2 H₂O → 2 NaOH + H₂. Magnesium and other mid-list metals react only with steam, and metals below hydrogen do not react with water at all, which is why copper plumbing works.
Metals plus acid. Any metal above hydrogen in the series displaces hydrogen from an acid, producing a salt plus hydrogen gas: Zn + 2 HCl → ZnCl₂ + H₂. The bubbles you see are hydrogen. Copper, silver, mercury, platinum, and gold sit below hydrogen and give no reaction with hydrochloric acid — a fact used to test whether a metal is one of the coinage metals.
Halogen displacement. A halogen higher on the list (closer to fluorine) displaces one lower down from its salt. Chlorine will pull bromide and iodide out of solution; iodine cannot displace anything except astatine.
Where students go wrong here: forgetting to write hydrogen as H₂ rather than H, and forgetting that when an active metal meets water the products are a hydroxide and hydrogen gas, not "metal oxide plus hydrogen." Another frequent slip is treating the acid HCl as though the metal replaces the chlorine; the metal always replaces the hydrogen, because hydrogen behaves as the cation in an acid.
When a reaction produces a gas, that gas escapes the container, which is exactly the same driving force that makes a precipitate reaction go forward: something leaves the solution.
Active metals plus water. The metals at the very top of the series — lithium, potassium, calcium, sodium — react with liquid water to produce a metal hydroxide plus hydrogen gas. For example, 2 Na + 2 H₂O → 2 NaOH + H₂. Magnesium and other mid-list metals react only with steam, and metals below hydrogen do not react with water at all, which is why copper plumbing works.
Metals plus acid. Any metal above hydrogen in the series displaces hydrogen from an acid, producing a salt plus hydrogen gas: Zn + 2 HCl → ZnCl₂ + H₂. The bubbles you see are hydrogen. Copper, silver, mercury, platinum, and gold sit below hydrogen and give no reaction with hydrochloric acid — a fact used to test whether a metal is one of the coinage metals.
Halogen displacement. A halogen higher on the list (closer to fluorine) displaces one lower down from its salt. Chlorine will pull bromide and iodide out of solution; iodine cannot displace anything except astatine.
Where students go wrong here: forgetting to write hydrogen as H₂ rather than H, and forgetting that when an active metal meets water the products are a hydroxide and hydrogen gas, not "metal oxide plus hydrogen." Another frequent slip is treating the acid HCl as though the metal replaces the chlorine; the metal always replaces the hydrogen, because hydrogen behaves as the cation in an acid.
When a reaction produces a gas, that gas escapes the container, which is exactly the same driving force that makes a precipitate reaction go forward: something leaves the solution.
Key terms
- Single-replacement reaction.
- A reaction in which a free element takes the place of an element in a compound, following the pattern A + BC → AC + B (metal replaces metal) or A + BC → BA + C (nonmetal replaces nonmetal).
- Double-replacement reaction.
- A reaction in which two ionic compounds exchange cations, AB + CD → AD + CB; it occurs only if a precipitate, a gas, or water forms.
- Activity series.
- A ranking of metals (and separately halogens) by reactivity; an element displaces any element listed below it but not any element above it.
- Precipitate.
- An insoluble solid that forms and drops out of solution when two aqueous solutions are mixed; labeled with (s) in the equation.
- Solubility rules.
- A set of generalizations predicting whether an ionic compound dissolves in water, used to identify which product of a double replacement is a precipitate.
- No reaction (NR).
- The correct answer when the free element is below the element it would replace, or when both double-replacement products are soluble; the ions simply remain unchanged in solution.
- Spectator ion.
- An ion that appears in identical form on both sides of the equation and does not participate in forming the precipitate, gas, or water.
- Polyatomic ion.
- A charged group of covalently bonded atoms, such as nitrate NO₃⁻ or sulfate SO₄²⁻, that stays intact through a replacement reaction and needs parentheses when more than one is required.
Worked example
Predict the products, write balanced equations with states, and identify any case of no reaction: (a) Al(s) + CuCl₂(aq), (b) Cu(s) + AlCl₃(aq), (c) Pb(NO₃)₂(aq) + KI(aq).
Part (a). Element plus compound, so this is single replacement. Aluminum is a metal, so it would replace the metal copper. Check the activity series: Al sits above Cu, so the reaction proceeds. Now build formulas from charges, not by copying subscripts. Aluminum forms Al³⁺ and chloride is Cl⁻, so the new compound is AlCl₃, not AlCl₂. The displaced copper leaves as the free metal Cu(s). Unbalanced: Al + CuCl₂ → AlCl₃ + Cu. Balance chlorine with 3 and 2: 2 Al(s) + 3 CuCl₂(aq) → 2 AlCl₃(aq) + 3 Cu(s). Check: 2 Al, 6 Cl, 3 Cu on each side.
Part (b). Same two elements, reversed roles. Copper is below aluminum on the activity series, so copper cannot displace aluminum. The answer is no reaction. Writing CuCl₂ + Al as products here would be a serious error — the metal simply sits in the solution unchanged.
Part (c). Two ionic compounds, so this is double replacement. Separate the ions: Pb²⁺, NO₃⁻, K⁺, I⁻ — the bare signs on potassium and iodide are charges of 1+ and 1−. Swap partners. Lead pairs with iodide: Pb²⁺ with I⁻ gives PbI₂. Potassium pairs with nitrate: K⁺ with NO₃⁻ gives KNO₃. Now apply the solubility rules. Iodides are soluble except with silver, lead, and mercury — lead is an exception, so PbI₂ is an insoluble solid, a bright yellow precipitate. All nitrates are soluble, so KNO₃ stays aqueous. Because a precipitate forms, the reaction really happens. Balanced with states: Pb(NO₃)₂(aq) + 2 KI(aq) → PbI₂(s) + 2 KNO₃(aq). Check: 1 Pb, 2 N, 6 O, 2 K, 2 I on each side.
Part (b). Same two elements, reversed roles. Copper is below aluminum on the activity series, so copper cannot displace aluminum. The answer is no reaction. Writing CuCl₂ + Al as products here would be a serious error — the metal simply sits in the solution unchanged.
Part (c). Two ionic compounds, so this is double replacement. Separate the ions: Pb²⁺, NO₃⁻, K⁺, I⁻ — the bare signs on potassium and iodide are charges of 1+ and 1−. Swap partners. Lead pairs with iodide: Pb²⁺ with I⁻ gives PbI₂. Potassium pairs with nitrate: K⁺ with NO₃⁻ gives KNO₃. Now apply the solubility rules. Iodides are soluble except with silver, lead, and mercury — lead is an exception, so PbI₂ is an insoluble solid, a bright yellow precipitate. All nitrates are soluble, so KNO₃ stays aqueous. Because a precipitate forms, the reaction really happens. Balanced with states: Pb(NO₃)₂(aq) + 2 KI(aq) → PbI₂(s) + 2 KNO₃(aq). Check: 1 Pb, 2 N, 6 O, 2 K, 2 I on each side.
Practice questions
Which mixture will produce a reaction?
- Ag(s) + Zn(NO₃)₂(aq)
- Cu(s) + HCl(aq)
- Mg(s) + Fe(NO₃)₂(aq)
- I₂(aq) + NaBr(aq)
Answer: Mg(s) + Fe(NO₃)₂(aq)
Check each free element against the element it would replace. Silver is far below zinc, so nothing happens. Copper is below hydrogen, so it does not displace hydrogen from an acid — this is why copper does not dissolve in hydrochloric acid. Iodine is below bromine among the halogens, so it cannot displace bromide. Magnesium, however, sits well above iron, so it displaces iron: Mg(s) + Fe(NO₃)₂(aq) → Mg(NO₃)₂(aq) + Fe(s). Notice that magnesium and iron(II) are both 2+, so the nitrate subscript happens to stay the same here — but you should still verify it from the charges rather than copying.
Barium chloride solution is mixed with sodium sulfate solution. Predict the products, decide whether a reaction occurs, and write the balanced equation with state symbols. Explain your reasoning about solubility.
Answer: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2 NaCl(aq); a reaction occurs because barium sulfate is insoluble.
Separate the ions: Ba²⁺, Cl⁻, Na⁺, SO₄²⁻, where the bare signs on chloride and sodium mean 1− and 1+. Swapping partners gives barium with sulfate and sodium with chloride. Build from charges: Ba²⁺ with SO₄²⁻ gives BaSO₄ (subscripts reduce to one each), and Na⁺ with Cl⁻ gives NaCl. Now test the products. Sulfates are generally soluble, but barium sulfate is one of the standard exceptions, so it precipitates as a white solid. Sodium chloride is soluble because all Group 1 salts dissolve. Since one product leaves the solution as a solid, the reaction proceeds. Balancing requires a coefficient of 2 in front of NaCl to account for the two chlorides. Sodium and chloride are the spectator ions here.
A student writes: Al(s) + 2 HCl(aq) → AlCl₂(aq) + H₂(g). Identify the error and give the corrected balanced equation.
Answer: The formula AlCl₂ is wrong; aluminum forms Al³⁺, so the product is AlCl₃. Corrected: 2 Al(s) + 6 HCl(aq) → 2 AlCl₃(aq) + 3 H₂(g).
The reaction itself is legitimate — aluminum is above hydrogen on the activity series, so it displaces hydrogen from the acid. The mistake is in formula writing: the student carried over a 2 from somewhere instead of pairing Al³⁺ with Cl⁻, which requires three chlorides. Once the formula is fixed, balance with coefficients only. Six chlorides on the right require 6 HCl on the left, which supplies 6 hydrogen atoms, or 3 H₂ molecules. Two aluminum atoms appear on each side. This is the classic sign of a formula error: an equation that seems impossible to balance until you rebuild a product from its ionic charges.
FAQ
- How do I know whether a free element replaces the metal or the nonmetal?
- Match like with like. A free metal replaces the cation (the metal) in the compound, so Zn + CuSO₄ gives ZnSO₄ + Cu. A free nonmetal, almost always a halogen, replaces the anion, so Cl₂ + 2 NaI gives 2 NaCl + I₂. Hydrogen counts as the cation in acids, so a metal reacting with HCl replaces hydrogen, never chlorine.
- Do I have to memorize the whole activity series?
- Most courses expect you to use a provided list, but knowing the general order helps enormously: the alkali and alkaline earth metals at the top, then Al, Zn, Fe, then hydrogen, then Cu, Ag, Hg, Pt, Au at the bottom. The single most useful fact is where hydrogen falls, because it tells you instantly which metals fizz in acid and which do not.
- Why is 'no reaction' so common in double replacement problems?
- Because mixing two soluble ionic compounds usually just mixes their ions. Unless one pairing is insoluble, forms a gas, or forms water, nothing has changed chemically — the same ions are still floating in the same solution. You need the solubility rules to spot the exceptions, and it is completely normal for a problem set to include several NR answers.
- What is the difference between a precipitate and a gas as a driving force?
- Both remove ions from solution, which is what makes the reaction go forward. A precipitate is an insoluble solid, labeled (s), such as AgCl or PbI₂. A gas, labeled (g), escapes the container — hydrogen from a metal-acid reaction, or carbon dioxide when a carbonate meets an acid and the carbonic acid formed decomposes into water and CO₂.
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The Crimsora tutor teaches Predicting Products & the Activity Series live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.