What happens in a double replacement reaction

A double replacement reaction (also called a double displacement reaction) is when two compounds swap partners. The cations—the positively charged ions—trade places with each other, and the anions—the negatively charged ions—do the same. You start with two compounds, the ions rearrange, and you end up with two new compounds.

The basic pattern is: AB + CD → AD + CB. Compound AB breaks apart, compound CD breaks apart, and the pieces recombine in a new order. The atoms themselves don't change; only which atoms are bonded to which atoms changes.

The most common double replacement reactions happen in water solutions. When you mix two ionic compounds dissolved in water, the ions float freely and can meet new partners. If the conditions are right—usually meaning one of the new products is insoluble, a gas, or water itself—the reaction actually happens and you see a visible change, like a solid forming or bubbles appearing.

Key Takeaways

  • In a double replacement reaction, the cations and anions of two compounds swap places to form two new compounds.
  • Write the formulas of both starting compounds, then swap the cations to predict what the products will be.
  • Use the periodic table and a solubility chart to determine the correct charges on ions so you can write correct formulas.
  • Balance the equation by adjusting coefficients (the numbers in front) until the same number of each type of atom appears on both sides.
  • A double replacement reaction only occurs visibly if one product is insoluble, a gas, or water.

Identify the ions in each starting compound

Before you can swap anything, you need to know what ions make up each compound. Every ionic compound is made of a cation (positive) and an anion (negative). Sodium chloride (NaCl) contains Na+ and Cl−. Silver nitrate (AgNO3) contains Ag+ and NO3−.

Use the periodic table to find the charge on each element. Metals in Group 1 (like sodium and potassium) are always +1. Metals in Group 2 (like calcium and magnesium) are always +2. Aluminum is +3. For nonmetals, fluorine is always −1, oxygen is usually −2, and nitrogen in a nitride is −3. Polyatomic ions like nitrate (NO3−), sulfate (SO42−), and carbonate (CO32−) have fixed charges that you need to memorize or look up.

Write down the cation and anion of each compound separately. For calcium carbonate (CaCO3), write Ca2+ and CO32−. For potassium sulfate (K2SO4), write K+ and SO42−. The subscripts in the original formula tell you how many of each ion you need to balance the charges, but for now, focus on identifying what the ions are.

Swap the cations and write the new formulas

Once you know the ions, swap the cations. If you start with sodium chloride (Na+ and Cl−) and silver nitrate (Ag+ and NO3−), the sodium will pair with the nitrate, and the silver will pair with the chloride. Your products are sodium nitrate (NaNO3) and silver chloride (AgCl).

To write the formula of each product, use the criss-cross method. The charge on the cation becomes the subscript on the anion, and the charge on the anion becomes the subscript on the cation. If you're combining Ca2+ (charge +2) with Cl− (charge −1), the formula is CaCl2 because you need two chloride ions to balance one calcium ion. If you're combining Al3+ with SO42−, the formula is Al2(SO4)3 because you need two aluminum ions and three sulfate ions to balance the charges.

Write out the complete unbalanced equation with both products. For the sodium chloride and silver nitrate example: NaCl + AgNO3 → NaNO3 + AgCl.

Balance the equation by counting atoms

Now you have the correct formulas, but the equation probably isn't balanced. A balanced equation has the same number of each type of atom on the left side (reactants) as on the right side (products). You balance by adding coefficients—small whole numbers written in front of each compound.

Count the atoms of each element on both sides. In NaCl + AgNO3 → NaNO3 + AgCl, count sodium (Na): one on the left, one on the right—balanced. Count chlorine (Cl): one on the left, one on the right—balanced. Count silver (Ag): one on the left, one on the right—balanced. Count nitrogen (N): one on the left, one on the right—balanced. Count oxygen (O): three on the left, three on the right—balanced. This equation is already balanced, so no coefficients are needed.

If the equation is not balanced, add coefficients to the left or right until all atoms match. Start with the element that appears in the fewest compounds. Never change the subscripts inside a formula—only add coefficients in front. For example, if you had 2 NaCl + AgNO3 → NaNO3 + AgCl, you would need to add a coefficient of 2 in front of AgNO3 and AgCl to balance the sodium and chlorine.

Check that the products make sense

A double replacement reaction only produces a visible result if one of the products is insoluble (forms a solid precipitate), is a gas that escapes, or is water. If both products stay dissolved in water, the reaction happens at the molecular level but you won't see anything change.

Use a solubility chart to check whether each product is soluble or insoluble in water. Most nitrates, chlorides, and sulfates are soluble. Most carbonates, phosphates, and hydroxides are insoluble. Silver chloride (AgCl) is insoluble, which is why the reaction between sodium chloride and silver nitrate produces a white solid. If you predicted two soluble products, the reaction probably won't happen visibly, and you may need to reconsider whether the equation you wrote is correct for the conditions described in the problem.

Common mistakes to avoid

The most frequent error is getting the ion charges wrong. Double-check the periodic table for single-element ions, and memorize the common polyatomic ions (nitrate, sulfate, carbonate, phosphate, hydroxide, ammonium). If you use the wrong charge, your formulas will be wrong, and everything that follows will be wrong.

Another common mistake is changing the subscripts while balancing. You can only add coefficients in front of compounds. If you write CaCl instead of CaCl2, you've changed the identity of the compound, and the equation is no longer describing a double replacement reaction.

A third mistake is forgetting to balance polyatomic ions as a unit. If you have two sulfate ions (SO42−) on the left side, you must have two sulfate ions on the right side. Don't try to balance the sulfur and oxygen separately; count the whole polyatomic ion as one unit.

Worked example: Lead nitrate and potassium iodide

Lead nitrate Pb(NO3)2 and potassium iodide (KI) react. First, identify the ions: Pb2+, NO3−, K+, and I−. Swap the cations so lead pairs with iodide and potassium pairs with nitrate. Lead is +2 and iodide is −1, so you need two iodides: PbI2. Potassium is +1 and nitrate is −1, so you need one of each: KNO3.

Write the unbalanced equation: Pb(NO3)2 + KI → PbI2 + KNO3. Now balance. On the left: 1 Pb, 2 N, 6 O, 1 K, 1 I. On the right: 1 Pb, 1 K, 1 N, 3 O, 2 I. Potassium and iodine don't match. Add a coefficient of 2 in front of KI and KNO3: Pb(NO3)2 + 2 KI → PbI2 + 2 KNO3. Now count again: left side has 1 Pb, 2 N, 6 O, 2 K, 2 I; right side has 1 Pb, 2 K, 2 N, 6 O, 2 I. Balanced. Lead iodide is insoluble, so a yellow solid forms.

Frequently Asked Questions

What's the difference between a double replacement and a single replacement reaction?

In a single replacement, one element bumps another element out of a compound. In a double replacement, two compounds swap ions with each other. Single replacement: A + BC → AC + B. Double replacement: AB + CD → AD + CB. Single replacement usually involves a metal and a compound; double replacement involves two compounds.

Do I need to memorize polyatomic ions?

Yes, the most common ones: nitrate (NO3−), sulfate (SO42−), carbonate (CO32−), phosphate (PO43−), hydroxide (OH−), and ammonium (NH4+). Your textbook or teacher will provide a list. You can't predict polyatomic ion charges from the periodic table alone.

What if I can't figure out whether a product is soluble?

Your textbook or teacher should provide a solubility chart. If you don't have one, ask. Without knowing solubility, you can't tell whether the reaction actually happens visibly, but you can still write and balance the equation correctly.

Can a double replacement reaction produce a gas?

Yes. If one product is a gas like carbon dioxide (CO2) or hydrogen sulfide (H2S), it escapes from the solution and you see bubbles. For example, hydrochloric acid and sodium carbonate produce sodium chloride, water, and carbon dioxide gas.

Why do I have to use the criss-cross method?

The criss-cross method ensures the charges balance in the formula. If you combine a +2 ion with a −1 ion, you need two of the −1 ions to balance one +2 ion. The method is a shortcut to get the subscripts right without having to think about it each time.