The chemical reaction K₂SO₄ + BaCl₂ ⟶ 2KCl + BaSO₄↓ (aq.) (aq.) (aq.) (solid) is an example of:

Correct — D, Double displacement reaction. Look at what changes partners. On the left the potassium is paired with sulphate and the barium with chloride; on the right the potassium has gone off with the chloride and the barium with the sulphate. Two pairs of ions have simply swapped, which is exactly NCERT's definition: reactions in which there is an exchange of ions between the reactants are called double displacement reactions. The barium sulphate that results is insoluble, so it drops out of the solution as a white solid — which is what the downward arrow in the printed equation and the label 'solid' beneath it are recording, while everything else stays dissolved. That gives the reaction a second, equally correct name: it is also a precipitation reaction, since any reaction that produces a precipitate can be called one. NCERT builds this very equation in Activity 1.10 using sodium sulphate and barium chloride; the paper has swapped potassium for sodium, and nothing else changes, because the driving force is the insolubility of barium sulphate.

• (a)Addition reaction — In an addition reaction two or more species combine into a single product. Here there are two products, not one, so nothing has been added together. Addition is the characteristic reaction of unsaturated carbon compounds, such as the hydrogenation of vegetable oil.
• (b)Displacement reaction — A displacement reaction needs a free element to push another element out of its compound — an iron nail dipped in copper sulphate, where iron takes the sulphate and copper is released as the metal. In the printed equation no element appears free on either side; only ions change partners.
• (c)Decomposition reaction — Decomposition means a single reactant breaking into two or more products. This equation begins with two compounds, so the direction is wrong. Heating calcium carbonate into calcium oxide and carbon dioxide is the standard example of decomposition.

NCERT sorts chemical reactions by what happens to the reactants. In a combination reaction two or more substances form one product; in a decomposition reaction one substance breaks into several; in a displacement reaction a more reactive element pushes a less reactive one out of its compound; and in a double displacement reaction two compounds exchange ions. When one of the products of that exchange is insoluble, it separates as a precipitate, and the reaction is a precipitation reaction as well. Barium sulphate is the standard example of an insoluble product, which is why this pairing appears in every textbook.

The reliable method for a reaction-type question is to count the species on each side before doing anything else. One reactant means decomposition; one product means combination or addition; two reactants and two products with an element free on both sides means displacement; two reactants and two products with everything in compound form means double displacement. Applying the count here settles the answer in a moment. The arrow pointing downwards after BaSO₄ is the standard symbol for a solid that separates out, and the state labels in the printed equation are the paper's own way of telling you which species stay in solution. The insolubility of barium sulphate is also why it is used as the barium meal in X-ray imaging of the digestive tract: barium ions are toxic, but the sulphate is so insoluble that almost none is absorbed.

• NCERT's definition: reactions in which there is an exchange of ions between the reactants are called double displacement reactions.
• Any reaction that produces a precipitate can also be called a precipitation reaction, so this equation carries both names.
• NCERT's Activity 1.10 uses the same reaction with sodium in place of potassium: Na₂SO₄(aq) + BaCl₂(aq) gives BaSO₄(s) plus 2NaCl(aq).
• Barium sulphate is insoluble in water, which is what drives the exchange and produces the white solid.
• A displacement reaction, by contrast, needs a free element: an iron nail in copper sulphate solution displaces copper metal.

• Calling any reaction that makes a solid a displacement reaction; displacement needs a free element, not just a change of partners.
• Overlooking the downward arrow, which is the equation telling you a precipitate has formed.
• Assuming the two names are alternatives — this reaction is both a double displacement and a precipitation reaction.

Usually by printing an equation and asking for its type, or by naming a reaction type and asking for an example, or by asking what the downward arrow in an equation signifies.

• practice — not a real PYQ

  Which one of the following is a double displacement reaction?

  • (a)2H₂ + O₂ → 2H₂O
  • (b)CaCO₃ → CaO + CO₂
  • (c)AgNO₃ + NaCl → AgCl↓ + NaNO₃
  • (d)Zn + CuSO₄ → ZnSO₄ + Cu

  Answer(c) AgNO₃ + NaCl → AgCl↓ + NaNO₃ — the ions swap partners and insoluble silver chloride separates. The first is combination, the second decomposition, and the fourth displacement, since free zinc pushes out copper.

• practice — not a real PYQ

  In a chemical equation, a downward arrow written after a product indicates that the product is

  • (a)a gas escaping
  • (b)an insoluble solid separating out
  • (c)absorbing heat
  • (d)a catalyst

  Answer(b) an insoluble solid separating out — the downward arrow marks a precipitate, while an upward arrow marks a gas being given off.