A double displacement (or double decomposition) reaction follows the general form AB + CD → AD + CB, where two compounds in solution exchange their constituent ions (cations and anions swap partners) without any change in oxidation state — this distinguishes it from redox-based reactions like displacement or combination reactions.

The most common and reliable visual confirmation that a double displacement has occurred is the formation of a precipitate — an insoluble solid that forms and settles out (or clouds the solution) when two soluble ionic solutions are mixed. This happens because one of the newly formed ion pairs (AD or CB) has very low solubility in water, so it comes out of solution as a solid.

Example: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ (white precipitate) + 2NaCl(aq). Here, sodium and barium ions simply exchange partners; the insoluble barium sulfate is the visible proof that a double displacement (specifically, a precipitation reaction) has taken place. Reactions of acids with bases forming an insoluble salt, or many "test-tube" qualitative-analysis reactions (like testing for chloride, sulfate, or carbonate ions), rely on exactly this kind of observation.

The other observations belong to different reaction types:

• Release of hydrogen gas when a metal reacts with an acid (e.g., Zn + 2HCl → ZnCl₂ + H₂↑) is characteristic of a single displacement reaction, where a more reactive element displaces a less reactive one (here, hydrogen) — this involves a change in oxidation states, unlike double displacement.
• Formation of a single product from two reactants (A + B → AB) is the definition of a combination (synthesis) reaction, not a double displacement.
• Colour change due to displacement of a metal (e.g., a blue copper sulfate solution turning colourless/pale as iron displaces copper) is again a feature of a single displacement reaction, driven by differences in reactivity/reduction potential between two metals, not an ion-exchange process.

Therefore, precipitate formation is the hallmark observation specifically confirming a double displacement reaction.