What is a Precipitation Reaction? (Definition & Examples)

If you mix two perfectly clear, transparent liquids together in a test tube, and suddenly the liquid turns cloudy and a powdery solid material sinks to the bottom, you have just witnessed a Precipitation Reaction.

Question (Click to Flip)

What is the liquid left above the precipitate called?

Answer

Once the heavy solid precipitate sinks to the bottom of the test tube, the clear liquid remaining on the top is scientifically called the Supernatant liquid.

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Key Facts

Precipitation reactions are extremely important in environmental engineering. Water treatment plants use massive precipitation reactions (adding alum) to force toxic heavy metals dissolved in dirty water to turn into solid chunks, which can then be easily filtered out.

The Scientific Definition

A Precipitation Reaction is a chemical reaction that occurs in an aqueous solution (water) where two soluble salts react to form an insoluble solid product. This insoluble solid is known as a 'Precipitate'.

It is almost always a type of Double Displacement Reaction, where the ions of two compounds instantly swap partners.

Famous Example 1 (The White Precipitate)

The most common example taught in Class 10 involves mixing Barium Chloride and Sodium Sulphate. BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) ↓ + 2NaCl(aq)

What happened?: Both starting liquids were clear. But when they mixed, the Barium and Sulphate ions joined to form Barium Sulphate (BaSO₄).
Because Barium Sulphate cannot dissolve in water, it instantly appears as a dense White solid (Precipitate) and sinks to the bottom. (The downward arrow '↓' indicates a precipitate).

Famous Example 2 (The Yellow Precipitate)

Another visually stunning reaction is mixing Lead Nitrate with Potassium Iodide. Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) ↓ + 2KNO₃(aq)

Here, the newly formed Lead Iodide (PbI₂) is highly insoluble in water and instantly forms a very bright, beautiful Yellow precipitate.

Questions and Answers

What is the liquid left above the precipitate called?+

Once the heavy solid precipitate sinks to the bottom of the test tube, the clear liquid remaining on the top is scientifically called the **Supernatant** liquid.

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