Source: Precipitation, Rgee’s 6th Grade Blog

What do you think of when you hear the word precipitation? You most likely have learned that rain, snow, and sleet are all types of precipitation.

In chemistry, the word precipitation has a different meaning: instead of something that falls out of the sky, a precipitate is something that falls out of a solution. One of the ways a recipitate can be formed is when two soluble compounds interact. A soluble compound is one that will dissolve in water. Insoluble compounds do not dissolve in water.

The picture below shows two soluble compounds combining to form an insoluble compound, also known as a precipitate. Use the information in the picture to identify if each compound is soluble or insoluble.

Watch the video below showing another example of a precipitation reaction and how to write the chemical equation for a precipitation reaction. In your notes, answer the following questions as you watch the video.

1. What is the evidence that a chemical reaction has happened?

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   The water starts out clear and then the color changes to yellow.

2. Solid KI and solid Pb(NO₃)₂ are placed into water. How does the water help the reaction take place?

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   The water dissolves the compounds, which mean the ions of each compound are separated from each other. The ions are able to recombine and form a new compound, PbI₂.

3. Is KI soluble or insoluble? How do you know?

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   We can tell that KI is soluble because it readily dissolves in water.

4. Is PbI₂ soluble or insoluble? How do you know?

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   PbI₂ is insoluble. We can tell because it separates out from the water. It does not dissolve.

5. What is the evidence that the ions of the reactants “swap places” to form new compounds as is indicated in the chemical equation?

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   The color change indicates that chemical reaction has occurred. In a chemical reaction, the products are different than the reactants, so the ions must have switched places.

The picture below shows the same reaction between KI and Pb(NO₃)₂.

Source: Precipitation Reactions, Prentice Hall

The bright yellow color change makes this a fun chemical reaction to see! Let’s look at the molecular level to understand the chemical reaction better.

Answer the following questions.

1. What does the abbreviation aq that follows the chemical formula mean?

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   Aq stands for an “aqueous” solution which means a solid ionic compound dissolved in water.

2. How do the drawings represent that KI and Pb(NO₃)₂ are dissolved in water?

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   Each compound is shown split apart into its ions. Different sizes and colors of circles represent the different ions.

3. In the final drawing of the products of the reaction, why are NO_3- and K⁺ ions still drawn freely floating in the water and not connected to each other?

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   KNO₃ is soluble in water. The reaction happens in water, so the K⁺ and NO_3- are dissolved in water and not bonded to each other. If the water evaporated, the ions would bond together.