Unit 2: Advanced Basics

Section 1: Precipitation Reactions

• Introduction
• What is A Precipitation Rection?
• Selective Precipitation

Introduction

In Chemical Reactions there are many different types of reactions, but for our purposes there are three different types of reactions. Section 1 deals with Precipitation reactions. Section 2 deals with acid/base reactions. Section 3 deals with oxidation/reduction reactions.

What Is A Precipitation Reaction?

A precipitation reaction occurs when two solutions are mixed together and a solid seperates from the solution. The solid part that forms and seperates form the solutions is called the precipitate. But what is the precipitate that forms? To understand what the precipitate is that forms, we first must understand the basic rules and how to apply those rules.

First, the rules that we must discuss are the rules of solubility of salts in water. Solubility of salts is a fancy term for the ability for substances to ionize in water. For example, NaCl when placed in water will break up into Na⁺ and Cl^-. But precipitation occurs when salts DON'T ionize in water, so we are using the rules of solubility of salts to determine what will NOT ionize in water.

The second important point that must be made is how substances behave when dissolved in water. When salts are dissolved in water, the salts break up into ions and then move around completely independent of each other. For example, NaCl, when placed in water will break up into Na⁺ and Cl^- and the Na⁺ and Cl^- will move around in the water independently. The point of this is to understand that when two substances are dissolved in water, the substances must be thought of as ions moving around, not substances moving around. For example, AgNO₃ and KCl, when placed in water produce Ag⁺, NO₃^-, K⁺, and Cl^-.

So now that we understand the rules of solubility and what happens when salts are placed into water, we can begin to understand why precipitation occurs. Precipitation occurs because two SOLUBLE substances ionize and then the ions form a substance that is INSOLUBLE. The insoluble substance is the precipitate that is formed.

Selective Precipitation

Selective Precipitation is basically a processs to seperate ions out of a solution by creating precipitates. For instance, if there was a solution containing Ag⁺ and Fe⁺³ ions, NaCl could be added to this solution and AgCl would precipitate out and the NaOH could be added and Fe(OH)₃ would precipitate out. (It is important to understand what selective precipitation is, but don't worry about how to use it.)