Lesson 4.1 basics of solutions


Lesson 4.1 Basics of Solutions

Suggested Reading:

 Zumdahl Chapter 4 Sections 4.1 and 4.2

Essential Question:

 What are the basic characteristics of aqueous solutions?

Learning Objectives:

Describe the basic properties of aqueous solutions.

Distinguish between the terms solute, solvent and solution

Differentiate between strong and weak electrolytes.

Chemical reactions are the heart of chemistry. Some of the most important chemical reactions involve ions in aqueous (water) solution. Therefore, before we review the major types of chemical reactions we will review the basics of solutions. Solutions can be made from a variety of solvents, but aqueous solutions are by far the most common.

Solution Vocabulary

Solute: a substance dissolved in liquid to form a solution.

Solvent : a dissolving medium in a solution.

Solution: A homogenous mixture.

In looking at the definition above, we can see that any homogenous mixture is a solution, but the terms solute and solvent are only used with liquid solutions.

So why is water the most common solvent?

Water is the most common solvent because it is able to dissolve a lot of stuff.

This ability is the result of the nature of the water molecule.

Recall that water has a bent geometry and consists of an oxygen atom covalently bonded to two hydrogen atoms. However, the electrons in the covalent bonds are not shared equally. The oxygen has a greater pull on the electrons, so they spend more time on the oxygen side of the molecule.

Because of this unequal sharing of electrons, the O has a slight negative charge and the hydrogen has a slight positive charge. Water is said to be a polar molecule as a result.

Polar molecule: a molecule that has a permanent dipole moment.

Dipole moment: a property of a molecule whose charge distribution can be represented by a center of positive charge and a center of negative charge.

It is this polarity that gives water the ability to dissolve stuff. When a an ionic compound is dissolved in water the negative oxygen interacts with the cation and the positive hydrogen interacts with the anions causing the solid to fall apart. This interaction between the water molecules and cations and anions of the solid is called hydration. Hydration leads to dissolution. Nonionic substances with polar bonds are dissolved by hydration as well. However, the water interacts with positive and negative charges resulting from the dipole moment of the molecule instead of cations and anions (see the image below).

Hydration: the interaction between solute particles and water molecules.

Some compounds are more prone to hydration than others. Thus, the solubility of ionic substances varies greatly.

Solubility : the amount of a substance that dissolves in a given volume of solvent at a given temperature.

When a substance is dissolved in water, the ions become dispersed in the solvent and move independently.

Ionic Theory of Solutions

Water that flows from the faucet in not pure, rather it is a solution of water with small amounts of dissolved substances in it. When I distill water for use in your chemistry labs, I am removing these dissolved substances, so they don't participate in the chemical reaction we are investigating. Although pure water is electrically neutral, these substances make the solution an electrical conductor.

Chemists began studying the electrical conductivity of water in the early nineteenth century. In 1884, Swedish chemist Svante Arrhenius proposed the ionic theory of solutions to account for this conductivity. He said that certain substances produce freely moving ions when they dissolve in water, and these ions conduct an electric current in aqueous solution.

Arrhenius submitted his ionic theory as part of his doctoral dissertation to in 1884. It was not well received and he barely passed! In 1903, however, he was awarded the Nobel Prize for his theory

Electrical conductivity in aqueous solutions works like this: When a substance such a NaCl is dissolved in water the ions go into solution as freely moving ions. Now suppose you dip electrical wires that are connected to the poles of a battery into a solution of NaCl. The wire that connects to the positive pole of the battery attracts the negatively charged chloride ions in solution because of their opposite charges. Similarly, the wire connected to the negative pole of the battery attracts the positively charged sodium ions. Thus, the ions in the solution begin to move, and these moving charges form the electric current in solution. We will study this more later.

Electrolytes and Nonelectrolytes

Substances that dissolve in water can be divided into two broad categories, electrolytes and nonelectrolytes.

electrolyte : a substance that dissolves in water to give an electrically conducting solution.

In general, ionic solids that dissolve in water are electrolytes due to the freely moving nature of the cations and anions.Certain molecular substances are also electrolytes. An example is hydrogen chloride gas, HCl(g).Hydrogen chloride gas dissolves in water, giving HCl(aq), which in turn produces hydrogen, H + , and chloride ions, Cl . nonelectrolyte : a substance that dissolves in water to give a nonconducting or poorly conducting solution.

A common example of a nonelectrolyte is sucrose, C






, which is ordinary table sugar. Another example is methanol, CH


OH, a compound used in car window washer solutions. Both of these are molecular substances that are able to dissolve in water. Since molecules are electrically neutral, they do not carry an electric current, so the solution is electrically nonconducting.

Strong and Weak Electrolytes

When electrolytes dissolve in water they produce ions, but to varying degrees.

strong electrolyte : a electrolyte that exists in solution almost entirely as ions.

Most ionic solids are strong electrolytes since they usually dissolve

completely in water. An example is NaCl:

weak electrolyte : an electrolyte that dissolves in water to give a relatively small percentage of ions.

These are usually molecular compounds. Ammonia, NH


, is an example.

When you buy "ammonia" at the store, you are buying an aqueous solution of ammonia. Ammonia molecules react with water to form ammonium ions,



+ , and hydroxide ions, OH .

However, these ions, NH


+ and OH , react with each other to give back ammonia and water. Both reactions, the original one and its reverse, occur simultaneously. We denote this with a double arrow.

As a result of this forward and reverse reaction, just a small percentage of the ammonia (about 3%) have reacted at any given moment to forms ions.

Thus, ammonia is a weak electrolyte. Most soluble molecular substances are either weak or nonelectrolytes. An exception is HCl(g) as noted earlier.