(POS) What is a Solution? Lesson

What is a Solution?

A solution is a mixture of two or more substances, but it's not just any mixture. A solution is a homogeneous mixture. In a homogeneous mixture, the dissolved particles are spread evenly through the mixture. The particles of the solution are also too small to be seen or to settle out of the mixture. In a solution, the properties are uniform meaning they are the same throughout.

When a solution is said to have uniform properties, the definition is referring to properties at the particle level. What does this mean? Consider brass as an example, which is an alloy made from copper and zinc. An alloy is a homogeneous mixture formed when one solid is dissolved in another. To the naked eye, a brass coin seems like it is just one substance, but at a particle level, two substances are present (copper and zinc). Now consider a handful of zinc filings and copper pieces. Is this now a homogeneous solution? The properties of any scoop of the mixture you are holding would not be consistent with any other scoop you removed from the mixture, so the combination of zinc filings and copper pieces in a pile does not represent a homogeneous mixture.

All solutions have two parts: the solute and the solvent. The solute is the substance that dissolves, and the solvent is the substance that dissolves the solute. Particles of solvent pull apart particles of solute, and the solute particles spread throughout the solvent. Saltwater is an example of a solution. In a saltwater solution, salt is the solute and water is the solvent.   In addition, it is important to know that the solvent is the substance present in the greatest amount. The solute, then, is the substance present in the least amount. For example, when you are making a cup of hot chocolate, you take a teaspoon of cocoa powder and dissolve it in a cup of hot water. Much less cocoa powder is used than water. The cocoa powder is the solute and the water is the solvent.

Many solutes can dissolve in water. In fact, so many solutes can dissolve in water that water has been called the universal solvent. Even some rocks can dissolve in water.  A solute that can dissolve in a given solvent, such as water, is said to be soluble in that solvent. Conversely, a solute that cannot dissolve in a given solvent is said to be insoluble in that solvent.

Although most solutes can dissolve in water, some solutes are insoluble in water. Oil is an example. Did you ever try to mix oil with water? No matter how well you mix the oil into the water, after the mixture stands for a while, the oil separates from the water and rises to the top.

Like salt water in the ocean, many solutions are normally in the liquid state. However, matter in any state can form a solution. An alloy, which is a mixture of a metal with one or more other substances, is a solid solution at room temperature. For example, the alloy bronze is a solution of copper and tin. Matter in the gaseous state can also form solutions.  Air in the atmosphere is a gaseous solution. It is a mixture that contains mainly nitrogen and oxygen gases, with very small amounts of several other gases.  Because air is a solution, it is homogeneous. In other words, no matter where you go, the air always contains the same proportion of gases.

Concentration

Concentration is the measure of how much a given substance is mixed with another substance. Solutions can be said to be dilute or concentrated. A concentrated solution is one in which there is a large amount of solute in a given amount of solvent. A dilute solution is one in which there is a small amount of solute in a given amount of solvent. A dilute solution is a concentrated solution that has been, in essence, watered down. Think of the frozen juice containers you buy in the grocery store. In order to make juice, you mix the frozen juice from inside these containers with about 3 or 4 times the amount of water. Therefore, you are diluting the concentrated juice.

In its most general form, concentration describes the number of items in a given area or volume. The units generally depend on the types of items being counted. For example, if we were to calculate the concentration of people that live in a city, we would divide the total number of people by the area. If we measured the area in square miles, the units of concentration for this measurement would be people per square mile. Or, we could determine the concentration of fish in a lake by dividing the total number of fish by the volume of the lake. If we measure the volume in cubic meters, the concentration would have units of fish per cubic meter.

To express the concentration of a solution, we can perform a similar calculation. The amount of solute can also be measured by mass or by the total number of particles. We can then divide this value by either the amount of solvent or the total amount of solution. These values may have units of mass, volume, or number of particles. Depending on how each component is measured, we get different ways to measure concentration.

For example, the concentration of salt water is measured in parts per thousand. In the diagram below you can see a comparison of the amount of solute (salt) in the given bodies of water. The diagram shows which bodies of water have the most concentrated amounts of salt.

Amount of Solute description Links to an external site.

There are different ways to describe the concentration of a solution, but weight percent is commonly used to describe acids and bases. Weight percent can be calculated by dividing the mass of a solute by the mass of the entire solution. In a chemistry course, the various methods of calculating concentration are learned.

Conductivity

Conductivity is the measure of a solution's ability to conduct electricity. Pure water does not conduct electricity very well.  However, when certain substances are dissolved in water, the solution does conduct electricity. Depending on the type of solute dissolved will determine how well electricity will be conducted.  

Solutions can be nonelectrolytes (nonconductors), weak electrolytes (poor conductors), or strong electrolytes (good conductors). What are electrolytes? Have you heard this term used among athletes?   Millions of people in the world jog for exercise. For the most part, jogging can be a healthy way to stay fit.  However, problems can also develop for those who jog in the heat. Excessive sweating can lead to electrolyte loss which could be life-threatening.  Early symptoms of electrolyte deficiency can include nausea, fatigue, and dizziness.  If not treated, individuals can experience muscle weakness and increased heart rate (which could lead to a heart attack).  Many sports drinks can be consumed to restore electrolytes quickly in the body.

An electrolyte is a compound that conducts an electric current when it is in an aqueous (water-based) solution. In order to conduct a current, a substance must contain mobile ions that can move from one electrode to the other.  All ionic compounds are electrolytes.  When ionic compounds dissolve, they break apart into ions which are then able to conduct a current (conductivity). We will be discussing ions and ionic compounds in detail in the next module but for now, understand that ions are charged atoms. Ions can either have a positive or a negative charge.  

A strong electrolyte is a solution in which a large fraction of the dissolved solute exists as ions. Strong acids and bases have a high conductivity because they produce positive and negative ions readily. Some other polar molecular compounds become electrolytes upon being dissolved into water, but do not ionize to a very great extent.  Gaseous nitrous acid ionizes in solution into hydrogen ions and nitrite ions, but does so very weakly. Aqueous nitrous acid is composed of only about 5% ions and 95% intact nitrous acid molecules.  A weak electrolyte is a solution in which only a small fraction of the dissolved solute exists as ions.  Weak acids or bases ionize only partially so they form solutions with low conductivity.

As you can imagine, a nonelectrolyte is a compound that does not conduct an electric current in either aqueous solution. These compounds when dissolved in water do not produce ions.  Since there are no charges, there is nothing to carry the electrical current.

Interesting Facts about the Roles of Electrolytes in the Body:

Several electrolytes play important roles in the body.  Here are a few significant electrolytes:

• Calcium - in bone and teeth. Also important for muscle contraction, blood clotting, and nerve function.
• Sodium - found outside the cell. Mainly involved in water balance as well as nerve signaling.
• Potassium - major positive ion inside the cell. Important for the proper functioning of the heart, muscles, kidneys, and nerves.
• Magnesium - in bone and cells. Involved in muscle, bone, nervous system, and takes part in many biochemical reactions.

View the video below in which you can watch a simulation that reviews the concepts of concentration and conductivity.

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