Reviewed by: BD Editors

Solution Definition

A solution is a homogeneous mixture of solvent and solute molecules. A solvent is a substance that dissolves another substance by pulling the molecules apart through electrochemical interactions. The solute then diffuses through the solvent until the concentration is equal in all parts of the solution. A solution can be liquid, solid, or gaseous. Further, a solution can be a mixture of liquids, gases and solids. In some cases, like seawater, the solution consists of a great many different types of solutes, like salts, oxygen, and organic molecules.

Types of Solution

Polar and Nonpolar Solutions

A polar solution is created when a polar solvent dissolves a polar solute. The opposing charges on the solvent molecules interact with the opposite charges on the solute molecules, which distributes them throughout the solvent. In a polar solution, the bonds are statically charged, in that they do not change. This is not the case of a nonpolar solution.

In a nonpolar solution, the same principle of opposite charges acting on each other causes the solvent to dissolve the solute. However, nonpolar molecules do not have static charges. Instead, the electrons sometimes group on the same side of the molecule. This negative area pushes the electrons on other molecules away, and creates positive areas of charge. These induced charges move throughout the solution, stirring it and moving the solute molecules about.

Acid and Basic Solutions

In biological systems, the acidity of a solution is important. If a solution is too acidic or too basic, the proteins in a cell will lose their form, and will not function properly. Water is the solvent in most biological systems, and many chemical can change the acidity of water. Cells have many mechanisms to keep the acids and basis of their cells balanced. Cells actively move free radicals (charged molecules) and acidic protons out of the cell when needed. Some cells can also produce buffers, or chemicals that tend to keep a solution at a certain acidity. Scientists use the pH scale to measure acidity, which is a function of the concentration of protons present in water. The more protons, the more acidic a solution is.

Examples of Solution

Nutrients in the Soil

Plants need access to nutrients and minerals in the soil to survive. To get these nutrients and minerals, plants must diffuse the nutrients across the membranes of their roots. In order to do this, the nutrients must be dissolved by water. The solution then bathes the roots, and proteins embedded in the root membranes can transport the nutrient into the cells. Once the cells have the nutrients, more water floods the cells. This mechanism in plants allows water and nutrients to flow all the way from the roots to the top leaves, even in the tallest trees. At the leaves, the plant transpires the water into the air, allowing the osmotic pressure to continue forcing nutrients and water up the leaves. This is all possible because water is an excellent solution which creates the solutions necessary for life.

Flower Nectar

Many flowers produce a solution in their flowers that attracts bees, birds, and other pollinators. The solution uses a solvent of water and a solute of sugar. Water is a polar solvent, and sugar is a polar solute. Together, they make a polar solution. This sugar-water solution provides an easily digestible source of nutrition for pollinators. Bees use the solution to create honey, another more viscous solution used to feed their young. Hummingbirds and some other pollinators simply use the solution for energy. Although it might not seem like much, the bonds of sugar pack an enormous amount of energy. In fact, even the human body relies on glucose for energy. Most of the sugar we eat come from fruits and vegetables, and is in a more complex form that our body must break down.

  • Solute – The molecules that become suspended by solvent in a solution.
  • Solvent – The material or substance that dissolves the molecules of solute into a solution.
  • Polarity – The amount of positive and negative charges a molecule has, and how they interact.
  • Concentration – The amount of dissolved solute per volume of solvent in a solution.


1. You are drinking a soda. In it, there is sugar, water, and carbon dioxide. Which of these are solutes in this solution?
A. Sugar
B. All Three
C. Sugar and Carbon Dioxide

Answer to Question #1
C is correct. The water acts as a solvent in this case, distributing and holding the sugar and water. However, after you open a soda, the carbon dioxide will exist in a higher concentration in the soda than it does in the atmosphere. As such, the CO2 has a tendency to leave the can, making your soda go flat. The carbon dioxide can easily diffuse out of the water, because it is not a polar molecule. The sugar on the other hand exists as a dissolved solid and is a polar molecule, and will stay in the solution until the water is boiled off.

2. If you’ve ever had a fish tank, you know that bubblers are important. A bubbler is a small device that generates bubbles on the bottom of a tank, and lets them rise to the top. While they rise, the bubbles diffuse oxygen into the water. The fish need this oxygen to survive. If the tank water is the solution, what is the solute and solvent?
A. Solvent=H2O , Solute=Oxygen
B. Solvent=Oxygen , Solute= H2O
C. Solvent=Oxygen , Solute=Oxygen

Answer to Question #2
A is correct. The oxygen is a polar molecule that can be dissolved by the solvent, water. While “tank water” is the solution, pure water is the solvent. Tank water includes all the dissolved solutes in the tank, including oxygen, minerals, carbon dioxide, and many others.

3. The concentration of protons dissolved in water is a measure of a solution’s acidity. Scientists developed the pH scale as a quick reference to the amount of protons in a solution. Acids are closer to 0, while basic solutions are closer to 14. Below are the pH numbers of different solutions. Which one has the most dissolved protons?
A. 12
B. 4
C. 5

Answer to Question #3
B is correct. The closer to 0, the more acidic. Therefore, the solution with the lowest number will have the highest proton concentration. Of these, a pH of 4 is the lowest. The solution with a pH of 12 will have almost no free hydrogens compared to the solutions of 4 and 5. However, the solution with a pH of 4 will have many, many more protons than a solution with a pH of 5.

Cite This Article

MLAAPAChicago Editors. "Solution." Biology Dictionary,, 13 Dec. 2016, Editors. (2016, December 13). Solution. Retrieved from Editors. "Solution." Biology Dictionary., December 13, 2016.

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