Isotonic Solution: Definition & Example

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  • 0:03 What is an Isotonic Solution?
  • 1:28 A Comparison of Solutions
  • 4:03 What This Does for Our Cells
  • 4:30 Lesson Summary
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Lesson Transcript
Adrienne Brundage
Expert Contributor
Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

An isotonic solution refers to two solutions having the same osmotic pressure across a semipermeable membrane. This state allows for the free movement of water across the membrane without changing the concentration of solutes on either side.

What is an Isotonic Solution?

Imagine you're in the hospital and the nurse hooks you up to an IV. What is in that IV? Is it just water? No way. If you were pumped full of pure water, your blood cells would burst. How horrible would that be? That IV is full of saline, a liquid with the same concentration of solutes as your blood cells. Why is this important? Because you want your blood cells to sit in an isotonic solution.

An isotonic solution is when two solutions, separated by a semipermeable membrane, have equal concentrations of solutes and water. Imagine you're at a party and there are an equal number of guests in the living room and in the kitchen. It doesn't make much of a difference where you stand because you are equally as comfortable in either room. You have just as much space, you can move just as easily, and you have equal access to food. You don't spend a lot of energy trying to get out of one room or into another. This party is like an isotonic solution; everything is equal from room to room.

Now, compare this with a party where the living room is packed full of guests, while there are only a few in the kitchen. I don't know about you, but I would be trying to get to the kitchen as fast as possible. The concentration in each room is different, so people are moving around trying to equal things out. Nature likes equality and that is apparent when it comes to solutions.

A Comparison of Solutions

Let's talk about membranes for a moment. A semipermeable membrane is one that lets some things across while keeping others out. Your blood cells, for example, have a semipermeable membrane. This allows things like water to enter and exit freely, but keeps other things, like fats and proteins, out of the cell. This membrane keeps your blood cells nice and supple and able to do their job without being stuffed full of whatever happens to be floating around in your bloodstream.

The semipermeable membrane is what keeps two solutions apart, allowing only for water to pass through. When we talk about solutions, we can compare two solutions that are separated by a semipermeable membrane. A solution is a mixture of two or more substances, often in liquid form. If you dissolve salt in water, you are making a solution. The salt in this example is known as the solute, the thing dissolved in the water. The more salt, or solute, you add to the water, the higher the concentration of solute you have in your solution.

Water likes to have things equal on either side of a membrane. It doesn't want to have more solute on one side than on the other. So, if it is presented with a situation where on one side of the membrane there is more solute than on the other, water will rush across the membrane and try to dilute the solute until both sides are equal. Basically, water is constantly comparing two solutions to each other and determining if more water is needed on one side or the other.

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Additional Activities

An Egg-cellent Solution

In this lab experiment, students will use eggs to demonstrate different types of solutions including isotonic, hypertonic and hypotonic. To do this lab you'll need one bottle of corn syrup, three cups water, three eggs, saran wrap, one bottle of vinegar, and paper towels to clean up. Students should expect the eggs to behave like cells and change size based on the solution they are in. They can identify which solution is isotonic, hypotonic, or hypertonic based on the final size of the egg. Water will flow in and out of the egg equally in an isotonic solution, more water will flow into the egg in a hypotonic solution, and more water will flow out of the egg in a hypertonic solution.


What does it look like when a cell is in an isotonic solution? What about a hypertonic solution or a hypotonic solution? If you've read the lesson, you probably already have an idea. But, in this activity, we're going to put your knowledge to work by looking at one of the largest types of cells, an egg, in each solution. Follow the directions to carry out the lab, then answer the analysis questions.

  1. First we need to remove the shells from the eggs so water can flow freely into and out of the cell, while keeping the semipermeable membrane of the egg intact. To do this, soak the eggs in vinegar overnight and the shells should easily dissolve.
  2. Next place one egg in each cup and label them one, two and three. Note the size of the eggs in your notebook. You can weigh them, measure the circumference, or make a reference to a similar sized object.
  3. Fill the first cup with tap water.
  4. Fill the second cup with corn syrup and cover.
  5. Mix 2 tablespoons of corn syrup with one cup of water and pour the mixture into the third cup.
  6. Cover the cups with saran wrap and let them rest over night.
  7. The next day, carefully dump out the liquid and examine your eggs.


  1. Which solution was isotonic and how did you know?
  2. Why did some of the eggs change size?
  3. Which solution would be best for the human body? Why do you think that?

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