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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
Instructor: Adrienne Brundage
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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