Hypertonic vs. Hypotonic Solutions: Differences and Uses

A solution is a mixture of substances, where one substance (the solute) dissolves in another substance (the solvent). The concentration of these solute particles determines whether you can call the solution hypertonic or hypotonic.

A hypertonic solution has a higher solute concentration when you compare it to another solution or a reference point. Essentially, in a hypertonic solution, there are more solute particles and fewer water molecules.

By comparison, a hypotonic solution has a lower solute concentration when you compare it to another solution or your reference point. In other words, in a hypotonic solution, there are fewer solute particles and more water molecules.

Why is the difference worth noting? Solute concentration plays a vital role in the biological and physiological processes of humans and other organisms. For example, it affects the movement of water and solutes across your cell membranes.

Because of this importance, scientists developed terminology to describe the different types of solute concentrations.

Hypertonic and hypotonic solutions aren't just theoretical concepts. You can encounter both in your everyday life, such as in the following scenarios.

1. In Intravenous (IV) Solutions

Medical professionals carefully formulate IV solutions so they're compatible with your body's natural fluids.

To reduce edema (swelling), they might use a hypertonic IV solution to draw excess water out of your bloodstream and into your cells. On the other hand, a doctor might administer a hypotonic IV solution to increase the total volume of fluid in your body.

2. In Your Blood

Your red blood cells, which transport oxygen throughout your body, are a classic example of how hypertonic and hypotonic solutions affect cells.

If your red blood cells were to meet a hypertonic solution, water would exit the cells, causing them to shrink and become less efficient at transporting oxygen. This could lead to a decrease in your blood pressure and reduce the amount of oxygen your blood cells can carry.

In contrast, if your red blood cells were to meet a hypotonic solution, water would enter the cells, causing them to swell and potentially burst. This could cause the red blood cells to rupture in what's called hemolysis, an obviously harmful outcome.

3. In Plants

Plant cells also exhibit responses to hypertonic and hypotonic solutions. The plant cell wall, which is rigid and surrounds the cell membrane, plays a critical role here.

In a hypertonic solution, water moves out of the plant's cells, causing the cell membrane to pull away from the cell wall. This phenomenon is known as plasmolysis and can lead to reduced turgor pressure (which causes wilting) in plant cells.

In a hypotonic solution, water moves into plant cells, causing them to become turgid and maintain their rigidity, something that's absolutely essential for the health and growth of the plant.

Hypertonic solutions have a higher solute concentration and cause water to move out of cells, potentially leading to cell shrinkage. Hypotonic solutions, on the other hand, have a lower solute concentration and cause water to move into cells, potentially causing them to swell or burst.

There's also a third type of solution, called an isotonic solution. In an isotonic solution, the concentration of solutes is the same as the reference solution, resulting in no net movement of water.

Isotonic solutions help your body maintain its balance of fluid levels. For instance, medical treatments often use normal saline, a solution containing sodium chloride (table salt) dissolved in water.

Normal saline is isotonic to your body fluids, meaning it has a similar concentration of solutes as your blood. This makes it compatible with your cells. If it weren't compatible, it would instead be a hypertonic or hypotonic solution, which would have negative physiological effects.

To better understand how hypertonic solutions affects cells, imagine you have a semipermeable membrane (a membrane that allows certain molecules to pass through while blocking others) separating two containers.

In one container, you have a hypertonic solution with a high solute concentration, and in the other, you have a lower solute concentration, maybe pure water.

When a semipermeable membrane separates these two solutions, the hypertonic solution will exert osmotic pressure upon it. This means that water molecules from the low solute concentration side will move through the membrane into the hypertonic solution, trying to dilute the higher solute concentration.

This movement of water molecules is called osmosis. During osmosis, water moves out of the cells in a hypertonic solution, causing them to shrink. When this happens in your body, it can affect your muscle function and overall cell health.

Now consider you have a container with a hypotonic solution with a low solute concentration, and in the other container, you have a higher solute concentration, perhaps a salty solution.

When a semipermeable membrane separates these two solutions, the hypotonic solution will exert osmotic pressure in the opposite direction, in a process we call "reverse osmosis."

In other words, in a hypotonic solution, water moves into the cells. They gain water, causing them to swell or even burst. This can lead to an increase in turgor pressure. A plant with high turgor pressure, for example, will stand upright or even become rigid, whereas a plant with low turgor pressure will droop.

Osmosis is a passive process; it doesn't require energy to occur. Instead, it relies on the concentration gradient — or the difference in solute concentration between two areas — to drive the movement of water molecules in one direction or another.

Osmosis occurs in cells, but you can also watch osmosis at work in plenty of other everyday situations.

For example, when you soak a raisin in water, it absorbs water and swells, thanks to osmosis. Another example is the process of pickling, which relies on salt and osmotic pressure to draw moisture out of food, preserving it and discouraging the growth of bad bacteria.

In biology, maintaining equilibrium is crucial since the balance between solute and solvent concentrations helps cells and organisms survive. Isotonic solutions, which ensure that there is no net movement of water across cell membranes, therefore play an important role in maintaining equilibrium.

Here are some situations in which you'll find them at work.

This article was created in conjunction with AI technology, then was fact-checked and edited by a HowStuffWorks editor.