Scientists Say: Osmosis

This word describes how liquids can move across membranes to equalize the amount of a substance dissolved in them

On the left, there are few molecules per volume of solution; on the right are many more. A somewhat porous membrane separates them. If microscopic holes in the membrane are big enough (but not too big), the liquid will move from left to right — through that membrane. And it will continue to do that until the concentration of the particles per unit volume is equal in the solution on each side of the membrane.  

OpenStax College/Wikimedia Commons (CC BY 3.0)

Osmosis (noun, “Oz-MO-sis”)

The movement of a solution across a membrane — a barrier that blocks the flow of some, but not all, materials. The liquid in the solution will move from the side with fewer dissolved molecules toward the side with more. Movement will continue until the concentrations on both sides of the membrane are the same. Think of a solution low in water and high in salt next to a solution high in water and low in salt. The water will move from the high-water, low-salt solution toward the low-water, high-salt solution. The water will continue this flow until the concentration of salt in each solution is equal.

In a sentence

To get to Mars, astronauts may have to depend on osmosis to purify their pee — so they can drink it again.

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Power Words

(for more about Power Words, click here)

concentration  (in chemistry) A measurement of how much of one substance has been dissolved into another.

dissolve  To turn a solid into a liquid and disperse it into that starting liquid. For instance, sugar or salt crystals (solids) will dissolve into water. Now the crystals are gone and the solution is a fully dispersed mix of the liquid form of the sugar or salt in water

membrane  A barrier which blocks the passage (or flow through of) some materials depending on their size or other features. Membranes are an integral part of filtration systems. Many serve that function on cells or organs of a body.

osmosis   The movement of certain molecules within a solution across a membrane. The movement is always from the solution where the concentration of some chemical is higher to the solution where the concentration of that chemical is lower. This movement tends to continue until concentrations on each side of the membrane are the same.

permeable  Having pores or openings that permit liquids or gases to pass through. Sometimes materials can be permeable for one particular type of liquid or gas (water, for example) but block others (such as oil).

semi–   An adjective meaning “somewhat.”

solution   A liquid in which one chemical has been dissolved into another.

solute  A substance dissolved in another substance, forming a solution.

solvent  A material (usually a liquid) used to dissolve some other material into a solution.

Bethany is the staff writer at Science News for Students. She has a Ph.D. in physiology and pharmacology from Wake Forest University School of Medicine.

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