How to Explain Permeability

Low permeability of skin cells keeps in what needs to be inside and keeps out what needs to be outside.
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Permeability of a material is the ease with which fluids or molecules can migrate through it. You can explain permeability using examples that will demonstrate what it is, why it's useful to understand it, and what can change it. The permeability characteristics of various materials are essential in many science and engineering fields, so you can target examples from a specific field. Hands-on demonstrations or experiments are fun ways to help explain permeability.

You Don't Want a Permeable Roof

Cell membranes in biology, soft-drink bottles in the food industry, and rock layers and soil in geology all offer examples of ways the permeability characteristics of materials are essential and useful in our lives. Many uses of permeability characteristics are tied to the ease or difficulty with which water can pass through a material; this makes water a useful example fluid for explaining or demonstrating permeability of different materials.

Permeability and Water Management

You can use several common examples to show ways we use permeability or impermeability of different materials as we capture and use water, direct it, or fend it off. Imagine living in a desert area near the ocean, where drinkable water may be an issue because salty sea water permeates the soil and enters the groundwater supply. We can use permeability characteristics of filtering materials to remove salt and impurities from water through reverse osmosis. Impermeability of plastics is useful when we buy drinking water and carry it home. Partial impermeability of our own skin cell membranes allows our bodies to keep water where we need it.

Permeability is Relative

Permeability is all about the interaction between a material that acts as a barrier and molecules, whether liquid or gas, that come in contact with it. Permeability isn't an unchanging property, like the temperature at which water freezes or boils; it depends on the materials that are interacting. Water molecules may not be able to get through a material that a gas can diffuse through easily. You can choose examples to demonstrate important characteristics and explain why the materials interact the way they do.

You Knew Glass Beats Plastic

You can use plastic and glass bottles to compare permeabilities of different barrier materials used to contain a specific fluid. The carbon dioxide that gives soft drinks their fizz can diffuse out of plastic bottles over time, leaving the drink flat. Glass bottles don’t allow diffusion. Different barrier materials demonstrate different permeabilities to the same fluid.

Balloons Will Let You Down

Balloons are great for showing how one barrier material interacts differently with different fluids. Balloons are somewhat permeable to both helium and water, but at different rates. Fill a balloon with helium; it will deflate within a day or two. Water balloons can last longer, as long as you don’t throw them or sit on them. The permeability of a barrier material depends on what is trying to pass through it.

Permeability Isn't Permanent

Factors such as changes in temperature or pressure, thickness of the barrier material, and whether the barrier has pores, all can change how easily a fluid can pass through. You can use flooding as an example of how altered conditions can alter permeability. If water has saturated a normally permeable soil and more rain falls, the soil will be temporarily impermeable; water will collect on the surface and runoff will increase. If it rains too much, bring out the kayak. It's reasonably impermeable if you've maintained it.

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