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My wife and I enjoy cooking. It's fun, and we love flavorful food! But recently, I've noticed I'm more sensitive to onions. When I was younger, they would make my eyes water a little. Now, I can hardly cut an onion without crying. The worst is when I have to chop more than one. I have to leave the kitchen between cutting each onion. Unfortunately, I'm suffering from the effects of diffusion.
What are Diffusion and Effusion?
Diffusion is the movement of atoms or molecules. They migrate from areas of high concentration to areas of low concentration. We would say a substance moves along the concentration gradient. Diffusion happens most quickly for gases. But, it does occur in liquids and solids.
Effusion is the diffusion of a gas through a hole. Gases are very small. So, they can escape through the tiniest holes. For example, if you puncture a balloon with the tip of a safety pin, it will deflate in seconds. Remember, the gas pressure comes from the molecules hitting the container. When we puncture a container the molecule can pass through. From the ideal gas law and Avogadro's law, we know that as the moles of gas decrease, so does the volume. That's why our balloon deflates!
What Causes Diffusion and Effusion?
Gaseous atoms and molecules are in constant random motion. From the kinetic molecular theory, we know that gases are continually colliding with other particles and their container. Let's imagine a system with two gases. They start separated by a barrier. What happens if we remove it? At first, only the gases at the boundary are mixing. However, over time, there is more and more mixing. The random motion of the gases is causing them to move into the other side of the container. Eventually, both gases will have diffused. There will be an equal mixture of both gases throughout the container.
What Affects the Rate of Diffusion and Effusion?
Diffusion and effusion are physical processes. They are affected by anything that changes the particles' movement. The most common factor is temperature. Remember, the temperature is the average kinetic energy of the molecules. So, at higher temperatures, they are moving faster. The faster they move, the quicker they diffuse.
Another thing that affects the rate is the size of the molecule. Larger molecules move more slowly compared to smaller ones. We compare molecules by their molecular masses with Graham's law. We'll study it in another lesson. But, we can see that the rate of diffusion is an intensive property. Why? It depends on the molecules size. So, it is determined by the constituent elements.
Chemistry in the Real World: More Examples of Diffusion and Effusion
Gases:
Candles: When you buy a candle, you get it for the lovely fragrances. When making a candle, a candler combines the wax with perfume. When you light a candle, the heat causes the wax to melt. As the wax melts, the scent vaporizes. As it defuses, your room smells delightful.
Cologne and Perfume: When I was in middle school, a friend and I thought it would be funny to spay ourselves with a whole can of Axe body spray. We smelled putrid! Every room we entered eventually reeked. Cologne's and perfumes work similarly to candles. Your body heat causes some of the scent to vaporize. The vapor defuses, and people think you smell nice. But, people don't like it when you spray yourself with the whole bottle.
The Smell of Dinner Cooking: I love to cook in my crockpot. My favorite thing is the smells that fill my home. As the food heats, it begins to release gases. And soon everyone knows that we'll be having a roast for dinner!
Liquids:
Tea: Right now, I'm drinking a cup of tea. My tea is currently dark red. But, when I first poured the boiling water on the teabag, the water was clear. As I watched, the color seeped from the teabag. For a while, there were two layers of water. One was red, and the other was clear. Eventually, the tea diffused throughout the water. It's completely red. And now, I'm enjoying a nice cup of tea.
Solids:
Whiteboards and Chalkboards: Have you ever noticed what happens when you leave something on a whiteboard? If you wipe it off right away, it cleans easily. But, if you leave it on for a couple of days, it leaves a shadowy impression. The same thing happens on a chalkboard. In the case of the chalkboard, the chalk diffuses into the board's pores. You have to use a solvent to dissolve the chalk.
