Happy Easter peeps! Did you know Peeps are ideal for demonstrating ideal gas laws?
note: this is adapted from a past post, hence the old pictures from a couple of years ago. Also, if you want a more detailed explanation of the ideal gas laws, there’s a link to my more serious post at the end. But today, I wanna just do a shorter, Peeps-focused post. so, here ya go!
If anyone’s not familiar with Peeps, they’re just marshmallows in the shape of chicks. And they get really popular around Easter. Content-wise, marshmallows are sugar, water, corn syrup, and gelatin, whipped together to trap in air. Marshmallows get their “original” size by whipping with air(a mechanical process), getting locked into place by the gelatin. note: gelatin is a mix of polypeptides (protein strands) that come from chopping up the protein collagen (the same protein that gives your skin strength). Even though the air is trapped in your marshmallow, you can get it to try to escape by heating it up. Heating gives the molecules more energy, which makes them try harder to escape. The molecules start banging more forcefully agains the mallow trap. They can’t get out, but they do “deform” their walls, pushing them out, and making the marshmallow expand. And you get more molecules to join – trapped liquid water molecules in the Peep also gain energy and, as I’ll get into, these water molecules want to be gasses too, so they join in.
You’re dealing with 3 states of matter in these chicks – solid (sugar & gelatin), liquid (water trapped in & keeping things moist), & gas (the air molecules trapped in pockets). The difference between these states of matter is how much energy the molecules have & whether that’s enough for them to overcome the attractions from nearby molecules. All molecules are made up of units called atoms (individual carbons, hydrogens, oxygens, etc.) and those atoms are made up of even smaller parts called subatomic particles, which include positively-charged protons and neutral neutrons hanging out in a dense central core called the atomic nucleus, and negatively-charged electrons which whizz about that nucleus in an electron cloud. Imbalances in the number of electrons compared to protons and/or relative location of electrons within the cloud (e.g. a bunch of electrons spending most of their time in one area of the cloud) can lead molecules or parts of molecules to have full or partial charges. Since opposite charges attract, this can lead molecules to stick to one another.
more here: http://bit.ly/frizzandmolecularattractions
Even in solids, molecules are vibrating, trying to break free. If they can’t, they’re stuck in place, bound to the molecules they can’t escape. If they get some more energy, then can slide around, breaking off some bonds but quickly getting grabbed by other molecules. Only in a gas do the molecules have enough energy to fully break free. And since they’re free, they’re no longer as influenced by the molecules around them. Therefore, gases act a lot alike, no matter what the molecules they’re made up of are.
This has huge implications because it allows us to describe and predict the properties of any gas just by knowing a few things:
- how many molecules of gas there are (n) (measured in a quantity called the mole (mol), which is a value like a “dozen” but that means 6×10²³ things instead of 12 http://bit.ly/dimensionalanalysising)
- the temperature (T) – the higher the temperature, the more energy the molecules have and thus the faster they’ll move around – in these equations, we use temperature measured in Kelvins (K). The Kelvin system is the “same” as Celcius, but “zeroed” so that 0 K is “absolute zero” – you can convert from C to K by adding 273.15
- the volume (V) – the smaller the volume, the more the molecules are likely to bump into one another or into the walls of the container (volume is usually in L)
- the pressure (p)(usually in units called atmospheres)
These properties are interrelated, and we can describe the relationship mathematically with the ideal gas law: pV = nRT
We saw most of those letters above. The one newbie is R, which is just a constant called the “ideal gas constant,” – 0.0821 L × atm × mol⁻¹ × K⁻¹
There are different names for different arrangements of the variables:
Boyle’s law: P₁V₁ = P₂V₂
- says, if you keep temperature constant, pressure & volume are inversely related (one gets bigger as the other gets smaller) -> if you squish the same amount of stuff together more, you compress it, so it takes up less space
Charle’s law: V₁/V₂ = T₁/T₂
- says, if you keep pressure constant, gas expands when heated because the molecules have more energy to move around and venture farther away from one another -> take up more space
Avogadro’s law: V∝ n (volume is directly proportional to the # of molecules)
- if temp & pressure are the same, the same volume of gas will have the same number of gas molecules, no matter what those molecules are (you can only fit a certain amount of gas molecules into a certain volume -> to add more you have to increase the volume)
- if you increase the number of gas molecules (n), you increase the volume (V) -> because gas molecules can move farther apart from one another than liquid or solid molecules, the same number of gas molecules will take up more room than that number of solid or liquid molecule
Gay-Lussac’s Law-> P∝ T (pressure is directly proportional to temperature)
- at a constant volume, pressure & temperature are directly related -> higher temperature means molecules are more likely to run into each other and walls of container, putting pressure on them
When you microwave a Peep, it gets a lot bigger, demonstrating Charles’ law (the trapped air molecules heat up and start banging against their gooey case, pushing them out, and Avogadro’s law – water molecules enter the gas phase, so you add more moles of gas (increase n). When you remove the heat, the Peep shrinks back down (Charles’ law). And it hardens because you’ve evaporated out that water that was keeping it moist.
If you want some Peeps humor, Atlanta’s Emory University has a really funny website http://peepresearch.org/ . Also check out https://students.millikin.edu/~acs/HTML/peep_revenge.html
If you’ve been following along, this might remind you of our discussion of bread leavening. In bread, the bubbles are made “in place” by yeast as byproducts from fermentation, and locked into place by gluten proteins https://bit.ly/leaveningagents
Happy Easter everyone!