Is it dangerous to eat an orange on a hot air balloon?

What a strange question. I mean, why would I even ask that? Let me contextualize it a bit more. Please watch the following video before you continue reading.

Now that you’ve watched the video, I’ll ask my question again: Is it dangerous to eat an orange on a hot air balloon? Think for a bit about it before you continue reading. A few probing questions to get you started:

  1.  Are party balloons and hot air balloons made of the same material?
  2. What about the liquid in an orange peel makes the balloon pop?
  3. What’s really happening to the balloon?

Let’s dig a little deeper!

Explanation of Demo

To understand why this happens, we need to think about the properties of these two substances. Latex is a hydrocarbon polymer, meaning it consists of long chains of atoms containing only Carbon and Hydrogen atoms. Oranges have an essential oil in them called limonene, which is also a hydrocarbon. The chemical makeup of limonene is included to the right. Don’t worry about the structure – but notice how the only two elements in this structure are carbon (C) and hydrogen (H).

Cool, but why does any of that matter?

To answer that, let me ask another question that you might be more familiar with: Why does salt dissolve in water? If you’ve ever swam in the ocean, you are probably familiar with the fact that oceans are HUGE bodies of salt water. But the salt that we sprinkle on our steak dinners looks like a bunch of crystals, not like the water we swim in. So…how does that happen?

Our answer to that question lies in the properties of matter, or what makes salt (and water) the way they are. Salt is made up of ions, or charged atoms, that attract opposite charges. Our table salt is made up of particles that have repeating positive and negative ions (Na+ and Cl–) that attract each other like the poles of a magnet. This is why people often use the term “opposites attract“.

Image result for salt ion

To the right is a scientific model of water and salt – this is often what scientists use to represent atoms and molecules because they’re too small for us to see. Notice how the water (blue and red) has positive and negative charges. We call substances that have these positives and negatives polar, or charged, substances. Notice how the salt (grey and yellow) also has the exact same combination of positive and negative substances. The salt (NaCl) breaks apart into its positive and negative ions – when this happens, we say the salt has dissolved (or separated) in the water. Salt dissolves in water because the positive parts of water attract the negative charges in the salt, and the negative parts of water attract the positive charges in the salt, again, just like magnets. This can be seen in the diagram below.

Unfortunately, that can’t directly help us explain why the balloon pops. BUT it does tell us that a charged (or polar) substance like water can dissolve other charged substances (like salts). You may have heard the expression “like dissolves like” before, and if you haven’t, then it will be really important to understanding why the balloon pops.

Back to our demo: The latex in rubber isn’t a charged material – it is actually uncharged. When something doesn’t have a charge, we call it nonpolar (“not charged”). The oil in the orange peel and the latex in the balloon are both nonpolar substances (remember – they are both “hydrocarbons”!) Since “like dissolves like”, we know that the oil from the orange can dissolve the latex in the balloon. When the oil dissolves the latex, the balloon weakens and pops!

balloon.gif

Answering My Initial Question

Okay, so we know the following so far:

  1. Latex is a hydrocarbon. Limonene (the oil in an orange peel) is also a hydrocarbon.
  2. Hydrocarbons are nonpolar, meaning they do not have a charge.
  3. Salt is a charged substance, and so is water.
  4. Like substances dissolve like — so salt dissolves in water, and latex dissolves in limonene.

Our next question becomes — what is a hot air balloon made of?

Hot air balloons have to be A LOT stronger than the balloons we use at parties, or else they wouldn’t be able to lift people in the air and would be WAY too easy to pop! Instead of latex, hot air balloons are made of nylon, which is a much stronger material. Nylon is a polymer (just like latex), but it has atoms in addition to carbon and hydrogen that make it polar (charged). Thinking about the oils in our orange again, we know that is nonpolar (not charged), so the oils in our orange peel will NOT damage the nylon in our balloon. And thankfully, something like water (that is polar) is too weak to dissolve nylon – or else a small rain shower would be incredibly dangerous!

To answer our initial question: you are 100% safe to peel that orange in your hot air balloon!

One Step Further

As an NGSS-oriented science educator, I grapple with the implementation of this type of phenomenon in a middle-school chemistry unit. Analyzing the phenomenon of a popped balloon is pretty cool. Extending that to consider how that system transfers to analyzing a larger-scale balloon is also pretty cool. But we, as teachers, could take this one step further to create a 3D unit out of this phenomenon.

One way to extend this unit beyond asking my initial phenomena-based question is to ask more questions. Ask students what material could dissolve a nylon balloon, as well as in what situations a hot air ballooner might encounter such a substance. Ask if a hot air balloon might be more efficiently made with a different material – why was nylon picked in the first place? What are the properties of nylon? Where else do students encounter nylon? Might those contexts help us to explain why nylon is used in hot air balloons?

Once further probing questions are identified, have students investigate those questions. Experimentation is fueled by greater purpose, which is most often identified through providing a genuine solution to a problem that matters to them. Exploring and explaining the physical world is naturally motivating due to its immense explanatory power; providing students the opportunity to investigate, to manipulate, to discover, to create, to explain – this provides the footholds that position students on the verge of authentic scientific investigation.

Connection to NGSS and NYSSLS

Of course, as a future science teacher, we must remain informed about the standards our instruction must follow. The Next Generation Science Standards (NGSS) contain standards for learning across K-12 scientific disciplines; as I will be teaching in a 7th grade chemistry classroom starting next Monday, I will focus on the middle school physical science standards (MS-PS). These standards guide 21st-century learning that involve students in more inquiry-based methods of teaching.

My original question and the accompanying demo revolve around MS-PS1-2 in the New York State P-12 Science Learning Standards (NYSSLS), which states the following:

Students who demonstrate understanding can…analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred (p. 28).

Specifically, my topic revolves around the physical property of solubility, or the ability of a substance to dissolve another. Interestingly in this demo, a chemical reaction did not occur – solubility depends on the physical property of the substances. By the end of this lesson/unit, students should be able to tell that solubility is a physical property and, no matter how exciting a popped balloon may be, dissolving the balloon’s latex is not a chemical reaction.

While that standard has its purpose, that analysis must be motivated by a greater purpose. We must consider our standards, of course – but, just as is the case with learning, those standards require context not provided in the framework. Our job as educators is to position those standards in a context that matters to our students and to ourselves.