Sometimes chemistry blows up in your face

Perhaps I am being a bit dramatic with the title of this post...

For each chemistry lesson, I have tried to include three components:
1. Chemistry content
2. Practice with the scientific method in general, and with one aspect of it in particular
3. Something edible to make the food aspect of this unit practical, and not just baking-soda-and-vinegar-bubbly. I don't like the idea of wasting food as we learn something about science.

Let's check my track record so far:

• Our rock candy crystallized, but not on the sticks. It's still lurking on the shelf next to my sink.
  
• After we finished straining the applesauce, no one wanted to touch, let alone, eat it.
• Kids enjoyed eating frozen apple juice on a stick.
• Homemade salad dressing was a hit.
• Kids reacted (pun intended) as if fizzy lemonade was the coolest thing they'd ever seen and tasted.
• We never got around to eating the wrapped up potatoes and apples that didn't oxidize. And the ones that did oxidize...
  

So far, I seem to be batting .500 on the experiment edibility charts. What's your guess about the caramel sauce we tried to make this afternoon?

The question was whether sugar, when heated on the stove, goes through a physical or chemical change. Kids were split when they wrote their hypotheses, with a couple saying both type of changes would take place. The scientific method focus for the day was writing a procedure. As I did this demo, kids took notes on what materials I used and what steps I took. I collected these and will assess them in the next day or two and get feedback to those who are not quite on track with this skill.

Here's what happens when you heat sugar: first it melts into a liquid (its melting point is over 300 degrees) - physical change. Then it starts to change color -- chemical change -- and you want to take it off the heat while it is a light, straw color. The longer it heats the more the sugars break down and if it gets too dark, it won't taste sweet at all. (An interesting side note is that this dark, used-to-be-sugar compound, can be used to darken stews and gravies.) Once off the heat, add water in a 1:1 ratio with the amount of sugar you heated. Do so carefully! The sugar liquid is hot and will spatter if you pour haphazardly. Now you heat the mixture to boiling, boil for several minutes, take off the heat, and serve as a caramel topping. It's not as rich as caramel sauce, which is usually mixed with butter and cream, but it has a warm, sweet taste and a maple syrup consistency.

I have to mention that I did this experiment three times at home in the past couple of days, tweaking the process. The first batch got way too dark and hardened into a solid black sheet in the pan, presumably un-sweet. The second batch was the right color but I boiled it too long and I burned the roof of my mouth trying to eat it before it hardened to glass. I wanted a topping that would pour after if cooled so I didn't end the school day sending half the class to the school nurse with first degree burns on their tongues and palates.

Of course my efforts at home only demonstrated to me what should happen! In the classroom, the sugar took forever to heat and melt for two reasons. The plug-in burner is tediously slow to warm up and I was trying to make a larger batch than I had made at home. More sugar means less heated surface area to make contact with in the pan.

It's not very interesting to watch your teacher stir a bunch of sugar in a pan for 10 minutes.

Finally the sugar melted but the liquid clumped to the unmelted bits and it started to change color before it was all liquid. I guess physical changes and chemical changes can happen simultaneously when there's that much sugar.

You can't holler, "Hey sugar crystals! Can you guys wait until everyone's melted before you engage in a chemical reaction?"

The lesson continued on in super slo mo compared to my practice trials.

When you add water to clumpy sugar, the clumps don't dissolve right away...

...but eventually they do, and then you just have to wait another eon for it to boil.

Kids finished their procedures, wrote observations, cut up apples, but also got silly and noisy. In the meanwhile I watched the clock tick along, knowing there was no way we'd make it to tech lab by our scheduled time. Thankfully, Frank in the tech lab is an uber-flexible guy about things like this.

The happy ending is that they all loved the caramel sauce. A dozen apples were devoured in no time and they walked to the tech lab, smiling and licking their fingers.