Physical or Chemical Reactions: What is going on with Insta-Snow®?

We’ve had a number of questions from teachers about the science behind our popular Grow Snow Classroom kit, so we thought we’d help you take this cool science experience to the next level, from awesome hands-on experiment to dynamic classroom discussion.

The Grow Snow Classroom Kit:  what is it?

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The Grow Snow Classroom Kit contains a special powder called Insta-Snow.  Insta Snow ® is a specially designed substance that belongs to the class of compounds called “superabsorbent polymers.”  It is similar to the substance found in disposable baby diapers, but its special formula was created just for kid’s science kits like Grow Snow.  Insta Snow® absorbs water (lots of water–up to thirty times its weight!) instantly, expanding immediately into a fluffy, white substance that looks just like real snow (it is actually being used in the film industry for winter scenes as a stand-in for the real thing.)  Because the Insta Snow® reaction is so fast (you can’t get any faster than instantly) and so visually impressive (you won’t believe how much it expands) it is ideal for introducing critical thinking and the scientific method to students.  For instance, after your students try the initial Grow Snow experiment, you could ask them questions, help them to hypothesize, and watch them test their theories.  For instance, you could ask your students:

“You’ve seen what happens when we add water to Insta Snow® powder.  What do you think will happen if we add food coloring to the water before we pour the water on the snow?”  ((Result: the “snow” absorbs the dye as well, and becomes colored.)

Or,

“Insta Snow® powder absorbs water, but what do you think will happen if we use other liquids, like oil, soda pop, or salt water?”

The Grow Snow classroom kit paves the way for open-ended experimentation and provides an introduction for a number of science topics, but one question we’ve been asked more than once is,

“Is the Insta Snow® reaction a chemical reaction or a physical reaction?”

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It is an interesting question with an even more interesting answer, and in this blog entry we’d like to show you how to turn this question into a lively classroom discussion that effectively teaches these concepts along the way.

First, a review of the terms:  What are chemical and physical reactions?

Scientists came up with the terms “chemical” and “physical” reaction to describe and distinguish the ways they saw matter changing.

In a physical reaction, the form of matter changes, but the substance itself stays the same.  In other words, an ice cube may melt into a puddle of water, but both the ice cube and the puddle are still H2O.  Some other examples of physical reactions are carving something from a block of wood, squashing an aluminum can, or crushing a large sugar crystal into powder.  In each case, the chemical composition of the substance stayed the same, even if its form changed.  Physical changes can involve:

  • A change in state (such as melting/freezing or evaporating/condensing),
  • A change in shape (such as pulling a ductile metal like gold into a wire, or crushing or breaking a solid)
  • Mixing of two substances (such as sand and salt, or sugar and water)
  • Change in size (expansion)

In a chemical reaction, the fundamental substances change.  Often there is a rearrangement of atoms or a change in molecular bonds.   For instance, if you burn propane fuel, one molecule of propane reacts with five molecules of oxygen to make three molecules of carbon dioxide and four water molecules, as shown below:

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Another example is the old classroom volcano reaction, in which mixing baking soda (a base) with vinegar (an acid) make a salt, water, and carbon dioxide (in other words, lots of “lava” and lots of fun!)

While it sometimes obvious whether a reaction is physical (carving wood) or chemical (burning wood), it can sometimes be a little tricky.  How can you tell them apart?

Scientists have compiled a list of “oftens.”  That is, the two types of reactions often have distinguishing characteristics.

Chemical reactions

  • Often involve the formation of a new substance, like a gas or insoluble solid
  • Are often accompanied by the appearance of light or sound
  • Often involve a temperature change (creation or loss of heat)
  • Often involve a color change

Physical changes

  • Often involve a form change
  • Are often reversible

However, even defining a reaction by these “oftens” can sometimes be a little tricky.  For instance, melting an ice cube (a physical change) is a reversible reaction, since you can freeze the resulting water and regain your ice cube, but grinding a wooden baseball bat into sawdust (another physical reaction) is not reversible (unless you are Harry Potter.)

Gray Areas

Even more interesting are reactions that are gray areas.  For instance, some people would say that dissolving table salt (NaCl) in water is a physical reaction since you can just allow the water to evaporate and get the salt back out, but others would say that it is a chemical reaction, since you are breaking the ionic bonds of the sodium (Na+) and chloride (Cl) ions to each other, and instead forming ionic bonds between sodium ions and water molecules and between water molecules and chloride ions.

The Grow Snow reaction

The Grow Snow reaction is one of the chemical/physical reaction gray areas.  Grow Snow is a flexible lattice, sort of like a big, 3-dimensional net made of rope.  You could think of it like a fisherman’s net.  Associated with the “rope” are lots of ions, including sodium ions.  When a dry Grow Snow crystal meets water, water molecules wiggle into the crystal “net” and bind to the ions.  Like a fisherman’s net filling with fish, the crystal lattice stretches and spreads out to accommodate the water molecule “fish” trapped inside.

Figuring out the classification of the Grow Snow reaction is a little tricky because it has characteristics of both “often” lists.  Like a physical reaction, it is reversible (just let the hydrated “snow” sit out for a while, and you’ll see that the water evaporates, regenerating powder that can then be rehydrated) and involves a form change (expansion.)  However, like a chemical reaction, there is a temperature change (try holding Insta Snow® powder in your hand and pouring water onto it, and you’ll see what we mean)  and involves the formation of new bonds (sodium ions to water.)

So which is it, a chemical or physical reaction?  Even the scientists at Be Amazing have thought of different arguments, but we would probably say that if we had to choose, Grow Snow hydration is more of a physical reaction.  Of course, the most important thing isn’t telling your students the answer, but helping them to learn to think (and talk about) science.

The classroom experience

Demonstrate or let your students experiment with the Grow Snow reaction, and then teach them the terms “chemical reaction” and “physical reaction.”  Write the “often” lists on the board, and discuss some of the more straight forward reaction types.  Once they understand the concepts, ask your students how they would classify the Grow Snow hydration and why.  Encourage them to explain their thoughts, and refer them to the lists to help guide the discussion.  This is a great opportunity to introduce science thinking as well as science facts, and to help your students develop critical thinking skills.

Happy mentoring!

You can find our more information about the Grow Snow classroom kit by clicking here: Grow Snow Blister card

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