Marshmallows contain sugar, water, and gelatin and possibly other ingredients for flavoring and sweetening. The gelatin is soaked in water to form a gel and whipped to form the fluffy marshmallow.Gelatin is solid at room temperature; water is liquid. A gel is a mixture of a solid and a liquid (soft contact lenses are an example of a hydrogel). When you put a marshmallow in the freezer, the water freezes even though it is part of the gel, causing the marshmallow to become hard. When you heat it up again, the water melts and the marshmallow softens. However, being mixed with the gelatin should decrease the freezing point of the water so you have to get it colder than usual to freeze the water. Normally water freezes at 0 degrees C(or 32 degrees F); however when it is mixed with other chemicals, the hydrogen bonding network necessary to form ice is disrupted. This causes the colligative property known as freezing point depression.

The marshmallow, like many things, contains water.When the water freezes, all of the water molecules stop moving around and just "hold hands" and vibrate in place. The object feels hard to us. As it heats up, the molecules "let go" of each other and move around, making the marshmallow soft.

Not everything goes back to normal after the water inside it freezes. If you have ever frozen raw fruit, you have probably seen that it gets all mushy when it thaws. That's because the ice takes up more space than liquid water. It bursts the cell walls that gave the fruit its structure. So when the ice melts, the fruit is all squishy.

If you melted the entire marshmallow (not just the ice) do you think it would go back to its old shape? Why or why not? If you try this yourself, be careful, hot marshmallows can burn you.

The viscosity of a material is a measure of its resistance to flow. The viscosity of most materials increases as the Temperature goes down. This is because the viscosity is related to how quickly the atoms making up a substance are vibrating about their equilibrium positions. So when the T falls, then the atoms vibrate more slowly and that is ultimately related to INCREASING the materials resistance to flow.

Marshmallows are in a semi-frozen state at room temperature. Heat them up further and they melt (this is why the interior of a marshmallow turns to goo in a fire, while the outside singes and turns brown). Cool the marshmallow down in a freezer and it goes from being semi-frozen to fully frozen, and return it to room temperature, and it becomes semi-frozen again.