It’s Not Melting, It’s Dissolving

Time for another post where I get on my science grammar soapbox. Have you ever seen the movie The Wizard of Oz? Everyone has seen that movie, right? Recall the scene where Dorothy throws a bucket of water on the Witch, and the Witch dissolves into a puddle while screaming “I’m melting, I’m melting”? First, don’t ever watch that movie with me. Why? Because every time I see that scene, I scream “you’re not melting, you’re dissolving, get it right.” Ok, it is a movie, a movie that takes place where monkeys fly, there are witches, lions walk and talk like humans, and scarecrows come to life. No, I shouldn’t be looking for realistic science in it. However it drives me crazy that they can’t even get the simple difference between dissolution and melting correct.

In the exact same incorrect way, there is a saying that some people say when their children, dog, whatever, is hesitate to go outside in the rain. “You are not made of sugar, you won’t melt.” There are actually several things wrong with that statement. Sugar, as in table sugar, which is specifically sucrose (as opposed to all the other sugars that exist), does not actual melt at all. At 186°C (367°F), it decomposes to caramel. So even if that saying meant decomposes, if the temperature outside is high enough for sugar to decompose, you have much bigger problems then possibly getting wet. You would die of heat. However, if you were made of sugar, and you went out into the rain, you would not have to worry about melting, you would have to worry about dissolving.

Melting is a physical process where solid turns into a liquid due to heat applied to it. Stick ice into a glass at room temperature. Wait a while. You now have water in the glass. The ice melted into water. But you didn’t apply heat, you might argue. The melting point of ice, the temperature at which solid water, i.e. ice, becomes liquid water is 0°C (32°F). So by simply having ice at room temperature (around 22 °C (72 °F)), heat has been applied to it. The temperature is higher than what the ice needs to stay a solid. Similarly put solid chocolate in a pot and heat slowly to 30°C (86°F). You have liquid chocolate. It has melted. Now don’t waste that chocolate, go eat it with strawberries or cake. [Excuse me for a moment. . .]

Now take that glass of water you made by melting ice at room temperature, and pour just a little salt into it. The salt has dissolved into the water. The water, which is the solvent, has dissolved the salt, the solute, into a solution. When Dorothy throws water on the Witch, the Witch is the solute, the water is the solvent again, and now you have a witch solution in water. Based on the film, witch dissolves quite readily. [It would not matter if she threw boiling water on the Witch, it would still be dissolution because the water is mixing with the witch. The water was quite clearly not boiling anyway.] Other liquids can act as solvents to dissolve solutes, but water is the most common in everyday life. Wiping acetone on nails painted with nail polish removes the polish because acetone, a solvent, dissolves the hardened nail polish, the solute, into a solution. [It is a temporary solution in the sense that acetone readily evaporates, but it forms a solution with the polish long enough to transfer the polish to a cotton ball. The acetone then evaporates leaving behind the polish on the cotton.] An important distinction between melting and dissolving is that melting only involves one substance, water, chocolate, wax, etc. Dissolution involves two substances, water and salt, water and sugar, acetone and nail polish, etc. Dissolution can also involve applied heat, but it isn’t required. There is a much longer explanation for that, and it relates to the solute and solvent and numerous other factors.

To review, melting is one substance changing from a solid to a liquid, and one, and only substance is involved. It is a phase change that must involve a temperature (or pressure) change. Dissolution is one substance becoming part of a solution with a liquid, and two substances are involved. It is two substances becoming one, and temperature change is not necessary for it to happen.

I honestly don’t understand why some people don’t understand the difference. However ignorance of this appears to be wide spread. Evidently the U.S. Special Inspector General for Afghanistan Reconstruction does not know the difference. That Inspector General recently released a report concerning a half-million-dollar U.S.-built police training center in Afghanistan that was so badly constructed that it is literally “melting.” Nope, it is not. It is literally dissolving. If the center had been made of wax, then maybe it might melt. Based on the wording in this article and the accompanying photos, the building is quite clearly dissolving. That is still incredibly appalling construction. As an engineer, I would really like to see the design plans. However, if the Inspector General does not even know the difference between melting and dissolving, then perhaps the Inspector General would do well to have someone on staff who does. It would make for better and more accurate reports.

2 Replies to “It’s Not Melting, It’s Dissolving”

  1. I have to admit to using ‘You won’t melt, you aren’t made of sugar’ quite regularly, even if I do know the difference (as do the people I’m using it on).

    Personally I was far more upset when the movie claimed that Oz was all a dream, when it’s quite clearly established in the book series (one of my favourite childhood reads) that Oz is in fact a real place that Dorothy physically visited on several occasions before permanently going to live there.

  2. Yes, but what about when candy is put in your mouth. It is hot enough to melt it, but it also dissolves in liquid. Is it doing both?