MadSci Network: Chemistry
Query:

Re: Endothermic process

Date: Fri Apr 3 18:42:48 1998
Posted By: Kevin Tuttle, Undergrad student, Biology, Georgia Institute of Technology
Area of science: Chemistry
ID: 887924007.Ch
Message:

Could you please give an "elementary" explanation of what an endothermic process is. Remember, we are just grade 4/5/6 students. What is the reason that salt is needed in snow to make ice-cream? We know that the milk/sugar/etc. mixture freezes after snow and salt layers are put around the mixture. We also know that the snow gets almost to cold to handle, but what is the scientific explanation for this?

We also know that salt is put on slippery icy surfaces to melt the ice. How is this related to the same scientific explanation as when making ice-cream?
Certainly, but I have a preface: I will try to keep my explanation simple, but I probably fail at times, so it might be best for your teacher to read this and put it in his/her own words, rather than read it verbatim.

An endothermic reaction is any reaction that requires energy to proceed. For instance, freezing water into ice is exothermic as it gives off energy in the process (the freezer is cold in order to "bleed" energy off of the water and convert it to ice). On the other hand, letting the ice sit on the counter causes it to return to water. This is an endothermic reaction in that it requires energy.

Although heat and energy are different, for elementary school it can be convenient to treat them similarly. So think of it this way, if something is hot it means it has a lot of energy. Conversely, something cold has little energy. So, ice has less energy than water and therefore energy is required to convert ice to water (an endothermic reaction).

Energy always moves from an area of high energy (hot) to an area of low energy (cold). In doing this, it equalizes both objects' heat to a temperature in between. So, when ice melts on the countertop, the atmosphwere -- which is relatively hotter than the ice -- donates some of its energy to the ice. As the energy in the ice increases, it becomes water. Now, the air loses some heat in the process, however all of the air molecules in the area donate some of its energy to the colder area which makes the energy change in the air imperceptible.

Ok, as to the salt. This is my interpretation of it, anyway. If you take a glass of ice water and isolate it from everything -- not allow it to absorb any heat from the air (not possible in real life, this is just a thought experiment) -- it will remain in an equilibrium. That is, some water will be continuously freezing while an equal amount of ice is melting. Therefore, there will always be an equal amount of ice and water.

If you add some salt to this mixture, it has the effect of lowering the freezing point of the ice water. For instance, while the melting point of water is usually 0C (32F), adding salt may make its melting point -2C. Since the ice and water are at equilibrium, it is at the melting point that represents the line between ice and water. However, now the salt has lowered the melting point, therefore the temperature of the ice water drops. This has the effect of making a solution of ice and water which is colder than it would be normally. This extra-cold solution is important to making ice-cream.

Now, this is different for just ice (no water) exposed to the air, like you find on your sidewalk on a cold day. Well, you didn't give a location so I can't say for SURE that you see ice on a cold day, but let's pretend you do. Putting salt on ice melts it. This may seem inconsistent since we just saw that salt makes an ice and water solution colder. However, think of it this way: if you add salt to ice, it loweres its melting point. So, let's say it's 0C outside -- just cold enough to freeze water. Let's say the ice is also that cold. Now, if you add salt to the ice all of a sudden its melting point becomes, say, -10C. So, the air outside is warmer than the ice's new melting point and therefore, the ice melts. This is why you use snow with the salt rather than solid ice. The snow melts easier to water and allows an equilibrium to form.

I hope I didn't overindulge in jargon here, but if you need more information feel free to submit another question.


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