Common sense says that water is the coldest. We put ice in our drinks, we have been on frozen ponds or lakes. Of course water is the coldest, isn't it? Thinking a bit more on the subject your mind might venture to the ocean...oh wait, the ocean doesn't really freeze over, except at the North and South Poles...so maybe salt water is the coldest? Then your brain hurts and you quit thinking about it...inquiring minds will look for an answer. While pure water freezes at 0°C (32°F), salt water needs to be colder before it freezes and so it usually takes longer to freeze. The more salt in the water, the lower the freezing point. Very salty water freezes at around -21 °C, or about -6 °F. (Utah Lake is full of salt, but it freezes when it is about 25 degrees F. It must not be salty enough...) While you are waiting for your lake to freeze you can set up this type of experiment...a bottle rocket shot off under the icy layer. (Poor fishies!) A more practical experiment is to pour salt on your frozen sidewalk, or road. Salt will melt snow and ice when the temperature is below freezing. If there's a lot of ice, you need a lot of salt. The essential question is "why does salt water need to be colder than fresh water to freeze"? To answer this question we have to look at the water molecule. Whoa! Did you get a look at those massive Chlorine atoms!! They are huge! Since salt has been added to the water molecule does it change the structure of the water? Instead of a nice crystal like structure: Instead of space between frozen water molecules, the salt water adds more atoms to the structure. There are more bonds to freeze together... There are many more choices. The hydrogen atoms are attracted to the negative Chlorine (Cl) atoms and the oxygen atoms are attracted to the Sodium atoms (Na). So there is more movement, kind of like "pole" dancing between the molecules. Where there is vibration there is heat energy, so the environment has to get colder to freeze the salt water to a stand still. We learned that salt water freezes early on in our history, when people tried to travel across the globe, especially over the North Pole... I have great respect for ice now...
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Mrs. TaylorI love science! Everything about the world is interesting and never boring. I love to study plants, animals, insects, and people. My favorite subjects are my students who are the most unique organisms on the planet! Categories |