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TBR GChem Mistake Regarding Heat Capacity, Hope this helps

Discussion in 'MCAT Study Question Q&A' started by Chrisz, 05.17.14.

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  1. Chrisz

    Chrisz 2+ Year Member

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    Hi, I been wondering why heat capacity increases after salt being added to pure water for a while. Based on the graph (page 89) provided by TBR, it also shows the heat capacity of water decrease after salt is added. The slope is steeper compared to the graph before salt is added. However, TBR says salt salvation increases heat capacity. In several passages, it also states heat capacity increases after salt is added. Today, I found a web page posted by an associate chem professor. In the page, he says salt lowers heat capacity and temperature changes at a faster rate compared to pure water. He says heat capacity for pure water is 4.18 while salt water is 3.93. He says the heat capacity is lowered due to disruption of the structure of water.

    Here is the original webpage, if interested.
    http://www.madsci.org/posts/archives/2012-06/1340974447.Ph.r.html

    It is quite short. I think it would be helpful.
     
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  3. Czarcasm

    Czarcasm Hakuna matata, no worries. 2+ Year Member

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    Was this information presented in a passage or in the content material? It's odd they'd introduce the topic with that extra information. I like to think of heat capacity as unique to every substance at a specific phase. Sure things can deviate from the norm (ie. adding salt), but those peculiarities would be addressed in a passage. I think trying to memorize each one of those peculiarities would be counter productive, but focusing on understanding how to incorporate the new passage info to answering the questions would be of better use of your time. I wouldn't stress too much on who is right or wrong. If the passage is telling you one thing, but you know another, then definitely go with what the passage tells you. At the end of the day, as long as you know what heat capacity is, what it's used for, and the relevant equations, then you'll be alright. Anything more won't help much.
     
  4. Chrisz

    Chrisz 2+ Year Member

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    Actually, both in the content material and the passage. I think the TBR made up the passage itself. That is why we see the same mistake in two places. To be honest, I deeply believe it is a mistake on TBR's part. I deeply doubt that MCAT would make the same sort of mistake. Think about that. If you use TBR for review and it tells you a mistake, it does not necessarily mean the test maker would make the same sort of mistake. Yes, MCAT does provide some info in a passage, but it does not necessarily mean you dont need an inventory on your own. One more thing to clarify, I am not a big fan of memorization. Once I feel something is not right, then I would like to keep finding a second source (a reliable source) to see if the information given is correct.
     
  5. Czarcasm

    Czarcasm Hakuna matata, no worries. 2+ Year Member

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    I get you, but at the same time, if I see a passage that involves heat capacity, my first thought won't be "How does salt affect heat capacity?" Sure it might be a useful fact to know, ...if the passage is about salts and its effect heat capacity, but it isn't the norm. There's so much information covered on this test. The general chemistry component is only 20%, and even then only a minor fraction will be tested. If you can remember those details though, more power too you. Just be careful not to let it cloud your thinking on the actual exam.
     
  6. Chrisz

    Chrisz 2+ Year Member

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    Czarcasm, you are absolutely correct. I totally agree with what you saying. You know I am the type of curious person. Once I see something that does not sound reasonable, I like to know why and find extra info to satisfy my personal curiosity. But when it comes to test, I am totally a different person. Even if there is something I do not quite agree with, I just choose it for the grade purpose. The reason I posted here is that I feel it is good to let people know the second source I found. I actually found other sources on the internet earlier, they all saying more or less the same thing that specific heat will lower after salt is added. Their explanation were really more appealing and more elaborate than the one I posted. Since they did not disclose their personal info, such as who they are and what they do, I kept the info to myself instead putting on here. Today, I found this guy saying the same thing. I feel he is a reliable source, That is why I posted here
     

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