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Definition of chemical reaction

Started over 2 years ago by Patricia Dwyer-Hallquist.

Sorry I couldn't pick a category that quite fits. My question has to do with how people view conversion of an anhydrous salt to its hydrate. An example is conversion of anhydrous cupric sulfate to cupric sulfate pentahydrate. I would argue that this is a chemical reaction because it involves formation of a new chemical substance. Anhydrous cupric sulfate and cupric sulfate pentahydrate should be considered different compounds because they have different chemical formulas, different colors, and different melting points. However, I have seen arguments that formation of the pentahydrate does not involve breaking or formation of bonds, and therefore is not a chemical reaction. I think the formation of the hydrate does involve bond formation. How do people normally teach this?


  • Carl Heltzel

    Posted over 2 years ago

    I agree with Kimberly's suggestion. In his article, Underwood brings up good points regarding teaching Chemical vs. Physical changes too early in the course. Personally, I think the distinction should be removed from any first-year chemistry course, including college-level. I would argue that all such changes are chemical; in the case of anhydrous cupric sulfate going to the pentahydrate (blue vitriol), hydrogen bonds are formed = chemical change. Different textbooks will claim opposing answers; why muddy the water for students when 'experts' don't agree? Getting students to understand the reaction at the particle-level is more important than imposing on them an unnecessary classification. I say students would be better off if the chemical/physical change conundrum was removed altogether. See the Association for Science Education's take: https://www.ase.org.uk/.../k3210aphysicalandchemicalchange/k3.2-1.0a-physical-and-ch...

  • Tricia Brown

    Posted over 2 years ago

    Wow, Jeremy. I love that approach. It's a great way to connect the Chemistry to an actual experience.

    To answer your question, Patricia, as far as teaching it, I tell my students that it's a physical change, not a chemical one since no bonds are broken or formed in the process.

  • Kimberly Duncan

    Posted over 2 years ago

    You might want to read "Rethinking Common Practices in High School Chemistry" by Kaleb Underwood in the November 2017 issue of Chemistry Solutions (http://bit.ly/RethinkingCommonPractices).

  • Jeremy Wolf

    Posted over 2 years ago

    I like to use this specific example to spark discussion about what is happening in the process of evaporating water from a hydrate on the particle level. I do this as a short lab with my students, where they dehydrate copper(II) sulfate and Magnesium sulfate. They notice the color change from the copper(II) sulfate, but no color change from the epsom salts. When we talk about the lab and the students claim that the dehydration of copper(II) sulfate was a chemical change because there was a color change, while the epsom salts was a physical change because there was no color change, the water just evaporated.
    It gets very Socratic from there and I just lead them through the process of thinking about what is happening on the particle level. This is one of the first reactions that the students do for my Chemistry I class and we can refer back to it throughout the year, when we talk about molar mass, chemical formulas, and later, when we talk about the differences between transition metals and main-group metals. The whole time, I don't tell my students whether the process is a chemical or physical change, because the debate drives deeper discussion and understandings. The students may pick a side and argue for their case. I will listen and counter with points from the other side or other students will. This also helps my students train to form evidence-based conclusions. It is awesome.
    PS - I never tell them my answer, but by the time we get there, it doesn't matter.