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Name: Hannah
Status: student
Grade: 6-8
Location: IA



Question:
How do polar molecules affect the cohesiveness of water?


Replies:
Hi Hannah,

I am a little puzzled by your question. The water molecule is itself highly polar. What other polar molecules would you be thinking of in this situation? I'm also unclear what you mean by "the cohesiveness of water". Could you possibly elaborate on your question a little more?

Regards,
Bob Wilson


Hi Hannah,

The short answer is that the mixture properties depend on what, and how much you add. There are many different kinds of polar molecules that have very different effects. It is hard to make many accurate broad generalizations. However, water really likes water, so most additives will reduce overall intermolecular attraction ("cohesiveness").

If you want more detailed information, keep reading.

Terms like 'cohesiveness' and 'polar' are useful to illustrate basic concepts, but they can be a bit too general/simplified when trying to understand complicated subjects like mixture properties. 'Cohesiveness' is a catch-all term that refers to many different kinds of attractions between molecules. Each type of attraction acts differently in different circumstances, so it is hard to accurately generalize overall 'cohesiveness'. 'Polar' is also a very general term. Molecules have different degrees of polarity -- one molecule may be more polar than another, rather than simply being classified as 'polar' or 'non-polar'. A molecule may be polar at one end, but non-polar at another. For example, soaps have a polar end and a non-polar end. One 'polar' molecule may have a very different effect than another.

I am also not sure if you are asking if the water itself is changing, or if the mixture properties are changing. A water molecule itself will not be changed by being in a mixture or being with just other water molecules. However, the mixture of water molecules and additives can change a lot depending on what and how much you have mixed to it.

Think about substances as a collection of individual molecules. Each molecule is surrounded by other molecules, and may be attracted or repelled by them. There are many bulk properties (like vapor pressure, boiling point, how much energy is required to evaporate [called heat of vaporization], droplet contact angle, etc.) that depend on intermolecular attractions. Some mixtures may have more self-attraction than the pure substance, while others may have less.

The bulk properties we can observe all depend on how 'happy' (more intermolecular attraction) the molecules are together. If they are 'happy', it will take more energy to break them apart. If they are 'unhappy' (less intermolecular attraction), it takes less energy to break them apart. Water is highly attracted to other water molecules, so it is 'happy' -- and has low vapor pressure, high heat of vaporization, and high boiling point.

Because water is so 'happy' by itself, most things you can add will generally reduce the overall intermolecular attraction of the mixture. We can observe changes in bulk properties that lead to this conclusion. Alcohols are "polar" molecules, and low molecular weight alcohols are miscible with water. An alcohol-water mixture typically has a lower boiling point and higher vapor pressure, indicating less overall attraction. However, the higher the molecular weight of the alcohol, the lower the polarity and the less miscible with water. These indicate lower overall intermolecular attraction. Alternatively, if you add a salt, e.g. sodium chloride, that dissociates in water into a pair of charged ions, the boiling point of the water-salt mixture rises. The vapor pressure changes in a complicated way (it is higher than fresh water at hear-boiling, but lower at near-freezing). This interaction is a little bit more complex than the alcohol.

So hopefully, this shows you how complex the situation is. The results depend on what specifically you add, and therefore overall generalizations are hard to make.

Hope this helps,
Burr Zimmerman


This is difficult to answer, because it is not clear what the term "cohesiveness" means. I do not know of a generally accepted definition of the term. This needs clarification.

Vince Calder



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