4 super tough chemistry questions:

1.) Water has a considerably higher boiling point and lower vapor pressure than other molecules of similar or larger molar mass. For example, dinitrogen (N2), methane (CH4), etc. all have lower boiling points and higher vapor pressures than water at the same temperature. How might this be explained?

2.) If the intermolecular forces between molecules of a substance were very small, what effect would you expect this to have on the boiling point and vapor pressure of the substance? What properties would you look for in a molecule in order to have a low boiling point and high vapor pressure? Should the atoms of the molecules be large or small? Why might this matter? Give two or three examples of molecules where you might expect the intermolecular forces to be very small.

3.) Why would contact with steam at 100 C produce a more severe burn than contact with liquid water at the same temperature?

4.) Orange growers often spray water on their trees to protect the fruit in freezing weather. Explain how the energy of the water --> ice phase transition could provide protection from freezing weather.

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  1. We will be happy to critique your thinking.

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  2. 1, 2, and 3 I have no answers. I've been staring at my monitor for 47 minutes now and I'm not coming up with any answer.

    As for no. 4, I think it's because water protects/shields the fruit.

    Actually, I think my answer for no. 3 is because steam has heat which is why it will produce a more severe burn than just plain water.

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  3. #4. The "protection" afforded by the ice is not so much from a shielding effect (I think a common misconception) as it is from the heat. Look at it this way,
    ice + heat ==> liquid water.
    So melting ice is an endothermic reaction; that is, you put heat in to melt the ice. So the reverse (turn the arrow around) must be an exothermic reaction; that is, heat must be given off.

    The steam question. Burns by steam (at 100 C) are worse than burns by water (at 100 C) because, in addition to the burns of the water, there is an excess of 2260 joules for every grams of steam that turns into water when it condenses. So that much more energy is emitted.

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