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As a teacher, I show students the particle model as an explanation of solids, liquids and gases, and changes of state. I did this today, explaining that the bonds in the solid break because the kinetic energy of the particles has increased so much that it overcomes the bonds between the particles. The bonds between the molecules in the liquid state are weaker. But... if this is true, then how come the level of kinetic energy that broke apart the solid bonds, is not also able to break apart the liquid bonds? Are the liquid bonds in fact stronger?

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    $\begingroup$ The situation is murky: at 30 °C, are the bonds in solid 1,4-dichlorobenzene mothballs, with high vapor pressure and boiling point of 174 °C, stronger than those of liquid gallium, with exceedingly low v. p. and b.p. of 2,400 °C? $\endgroup$ Mar 14, 2022 at 23:55
  • $\begingroup$ @DrMoishePippik ..... for the same substance .... $\endgroup$
    – Poutnik
    Mar 15, 2022 at 13:31

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The bonds between the molecules in the liquid state are weaker.

The bonds in the solid are the same kind of bond as in a liquid. We know the liquid still has bonds between particles because liquids have a constant volume. It would be better to say that the solid turns into a liquid because there are fewer bonds, or bonds break continuously (with the current neighbor) and form continuously (with the new neighbor). In solid and liquid water, we have hydrogen bonds between molecules. In solid and liquid sodium chloride, we have ionic bonds between ions. In solid and liquid iron, we have metallic bonds between atoms.

Here is a simulation of ice melting. Notice the change in density as you pass zero degrees celsius, water molecules starting to rotate as the temperature increases, and rare switches in position (you would have to play this faster to see diffusion of water molecules in the liquid).

enter image description here

Source: https://phet.colorado.edu/sims/html/states-of-matter-basics/latest/states-of-matter-basics_en.html

[...] how come the level of kinetic energy that broke apart the solid bonds, is not also able to break apart the liquid bonds?

If I want to buy a house, and then buy furniture for it, the house will be more expensive than the furniture. However, if I have just enough money to buy the house, I can't buy the furniture until I get more money because I already spent it on the house.

So at the melting point, we put in sufficient energy to switch from continuous bonds between neighboring particles to bonds that break every now and then, allowing particles to move but not to escape (become a gas). To turn this liquid into a gas, we need to break all the intermolecular bonds, which has an additional energetic cost.

Often, the energy required to go from solid to liquid is lower than the energy required to go from liquid to gas. For example, in ice, each water molecule makes 4 hydrogen bonds. In liquid water, each water molecule makes between 3 and 4 hydrogen bonds. In steam, each water molecule makes about zero hydrogen bonds (except for the rare and fleeting instances where it collides with another water molecule).

Are the liquid bonds in fact stronger?

No, bond by bond they are of similar strength. If the liquid has a lower density thant the solid, the particles are a bit further apart from each other, so the bond will be slightly weaker. Also, averaged over time, there will be a lower number of bonds.

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  • $\begingroup$ Ifthe energy required to go from solid to liquid is lower than the energy required to go from liquid to gas. Can we conclude that the number of hydrogen bonds in ice is less than in liquid water? $\endgroup$ Mar 16, 2022 at 8:09
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    $\begingroup$ No, but the difference in number of bonds between solid and liquid is less than the difference in number of bonds between liquid and gas. $\endgroup$
    – alexigirl
    Mar 17, 2022 at 16:28
  • $\begingroup$ @alexigirl Couldn’t have responded to that comment better myself. $\endgroup$
    – Karsten
    Mar 17, 2022 at 18:38
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As a teacher, I tell my students that in a solid the atoms are vibrating around a central point. The higher the temperature, the stronger they vibrate. But, if the vibration energy is too high, it is like with a rubber band or an elastic, the bond is broken, and the system acquires a new degree of freedom : translation. The solid becomes a liquid. I know it is oversimplified, but for my high school students, it is sufficient

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  • $\begingroup$ This makes it seem that once the bond is broken, it is like a snapped elastic band that can't reform. But Karsten Theis says that the bonds form and reform, and that they are of the same strength in the liquid as the solid. $\endgroup$
    – alexigirl
    Mar 17, 2022 at 16:27
  • $\begingroup$ I told you that it is a oversimplified message. Karsten Theis does not say that bonds have same strengths in solid and liquid. He says it is "slightly weaker" in the liquid. And there is a lower number of bonds $\endgroup$
    – Maurice
    Mar 17, 2022 at 17:01
  • $\begingroup$ You could go for Velcro instead of rubber bands if you want to address the temporary nature of bonds in your analogy, and the capability of forming again when the substance freezes. Magnets are great for that purpose in a physical model. You did say the rubber bands are sufficient for the learning outcomes you are going for in your specific scenario. $\endgroup$
    – Karsten
    Mar 17, 2022 at 18:43
  • $\begingroup$ I agree that simple analogies are what is needed at high school, but it would be great to avoid teaching things that will need to be unlearnt later. My understanding of $\endgroup$
    – alexigirl
    Mar 18, 2022 at 20:39
  • $\begingroup$ ... weaker bonds in the liquid is that the particles are. on average, slightly further apart, and electrostatic field strength decreases with distance. But would you therefore say that the bonds are weaker? At any time there will be some particles that are as close together in the liquid as they would be in the solid, would the bonding force then be the same as in the solid? $\endgroup$
    – alexigirl
    Mar 18, 2022 at 20:52

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