# If hydrogen bonds are chemical bonds, and these bonds are broken when water changes phases, why doesn't this classify as a "chemical" change?

This is more of a terminology question. How are phase changes considered a physical property, not a chemical property when hydrogen bonds break. A chemical property can only be observed during a chemical change. Aren't hydrogen bonds a type of chemical bond, and those bonds are being broken during a phase change from ice to liquid water, for example. I understand that no intramolecular bonds are being broken. Water molecules remain intact. But we teach that a chemical bond has to break during a chemical change, but phase changes are considered to be physical.

• Possible duplicate of Ripping apart plastic: is this a chemical or physical change? Nov 16 '17 at 6:18
• The question there makes use of a different example ("ripping a plastic sheet") but the answers to it should address most of your issues too... since your question (as I interpret it) is basically "Are physical and chemical changes sharply defined?". Cheers! Nov 16 '17 at 6:20
• The distinction between chemical and physical changes exists only in our heads. Nov 16 '17 at 7:03
• I respectfully disagree with @paracetamol because the other question focusses on breaking covalent bonds which we should all agree are sufficiently different from hydrogen bonds.
– Jan
Nov 16 '17 at 7:10
• @Jan Ah, but the physical-chemical change classification taught at school represent a false dichotomy (I don't have an issue with the type of bonding per se). But now that I've re-read the question I linked (and its answers), I realized that the answers to ron sensei's question don't make much of a case in this regard (i.e- dispelling the notion of rigid physical-chemical change classification). I stand corrected, accordingly I've withdrawn my close vote. Thanks for the pointer O:) Nov 16 '17 at 7:52

You must realize that the line between physical and chemical properties is very thin. Most bonds work via the same principle, which has mostly to do with electronegativity. When looked at in reactions it has probably also to do with the according entropy and enthalpy change. Hydrogenbonds are basically bonds based on electronegative attraction between two atoms (which is a simplified few of the reality). A phase change basically when a certain energy point has been reached: since heating op a substance is equal to giving it more kinetic energy (since heat is the same as the movement of individual atoms or molecules). When you reach a certain heat there is too much kinetic energy to form hydrogenbonds the same way it as it did before it changed phase. You probably can agree that molecules or atoms moving quickly is an obvious physical property. When you look at great detail to how hydrogenbonds work, it has something in common with many other way of two objects attracting each other (for example: the formula for the gravitional force between to large heavenly bodies, is very similar to the formula for the electrical force between an electron and a nucleus).

In simple: Many of these chemical properties seem very physical when you look at them in detail. So saying that a certain property should actually be chemical because the changing of this property seems to correspond with the changing of chemical bonds, is not a realistic claim looking at how "physical"/physic based these "chemical" bonds are.

First let's clear up a misconception in the OP's Question: Water molecules remain intact. - NO!!! This is absolutely wrong. Water is sort of an odd solvent because the "molecules" are not absolutely fixed.

If pure $\ce{D2O}$ is mixed with pure $\ce{^1H2O}$, then you get $\ce{D^1HO}$ which shows that the hydrogen oxygen bond is very liable. So a hydrogen bond can easily be transformed to one of the two covalent O-H bonds by having one of the original O-H covalent bonds become a hydrogen bond. So water is a vast network of hydrogen and covalent bonds which is forever changing.

We could consider $\ce{D2O}$ and $\ce{^1H2O}$ as different compounds since they have different freezing points and different boiling points. But the point here was to somehow label the hydrogen atoms in water. In pure $\ce{^1H2O}$ the hydrogen-oxygen bond is still very liable.

Now to the overall question. The point is that most pure substances will solidify if made cold enough. Warm the substance beyond its melting point and most turn into a liquid. Heat the liquid hot enough and the liquid boils. Furthermore thermodynamics dictates that it takes a certain amount of heat to convert a certain mass of the solid at the melting point to a liquid. Also it takes a certain amount of heat to convert the liquid to a vapor at the boiling point. That is the conceptual framework upon which thermodynamics is based.

There is some acknowledgement of how various forms of bonding effect the phase transitions. For instance consider sodium chloride. The bonding energy of one $\ce{Na^+}$ ion in the solid isn't between the one $\ce{Na^+}$ ion and one other $\ce{Cl^-}$ ion but between the attraction of the $\ce{Na^+}$ ion to all the $\ce{Cl^-}$ ions and the repulsion of all the other $\ce{Na^+}$ ions. The overall factor is called the Madelung constant.

• But this is an effect of water being a protic solvent. Water is also able to protonate substances with which it does not undergo hydrogen bonds such as sulphides or carbanions. Indeed, with a sufficient acidity of the corresponding alpha protons of a ketone, these can be deuterated in $\ce{D2O}$ — does that mean water hydrogen bonds to the carbon?
– Jan
Nov 16 '17 at 9:36

Suppose you heat carbon in an atmosphere of carbon dioxide. They react; carbon monoxide is formed in the gas phase. Cool the gas and the reaction, which is endothermic for formation of the carbon monoxide, reverses; you redeposit carbon (possibly on cooler parts of the furnace in industrial settings).

Suppose you put sugar into water and heat it up. The sugar dissolves, and more dissolves as you heat the system. Let it cool; the dissolution of the sugar, which is endothermic, tends to reverse and solid sugar reappears.

What is, empirically, the difference between these two processes? "Chemical reaction" versus "physical change" is a blurred distinction.

• The original post referred to phase change of a pure substance. With mixtures the situation may be different, I suspect. Otherwise why do we say that the sugar 'dissolves' - rather than 'melts' - in water? Nov 16 '17 at 18:12