# Is dehydrating copper(II) sulphate pentahydrate a chemical or physical change?

Is removing the waters of hydration from $$\ce{CuSO4 * 5H2O}$$ a chemical or physical change? Sorry for the trivial question. I get varying answers across the net.

• Does the crystal melt, or does it just change color? – LDC3 Oct 10 '14 at 4:53
• I heated it in the lab just until it turned white – Jack Oct 10 '14 at 4:56

The traditional answer is that it's a chemical change when bond-making and bond-breaking is involved. The material certainly changes appearance, and heating copper sulphate somehow releases bound water.

That said, bonding in metal-aquo complexes runs the gamut from very weak to exceedingly strong - exchange rates span 15 orders of magnitude (Fig. 1 in Jan Reedijk, Metal-Ligand Exchange Kinetics in Platinum and Ruthenium Complexes. Platinum Metals Rev. 2008, 52, 2–11. DOI: 10.1595/147106708X255987), and where do you draw the line between a "physical" electrostatic interaction (in, say, $$\ce{Na(OH_2)_6^+}$$) and a chemical bond (as seen in $$\ce{Cr(OH_2)_6^3+}$$?

You heated copper(II) sulfate until it turned white. This is a quite simple chemical reaction:

\begin{align} \ce{\underset{(blue)}{CuSO4.5H2O} &->[][100 ^\circ C] CuSO4.H2O + 4 H2O}\\ \ce{CuSO4.H2O &->[][> 200 ^\circ C] \underset{(white)}{CuSO4}} \end{align}

According to one text,[1] a change in color, release of a gas (and some other changes), are indications of a chemical reaction. Since water vapor is released and the solid changes color, this should be considered as a chemical reaction, not a physical change.

• This misses the point of the question. Changes in colour and release of gases are indications that a reaction may have taken place. However, passing an electric current through a grey tungsten filament makes it glow yellow-white but there's no reaction. Heating water to 100C makes it release a gas (steam) but there's no reaction. The asker has observed signs that a reaction may have taken place and is asking if one actually did. What is the chemical change you think the asker observed? – David Richerby Oct 10 '14 at 10:18
• @DavidRicherby There is a difference between the tungsten filament and the compound above in that the above compound has a different color at room temperature after heating. Also, there does not appear to be any water with the starting material, yet it is liberated when heated. As I answered the question as it was written, it is a chemical reaction. The OP did not ask what the reaction was. – LDC3 Oct 11 '14 at 0:32
• A good answer would explain both why there is a reaction and what the reaction is. If you're going to be minimalist, why not just answer "yes"? – David Richerby Oct 11 '14 at 1:01

By heating copper(II) sulfate pentahydrate until it was white and contained no more water, you undergo a chemical change. The chemical makeup of $$\ce{CuSO4.5H2O}$$ changed to $$\ce{CuSO4}$$. The change in color in this situation also indicates a chemical change, but a change in color doesn't always entail a chemical change.

The crystal structure shows that the copper is six coordinated and has four oxygen atoms from water molecules in a plane and the other two octahedral positions are each occupied by one oxygen from an $$\ce{SO4}$$ group.

The fifth water molecule has its oxygen bridging two $$\ce{CuSO4}$$ molecules, see the sketch, and is bonded to four oxygens, two from water and two to sulphate oxygens, presumably by hydrogen bonds. The H atoms scatter x ray so weakly that these are not seen.

As these bonds from this oxygen will be weaker than those of other oxygens bonded to the cation they should be broken first on heating so this should be considered to be a chemical reaction. Breaking the other bonds $$\ce{Cu-OH2}$$ bonds will follow on further heating to form the anhydrous solid.

Structure of $$\ce{CuSO4-5H2O}$$ determined by x-ray diffraction of crystals. The sulphate oxygens are shaded, the water oxygens as large open circles and the fifth water oxygen with a bold outline. The Cu atoms are at the centre of the octahedral structure and drawn with a dot in the centre of the circle.(After Beevers & Lipson, Proc. Roy. Soc. A 1934, v146, 570, image from A. Wells 'Structural Inorganic Chemistry', publ Oxford)

• This is not entirely correct to use crystal structure to predict the path for dehydration. According to DTA, copper(II) sulfate looses 4 water molecules first, not 1 as one would think looking at the crystal structure. I touched this issue in one of my answers to related question. – andselisk Apr 15 at 16:20
• You might well be correct, but energetically to lose four strongly bound waters before losing one other one does not seem to be what one would expect. Only ultra-fast time resolved methods could determine an answer, for example time resolved x-ray diffraction after fast laser heating of a small crystal. – porphyrin Apr 15 at 20:35

The definition of a chemical reaction: We call changes that change bonding form of atoms, which results changes in the structure and properties of matter, is called a chemical reaction. So this reaction isn't a chemical reaction. Copper (II) Sulphite is still Copper (II) Sulphite and water was'nt in bond with Salt it was trapped in crystal system of salt P.S: SO3 is Sulphite and SO4 is Sulphate

• Unfortunately, this is wrong. First, how do you think water is "trapped" within crystal lattice if there were no chemical bonds? Second, crystal structure of copper(II) sulfate pentahydrate consists of [Cu(SO4)2(H2O)4] octahedra with 1 water molecule dwelling in the lattice per copper, and the first dehydration step actually takes four water molecules out, not just one, suggesting that even the first coordination sphere is disturbed. – andselisk Apr 15 at 13:29