Why does copper sulphate (CuSO4) in water increase surface tension?

Question

So, recently for school, I did an experiment on surface-tension. We tested Water, Salt, Sugar, Dissolvable Panadol, Coffee Powder, Vinegar, Detergent, Flour, Methylated Spirits, Oil & Detergent, Copper Sulphate, Iron Sulphate, and Ammonium Chloride.

Our theory at the time was:

Copper sulphate has a high surface tension when mixed with water because it has a lot of hydrogen bond acceptors that are not used. As more hydrogen bonds increase the surface tension, the increased amount of possible hydrogen bonds thus increasing the surface tension. What the copper sulphate is doing is being a whole bunch of extra links connecting the hydrogen atoms of water molecules together.

My Current Theory:

As Hydrogen likes Oxygen, the large amounts of Oxygen bonds in CuSO4 strongly attract to the H2O, which increases the pull force. Also, both H2O & CuSO4 are polar substances, helping them stick together.

I have no idea if any of this is true; our chemistry teacher had no idea either. Does anyone have a better theory/proof as to why this happens?

Note; We did not use capillary tubes as the order time would exceed the time our assignment due date. Also, our data might just all be noise too, as our method was not the most error-free design possible.

Our results are here.

(Relevant Results:) | Average | Test1 | Test2 | Test3 | Test4 | Test5 | Control | 3.21 | 3.25 | 3.4 | 3.1 | 3.1 | 3.2 | Copper Sulphate | 3.94 | 3 | 4.7 | 3.5 | 4.5 | 4 |

Method:

1. Fill beaker with 40 mL of water (4th line)
2. Mix in approximately 3 mL amount of solute.
3. Using a pipette, drop water into a bottle cap until liquid is flat with the top.
4. Fill a pipette to 3 mL. Slowly drop in the water until it overflows, taking note of how much is used (i.e. emptied once & 2 mL used = 5 mL on top)
5. Record the results in a table. Repeat 4 more times.
6. Repeat the procedure with different solutes in different beakers.

Answer 1

From Sayok#6589 on Discord:

1. When dissolving CuSO4 in H2O, it does not remain as CuSO4. It's a soluble salt, so you make [Cu(H2O)6]2+ and [SO4]2-. The first coordination shell of either ions is filled with water, forming electrostatic/polar bonds between the water molecules and the ions.

2. Because the water molecule are forming bonds with charged ions, their polarity is slightly increased compared to a regular water molecule.

3. Surface tension of H2O is formed because of the polarity of the water molecules. They like each other and want to stick with each other. In general, the more polar a solution is, the more surface tension it can have.
4. Increasing the polarity of the water molecules because of the presence of the charged ions in solution thus results in a small increase in the surface tension.