[Kahagari]

I am confused???. Others say that it is only physical reaction while others say that there is a chemical reaction. so which one is it?

[chenbeier]

Its only physical. The water sugar mixture is still sweet and no changes.

[Kahagari]

ohhh..thanks 

[Borek]

Note: this is tricky in general.

Yes, the answer that there is only a physical change here is typically considered correct, and is a good approximation. However, there is no sharp border between the chemical and physical change. When sugar dissolves in water its molecules create hydrogen bonds with water molecules, these are way weaker than bonds between atoms in the molecule, which makes our assumption ("physical change only") reasonably correct. But for other compounds hydrogen bonds become stronger and stronger, then some other, comparable intermolecular bonds start to play an important role - at some point it is impossible to say whether the change is strictly chemical or strictly physical.

Take out lesson: there is a continuity of changes, that are not always possible to classify. In most cases we don't have to care about that.

Take out lesson 2: real world doesn't care about our classification attempts and will always fling at us examples that are not possible to classify.

[jeffmoonchop]

dissolution is endothermic due to the equilibrium shifting and the total energy is reduced. The hydrogen bonding between water and sugar molecules are stronger than the hydrogen bonds between sugar and sugar which is why it dissolves.

Keep adding sugar and there wont be enough water molecules to displace sugar molecules so the sugar will remain crystalline.

Start warming up the system and there will be more energy available to break the intermolecular bonds in sugar crystals so more will dissolve.

Cool it down and less energy will be available and more crystals will eventually appear.

Sugar is weird though because you need to consider high solubility producing high viscosity and wide metastable band widths.

Describe what is happening not whether its chemical or physical.

[Babcock_Hall]

When you say sugar, do you mean sucrose (table sugar), or some other sugar?

[Enthalpy]

Dissolution is endothermic due to the equilibrium shifting and the total energy is reduced. The hydrogen bonding between water and sugar molecules are stronger than the hydrogen bonds between sugar and sugar which is why it dissolves.

Many processes occur despite absorbing energy. This is a matter of entropy or of chemical potential, for which the enthalpy doesn't decide alone,except in extreme cases. Especially when the enthalpy change is small, as is often the case in dissolutions, it doesn't decide. For polyols and some sugars, dissolution absorbs heat, hence the new bonds are weaker than the old ones - but the solubility or miscibility can be quite high.

Keep adding sugar and there wont be enough water molecules to displace sugar molecules so the sugar will remain crystalline.

And what about solubilities in the ppm range? A number of solvent molecules per solute molecule won't explain them.