Chemistry - Why is sugar syrup so sticky?

The "sugar" you refer to is sucrose, which is a disaccharide (a fancy term we use for a type of sugar, composed of two other bits of sugar called monosaccharides; which in this case, are glucose and fructose). I felt I ought to have brought up the sucrose specification, since when you talk chemistry, "sugar" can refer to a whole lot of things.

Anyways have a look at this diagram:

Observe carefully, and you'll see that there are 8 -OH groups (hydroxyl groups) in a molecule of sucrose. Now why did I direct your attention to these particular groups? It's because they're capable of taking part in a fascinating phenomenon called hydrogen bonding (I believe you already know a bit about it, so I won't go into details).

Now when you add a solute to water, that is capable of H-bonding, it sorta causes all the water molecules and solute molecules to 'clump' together. The greater the amount of H-bonding the solute displays with water, the greater the tendency for all those interaction to put up some resistance to the flow of water. This ability to 'resist flow', that is inherent to every real fluid (to different degrees of course), is what you call viscosity.

Now 8 hydroxyl groups, can result in some pretty extensive H-bonding. To give you a rough idea as to how much that contributes to the viscosity of the sugar solution, think of glycerine (glycerol). Rub some of it between your fingers, it's pretty viscous right? Now the thing is, a glycerol molecule has only 3 hydroxyl groups. So imagine how much more viscous/sticky the sugar solution ought to be, when it's got nearly 3 times as many hydroxyl groups! [No, I'm not saying H-bonding is the only factor you should consider]. Of course, the more concentrated the sugar solution, the more the sucrose molecules and hence more the H-bonding; so it'll get stickier and stickier.

Now another thing you ought to consider is the geometry of the sucrose molecule. Now it isn't as important as the H-bonding bit, but it's still noteworthy. What you must know is that the sucrose molecule is not a planar molecule. It isn't some 2-dimensional structure. It's got all sorts of groups sticking out above and below the plane of the carbon rings. Now at low concentrations this isn't really an awfully important thing, but the story changes when you're dealing with concentrated solutions. That's when the 'jagged' physique of the sucrose molecule comes into play. It serves to compound the effect of H-bonding and further obstructs flow.

Want an analogy for that bit on the importance of the molecule's geometry?

Imagine you've got this large sack of coins. Dump them onto the floor and they spread out so well, that you wouldn't be wrong if you said they flowed. But suppose you had nothing better to do, and decided to glue a couple of screws on both faces of each coin. Put all your coins back in the sack, and proceed to dump out the whole lot. You can easily picture how well this 'modification' obstructs the flow of the coins, and probably now, you can appreciate the importance of molecular geometry in this regard better.