When you melt sugar or dissolve it in water, it’s sticky. So are honey and syrup, which are mostly dissolved sugars in water. Do you know why sugar is sticky? Here’s a look at the science of stickiness.
Hydrogen Bonds and Viscosity Make Sugar Sticky
When something is sticky, it displays a property called adhesion. Adhesion is a measure of how well particles stick to each other and surfaces. There are several factors that affect adhesion, but two are particularly important in making sugar sticky: hydrogen bonds and viscosity.
A sugar molecule consists of carbon, oxygen, and hydrogen atoms. In a sugar crystal (or any dry form of sugar), the molecules are bonded to each other in a crystalline lattice. Dry sugar isn’t sticky. But, when you dissolve sugar in water, the molecules disperse in the water. Even though they are chemically bound to atoms already, the oxygen atoms on the exterior of the sugar molecules have a slight negative charge and are attracted to the hydrogen atoms of water molecules and other sugar molecules. Meanwhile, the hydrogen atoms of the sugar molecules have a slight positive charge and are attracted to oxygen atoms in water and other sugar molecules. The attraction between hydrogen atoms and negatively charged atoms is called hydrogen bonding. It’s not a strong chemical bond, but it’s enough to make particles in sugar water, honey, and syrup want to stick to each other.
Dissolving sugar also makes sugar water more viscous than pure water. In other words, it doesn’t flow as easily as water. This is important because viscosity helps water wet surfaces better than it ordinarily would. So, sugar water sinks into every nook and cranny of a surface (like your fingertip). Since the sugar molecules are attracted to water and each other, when you dip your fingers in sugar water and try to separate them, you have to exert more force than usual. The sugar is sticky!
Other Factors That Make Sugar Sticky
In addition to hydrogen bonding and viscosity, other factors play a role in sugar stickiness. Dissolved sugar changes the surface tension and cohesion of water. As sugar dries, it forms microstructures on a surface, increasing surface area and making it harder to separate sugar molecules from each other. Another affect is called “stringing.” Stringing occurs when sugar water flows in to fill a gap caused by pulling particles apart. Stickiness is actually pretty complicated.
References
- Kendall, K. (1994). “Adhesion: Molecules and Mechanics”. Science. 263 (5154): 1720–5. doi:10.1126/science.263.5154.1720
- Maeda, N.; Chen, N; Tirrell, M; Israelachvili, J.N. (2002). “Adhesion and Friction Mechanisms of Polymer-on-Polymer Surfaces”. Science. 297(5580): 379–82. doi:10.1126/science.1072378