“He’s hot and sweet, but a little dense.”

Density. Solubility. Temperature. And how being sweeter also means being more dense.

density - higher temperatures dissolve more, making the solution more dense1-Minute NomNom

The hotter it gets, the sweeter it can be. That is simply what happens when we dissolve sugar in hot water, for example when we make some sugar syrup or a cup of tea. When we dissolve more sugar in water, it also makes the solution denser.

The difference in density between sugar solutions with different amounts of sucrose dissolved is not always apparent to us but it is clearly so in the video above.  Here, we see how sugar solutions of different densities can stack on top of each other. That is pretty amazing!

Why does this happen?

density increases with temperature as more dissolvesTo begin with, sucrose (i.e. table sugar) is very soluble in water. This is partly because of water’s fairly open molecular structure that accommodates the sugar particles. It is also because of the mutual attraction between the positively charged areas and negatively charged areas on both the sucrose and water molecules; this is strong enough to pull sucrose molecules away from each other (read all about the science in the 1-Minute NomNom “Sucrose yet so far“).

Water's molecular structure, hydrogen bonding, and sucrose molecules - find out more in the 1-Minute NomNom mentioned above

Water’s molecular structure, hydrogen bonding, and sucrose molecules – find out more in the 1-Minute NomNom mentioned above

We can increase the amount of sucrose dissolved by heating the water. Heat has two effects.

Turning up the temperature on water molecules increases their thermal energy and motion. As a result, they have energy to move further apart from each other. This make the gaps between them bigger (i.e. the open structure is more “open”) and makes it possible to accommodate even more sugar.

density and temperatureAt the same time, turning up the heat on sucrose molecules makes them vibrate more vigorously. Together with the higher thermal energy of the water molecules, this increases the mutual attraction between the sucrose and water molecules because…

“… [w]hen faster-moving water molecules attach to sucrose molecules, a higher proportion of these sucrose-water interactions have enough energy to pull sucrose molecules away from other sucrose molecules, so the rate of dissolving increases.” (Source: American Chemical Society)

To give a sense of how big a difference it can be, at 0°C, about 180 grams of sucrose can dissolve in 100 ml of water. At 20°C, it is close to 200 grams. At 100°C, it more than doubles to 490 grams.

hot water, coffee, and tea dissolves more sugar i.e. they are more dense

The more sugar that is dissolved in the water, the denser the sugar solution will be. Density is simply mass divided by volume. When we add more sucrose, we are adding more mass. But the volume does not change by the same amount because the sucrose molecules “fill in” the spaces between the water molecules.

When we are drinking our tea or coffee (or making sugar syrup), and we are adding more and more sugar, we may not see and feel it (other than the sweeter taste), but there is definitely an increase in mass and density!


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