When getting coffee, the people at Starbucks ask, “Do you want to leave room for sugar and cream?” Do I need to leave space if I just add sugar?

Question answered by Catherine Riddle, PhD, Radiochemist, Idaho National Laboratory

The chemistry of coffee is complex to start with so adding sugar, milk or both makes it even more so. There are many systems at work in a cup of coffee including density, inter-molecular spacing, temperature and concentration. Believe it or not, there is a lot of chemistry going on in your morning cup of Joe! I believe we all know why we need to leave room in our coffee cup when adding milk since we are increasing the volume by adding a substance that is less water soluble (regular grind is about 1 part coffee to 18 parts water). But what about leaving room for sugar? Does sugar really increase the volume of our delightful morning wake up drink or does it stay the same? The answer is, it depends. This is probably not the answer you were expecting and in chemistry answers can be dependent on a multitude of factors. We will explore our coffee conundrum in two parts, so let’s begin!

Why would adding sugar to coffee increase the volume? You may hear scientists say, “Dilution is the solution” and indeed it can be, however, the concentration of a solution is very important when adding a solid solute (sugar) to our solvent (coffee). Density is also at work in our coffee/sugar solution. Sugar has a slightly higher density (1.59g/mL) than coffee which is mostly water and will have a density of about 1g/mL. Just when you thought the chemistry lesson was over, we have the effect of temperature to consider in our solution! Temperature is very important when dissolving a solid solute into a solvent and generally the greater the temperature of our tasty solvent, the more solute it can dissolve. Even the simplest of activities such as stirring your coffee has a chemistry component. When you quickly stir coffee to dissolve the sugar you increase the kinetic energy which increases the temperature and the sugar dissolves faster. Now that we have considered some of the chemistry of coffee, let’s answer the volume increase portion of the question.

Yes, the volume of coffee will increase very, very slightly. However, you will need a definite sweet tooth if you see a volume increase because you’ve added a LOT of sugar; enough to taste like coffee flavored candy! If you add 2 tablespoons of sugar to 8 oz. of coffee, you will see a very slight increase in volume, however, if you add those 1 tablespoon to 8 oz. there is no increase in volume. Thus, concentration plays the larger role in our solution volume increase.

So the question becomes, what’s happening when the volume does not increase? Prepare to be amazed! The sugar molecules, as they dissolve, fit in between the water molecules so they don’t take up any additional space. The solution becomes denser without taking up additional volume due to inter molecular spaces. You can visualize it this way – Imagine you have a cup full of glass spheres and you pour fine sand in the cup. The glass spheres don’t rise up because the sand occupies the empty spaces between the spheres. But once the saturation level is reached, i.e. all the empty spaces have been filled, then the volume would start rising.

So it looks like the answer to this question depends on just how sweet you plan on making your cup of Starbucks best in the morning, so consider your solution concentration level and answer your barista’s “room for sugar” question accordingly!

Solutions

Solutions are mixtures of different compounds or elements. You encounter solutions every day without realizing it. The air you breathe—which contains water—is a solution of a liquid and a gas. A soda pop is a solution of a gas dissolved in flavored water. A sterling silver bracelet is a solution of two metals.

In this experiment, we’ll make a solution of a solid and a liquid.

Materials: Water, sugar, clear liquids, 6 clear cups, 1/2 teaspoon measure, 1 cup measure, clock

  1. Place 1/2 teaspoon of salt (solute) into each cup.
  2. Add 1 cup of water (solute) to one cup and check the time on the clock.  Observe the solute (sugar) dissolving in the solvent (water). When it appears to you that the sugar has completely dissolved,  check the clock again and calculate how many seconds it took for the solute to dissolve into the solvent. 
  3. Repeat Steps 1 and 2 with each of the clear liquids  (white vinegar, club soda, glass cleaner, lemonade, apple juice, rubbing alcohol for example).  Did the time to dissolve the solute and make a solution differ with each solvent?

For further study:  Repeat the experiment with different temperatures of water instead of different solvents.  You could also test whether stirring a solute caused it to dissolve faster.

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