Does Adding Sugar To Water Make It Freeze Harder?

Exploration Phase

Throughout my life I have noticed that ice cubes, popsicles, ice cream, Italian ice, etc. all have different consistencies. Some popsicles are harder than others, for example. I remember making popsicles in our freezer with varying amounts of success, both in taste and consistency. As I explored these things I had the following questions:

  1. Does pure water freeze harder than popsicles?
  2. Why is some ice cream harder than others?
  3. Why is some Italian ice smooth and others crystalline?
  4. How can we get something to freeze faster?
  5. Does adding something to water make it freeze harder?

For this exploration the question "Does adding sugar to water make it freeze harder?" was chosen.

Experimentation and Data Collection

The objective of the experiment was to see if the amount of sugar placed in water has any effect on how hard it is after freezing.

In this experiment an ice cube tray with space for eight ice cubes along one side was used. In the first ice cube space was filled with plain cold water from the tap. Sugar was then placed into each of the remaining spaces, increasing the amount from space to space. A small scoop was used to measure the sugar. Each scoop corresponded to about one teaspoon. The amounts, in terms of scoops, were: 0.25, 0.5, 1, 2, 3, 4, and 5 scoops respectively. The ice tray and sugar were used because they were readily available and sugar dissolving into water is familiar to most people. The experiment could have been done just used zero and five scoops to see if one was soft and the other hard, but it was hoped that a trend might be seen by using varying amounts.

Each cube was filled with water and the sugar was allowed to dissolve. The ice cube tray was placed into the freezer for two hours. After they were in the freezer for two hours each ice cube was tested by pushing on the top of the ice cube with a finger. The data that was collected is given below in tabular and graphical form.

CubeAmount of SugarObservation
#10 ScoopsFelt hard
#20.25 ScoopsFelt hard
#30.5 ScoopsFelt hard
#41 ScoopsDidn't break through the top, but felt softer. It was easier to scratch.
#52 ScoopsDidn't break through the top, but felt softer like the last one.
#63 ScoopsBroke through the top into a liquidy area
#74 ScoopsBroke through the top into a liquidy area
#85 ScoopsBroke through the top into a liquidy area
Very Soft
Very Hard

The hardness of the ice cube with varying amounts of sugar will help to determine if adding sugar makes the ice cube harder when frozen.

Making Sense

The data suggests that adding sugar to water makes the resulting ice cube softer. Comparing zero sugar to five scoops of sugar the data says that the cube with zero sugar is much harder than the cube with five scoops. There is a general trend: as more sugar is added, the ice cube seems to become softer. From the data it is predicted that a very large amount of sugar would not allow the formation of an ice cube at all, it would never freeze.

There is a lot of uncertainty with this result. There wasn't an objective scale of hardness used. It was just perception by a finger and an observation about being able to break the ice layer on top of the cube. The question asked about the hardness after the ice cube was frozen and clearly several of the ice cubes were not completely frozen after two hours in the freezer. A better question may have been about how fast the ice cubes freeze, rather than about how hard they are. There is no numerical data so the uncertainty can't be quantified.

The result does agree with previous experience. It seems from experience that the popsicles with the most sugar are the softer popsicles and the Italian ice with the most sugar is the softer variety. There was no class discussion of this experiment to draw from.

Scientists have studied how the freezing point, or the temperature at which a liquid freezes, changes with the amount of added material. The freezing point is an example of a colligative property which is a property that depends on the concentration of added material. There is even an equation that describes this effect. It says that the change in the freezing point can be found by multiplying the concentration by a constant that is associated with the liquid. Here is some more information if you are interested. The results of this experiment are consistent with the theory of colligative properties.

More information could be obtained by repeating the experiment using more accurate measuring tools, possibly adding even more sugar, and determining a way to better quantify how hard the ice cube is. The amount of time the ice cubes are in the freezer could be extended in order to attempt a more complete freeze. A few new questions are given below. Experiments to answer these questions could extend the current investigation.

  1. Would they all have completely frozen?
  2. Can you add enough sugar to keep it from freezing at all?
  3. Could we use something besides sugar?
  4. Do some substances have more effect than others?