Recently we acquired a set of spherical ice molds at the primary testing facility. While the idea of spherical ice is exciting all by itself, the packaging for the molds makes several bold claims about the properties of spheres. Having all of the equipment on hand to test those claims, we decided to do science to them.

Initial observation:

Spherical ice mold packaging claims, “Ultra-slow-melting Ice Sphere quickly cools your cocktail without watering it down.”

Question:

Does spherical ice a) melt slowly, b) cool quickly, and c) dilute the drink less?

Expectation:

Since a sphere will have less surface area, for the same volume of ice, this means there is less contact between the ice and the liquid being cooled. Therefore, I expect less melting, but also slightly slower cooling.

Equipment & Materials:

Procedure:

Mass an empty glass. Add a single sphere ice to the glass. Mass glass and ice. Add 3 fl. oz. of bourbon to glass. Mass glass with ice and bourbon. Place temperature probe into glass. Record temperature until the bourbon reaches an acceptable drinking temperature, or stops changing. Remove probe and mass again. Remove ice and mass glass with bourbon. Repeat steps 1 through 9 using an equal mass of ice cubes. Enjoy responsibly with a fellow doer of science.

Data:

To determine how much ice melt occurs during the course of cooling, a series of mass measurements were taken to facilitate determining the mass of ice before and after each trial. The table below gives each of these measurements.

Sphere (g) Cubes (g) Glass 322.8 322.0 +ice 445.1 436.1 +bourbon 523.5 514.2 After Cooling 523.4 514.1 -ice 420.9 423.3

Based on the raw measurements, the masses of each component were computed, as show in the table below.

Sphere (g) Cubes (g) Bourbon 78.4 78.1 Initial Ice 122.3 114.1 Final Ice 102.5 90.8 Ice Melted 19.8 23.3

During the cooling phase of the experiment, temperatures were recorded every half second. Below is a graph showing the 5 second average temperature until the temperature stabilized (<0.01ºF/s). The slight uptick at the end is from the removal of the probe. Both the sphere and the cubes start cooling the bourbon very rapidly. The sphere however slows considerably around 35-40 seconds and comes to a final temperature around 39ºF. The cubes maintained a faster cooling rate and reached a final temperature around 33ºF. The cubes achieved this additional cooling in approximate 30 fewer seconds, starting with 8g less of ice.

Using this temperature data, it’s possible to compute the rate of change over time. Below is a graph of the temperature change over time.

From this we can see that while both the sphere and the cubes managed the same peak level of cooling, the spheres peaked sooner and the cubes maintained a higher rate of cooling for a longer period of time. This resulted in more overall cooling by the cubes. If we look at the rate of cooling in conjunction with the amount of ice melted (see table below), we can see that while the cubes reached a lower final temperature than the sphere, and in less time than the sphere. The cubes did this by melting at a rate 43% greater than the sphere. In both cases the temperature change per gram of ice melted was nearly identical.

Sphere Cubes Initial Bourbon Temp (ºF) 67.96 67.9 Final Bourbon Temp (ºF) 39.17 33.26 Total Time (s) 176s 144s Average Cooling Rate (ºF/s) 0.164 0.241 Average Melt Rate (g/s) 0.113 0.162 Temp Change per gram of ice melted (ºF/g) 1.454 1.487

Surprise Results:

Not only does the shape of the ice affect the rate of cooling, but it also had a surprising effect on what temperature the cooling appears to stop.

Conclusion:

Based on these results, ice cubes not only cool faster, but are able to chill bourbon to a lower temperature. This increased cooling however comes at the cost of increased melting, and dilution of the drink. Therefore the ice spheres do appear to melt slower, chill the drink in a reasonable amount of time, and dilute the drink less. However ice cubes are able to cool a drink more rapidly and to a lower temperature.

Future Questions:

Did the ice cubes only seem to cool faster due to larger direct contact with the probe? Would continual stirring affect the results? How do these cooling rates compare to say perfect cubes of ice, whiskey stones , or stainless steel cubes ? How would using amounts of ice with equal surface areas affect the results?