# Why do my "steel ice cubes" have water in them?

I recently bought steel ice cubes. A better name is probably "steel cubes with unidentified liquid" which I presume is water. When I opened the package I was quite surprised that they contained some liquid. I thought it was going to be a solid cube of steel. This made me think how do steel cubes compare to plastic cubes filled with water (or regular ice cubes)?

Volume of cube: $$V=(2\ \mathrm{cm})^3=8\cdot10^{-6}\mathrm{m^3}$$
Specific heat capacity of ice $$C_i=2090\left[\mathrm{\frac{J}{kg ^\circ C}}\right]$$
Specific heat capacity of steel $$C_s=466\left[\mathrm{\frac{J}{kg ^\circ C}}\right]$$
Density of ice $$\rho_i=917 \left[\mathrm{\frac{kg}{m^3}}\right]$$
Density of steel $$\rho_s=7750 \left[\mathrm{\frac{kg}{m^3}}\right]$$

I used the lower boundary for density of steel (from source).

Ice cubes:
Heat capacity $$C=\rho_i\cdot V \cdot C_i \approx 15.33\left[\mathrm{\frac{J}{^\circ C}}\right]$$

Steel cubes:
Heat capacity $$C=\rho_s\cdot V \cdot C_s \approx 28.89\left[\mathrm{\frac{J}{^\circ C}}\right]$$

Even though I used the lower boundary for the density of steel it still has more heat capacity than ice. From this it seems to make most sense to produce cubes of solid steel since it would cool your drink more.

Why do my steel cubes (used for cooling drinks) contain (presumably) water?

Could it be that solid steel cubes transfer their heat too quickly? Hence manufacturers add water inside which does lower the heat capacity but make the cubes cool for a longer time period? Or am I interpreting the results from my calculations wrongly? Or maybe my calculations are wrong?

Sources where I get values:

• I think you are expecting steel to be better at heat storage than water. Actually, water is the best heat storage material known by volume. So your math is off. By volumetric heat capacity, water beats steel (though steel is #2 among common materials). See table here Oct 19, 2021 at 4:57
• I wonder how you can tell they have liquid in them, if they are made of steel which is not transparent? Oct 19, 2021 at 8:20
• @user253751 There must be air inside to allow space for the water to expand when it freezes. So OP must be able to hear the water when he shakes the cubes. Oct 19, 2021 at 10:30
• @WoJ all the solid ones are useless, but some are more decorative than others Oct 19, 2021 at 10:32
• It's possible that it's a saline solution instead of pure water. That would depress the freezing point (and increase cooling capacity) while not adding very much cost. Some electric ice cream makers use saline solutions, though they do need to actually freeze the liquid they're in contact with rather than just cool it, so cooling capacity is more of a concern for them. Oct 19, 2021 at 12:55

Water thaws at 0°C. The latent heat of fusion of water is $$L_i = 344000$$ J/kg, which means that to thaw the frozen water inside your cubes requires much, much more energy than you're accounting for. Steel, on the other hand, undergoes no such phase transformation (edit: at comfortable drinking temperatures), and so it cannot invoke the magic of phase transformations to cool your drinks.
To put some numbers on it: how cold would a cube of solid steel have to be to absorb as much heat as the same volume of melting ice? We want $$\rho_s C_s V \Delta T = \rho_i L_i V \quad \Rightarrow \Delta T = \frac{\rho_i L_i}{\rho_s C_s} = 87.3 {}^\circ \mathrm{C}.$$ So to match the performance of a melting ice cube at 0°C, your steel cube would have to start out at -87°C or so, which is clearly out of the range of household freezers.