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What makes the ice cream float in a root beer float?

A root beer float, also known as a black cow, is a beloved American drink consisting of vanilla ice cream added to root beer soda. The mixture of the creamy ice cream and the bubbly root beer creates a fun, frothy top and a delicious sweet and smooth beverage. But what makes the ice cream float on top of the root beer rather than sink or blend in completely? The answer has to do with the differences in density and carbonation between the soda and ice cream.

Density Difference Between Soda and Ice Cream

Density refers to how compact a substance is – how much mass is packed into a certain volume. Substances with a higher density sink below substances with a lower density when they are combined, while the less dense substances float above. This same principle applies to the ice cream and root beer in a float.

Ice cream has a lower density than root beer. Ice cream is an emulsion of air bubbles, tiny ice crystals, milk fat, and sugars. It contains a lot of air incorporated into it through churning and freezing, which gives it a light, foamy texture. Root beer, on the other hand, is mostly water mixed with sugar, caramel coloring, and sassafras extract. The liquid soda is denser and heavier than the frozen ice cream.

When the scoop of ice cream is dropped into the root beer, the lower density ice cream naturally floats above the higher density soda below. The ice cream has less mass per volume and is more buoyant than the root beer, allowing it to rise to the top rather than sink down.

Typical Density Values

Substance Density (g/mL)
Ice cream 0.5 – 0.9
Root beer 1.0 – 1.05

As you can see, even the densest ice creams with lowest air content and highest fat content have a lower density than typical root beers. This density difference ensures that the ice cream will always float on top.

Carbonation in Root Beer

In addition to the density difference, the carbonation of the root beer also helps the ice cream float. Root beer contains dissolved carbon dioxide gas that makes the beverage effervescent. These tiny bubbles of CO2 bring the lower density ice cream upward as they rise through the soda.

As the root beer is poured into the glass, the agitation releases some of the gas, forming bubbles that float up. When the ice cream is added, the bubbles cling to the rough, pitted surface of the ice cream, bringing the ice cream up with them as they ascend.

The ice cream essentially gets lifted from underneath by the stream of rising bubbles, further exaggerating the floating effect. As more bubbles continue forming, they help maintain the ice cream at the top of the drink.

Role of Bubble Adherence

The adherence of bubbles to the ice cream surface depends on:

  • Surface roughness – rougher ice cream surface allows better adhesion
  • Surface area – more surface area for bubbles to stick
  • Surface tension – bubbles cling to sites that lower surface tension

Smooth, hard-packed ice creams may not float as readily due to less bubble adhesion. The best floats use freshly scooped ice creams with an airy, porous texture.

Melting and Mixing Effects

While the ice cream initially floats, over time it will gradually start to melt and sink into the root beer, creating a blended layered effect. Several factors contribute to this melting and mixing:

Heat Transfer from Root Beer

The root beer is at a higher temperature than the frozen ice cream, so heat gradually transfers from the soda to the ice cream. This added heat melts the ice cream, making it less rigid and solid. As the ice cream softens and starts to liquefy, its density increases and it is more likely to sink.

Using freshly chilled root beer and minimizing float serving time reduces melting. Using hard ice cream with minimal cream also slows melting.

Dissolution of Ice Cream

As the ice cream melts, the liquid drips down into the root beer, dissolving slowly into the soda. The milk fats and sugars dispersing through the root beer also decrease the density contrast between the fluids, reducing floating.

Using higher fat ice creams or richer sodas minimizes dissolution. Alcoholic floats with spirits added also mix less readily.

Loss of Air

The air incorporated into ice cream is gradually lost after scooping, as the foam structure destabilizes. Loss of air bubbles increases the ice cream’s density, causing it to sink more.

Using ice creams with stabilizers like guar gum helps retain air bubbles longer, maintaining floatability. Smaller scoop sizes also help limit air loss.

Turbulence from Drinking

As you drink through a straw, the resulting suction and turbulence also disturbs and mixes the ice cream into the soda below. The act of consuming the float mingles the contents.

Using a spoon to gently eat the ice cream minimizes mixing, or opting for floats served in glasses with long spoons can prolong separation.

Ideal Conditions for Floating

Here are some ideal factors for the longest lasting ice cream float:

  • Very cold, freshly opened soda
  • Small scoop of airy, creamy ice cream
  • Ice cream with stabilizers to maintain air bubbles
  • Eating ice cream gently with a spoon rather than straw
  • Shorter duration between assembling and drinking float
  • Higher fat ice cream and richer soda formula
  • Alcoholic floats using spirits (minimize dissolution and melting)

Following these tips helps prolong the separation and floating layer. But the melting and mixing are inevitable, contributing to the unique, frothy texture that makes floats so delicious and fun to drink!

The Science Behind Floating Ice Cream

To summarize, the ice cream floats initially because:

– Ice cream has a lower density than root beer due to air content
– Root beer is carbonated, and rising bubbles lift the ice cream

Over time, the ice cream sinks and mixes due to:

– Melting from heat transfer
– Dissolving into root beer
– Losing air bubbles
– Agitation from drinking through a straw

Understanding the science illuminates what factors keep the ice cream floating longer before the inevitable mixing of the delicious ingredients. Next time you enjoy a frosty root beer float, think about the fluid mechanics at play!


A root beer float gets its iconic layered appearance from the difference in density between the melted ice cream and the soda, allowing the ice cream to float on top when initially served. The carbonation in the soda also lifts the ice cream by the rising bubbles adhering to its rough surface. Over time, heat transfer, dissolution, air loss, and agitation all cause the ice cream to melt, sink, and blend into the root beer to create the signature blended texture. Following best practices like using very cold sodas, creamy ice cream with stabilizers, and gently spooning to eat can prolong the separation. But the synergistic blend of the melting ice cream and soda is part of what makes the root beer float such a classic, crave-worthy, summertime treat!