Ice typically floats in water, but sometimes it sinks. This behavior, which goes against common expectation, involves the unique physical properties of water and how various factors can alter them.
The Fundamental Reason Ice Floats
Ice floats because, unlike most substances, its solid form is less dense than its liquid form. Density measures how much mass is in a given volume; an object floats if it is less dense than the fluid it displaces. As water freezes, its molecules arrange into an open, hexagonal crystalline structure. This creates more space between molecules than in liquid water. Consequently, a given mass of ice occupies a larger volume than the same mass of liquid water, making ice about 9% less dense.
Factors That Can Make Ice Sink
While pure ice typically floats in pure water, several factors can alter this behavior, causing ice to sink. These factors primarily involve changes in the density of either the ice itself or the liquid it is placed in.
Dissolved Substances in Ice
One common reason for sinking ice is the presence of dissolved substances within the ice. If ice is made from liquids other than pure water, such as sugary drinks or fruit juices, it can become denser. The dissolved solutes, like sugar, add mass to the ice without a proportional increase in its volume, thereby increasing its overall density.
Liquid Density
The density of the liquid surrounding the ice also plays a significant role. If ice is placed in a liquid that is less dense than the ice itself, it will sink. For instance, ice cubes made from water will sink in alcoholic beverages because pure alcohol (ethanol) is less dense than water, and therefore less dense than ice. Many common spirits, which are water-alcohol mixtures, often have densities lower than that of ice, causing water-based ice to sink.
Other Factors
Conversely, if the liquid is significantly denser than pure water, pure ice might float differently. For example, very salty water or highly concentrated sugar solutions are denser than pure water. While pure ice would float higher in such dense liquids, ice made from these solutions would be denser and could potentially sink if its density exceeds that of the surrounding liquid. Additionally, if heavy objects are frozen inside an ice cube, the overall density of the ice cube can increase enough to cause it to sink.
Real-World Scenarios and Other Considerations
In everyday situations, sinking ice is often observed in beverages other than plain water. For example, ice cubes made from pure water will typically float in soda, as soda is usually denser than water due to dissolved sugars and other ingredients. However, if the ice itself is made from a very sugary drink, like frozen soda cubes, those denser cubes might sink in plain water or even in less dense sodas.
When ice melts in a drink, the water level generally does not change if the ice was floating in pure water. This is because the floating ice displaces a volume of water equal to its own weight, and when it melts, it turns into exactly that volume of water. However, a slight change can occur if the ice melts into a liquid of different density, such as when freshwater ice melts into saltwater.
It is a common misconception that air bubbles trapped within ice cause it to sink. In reality, these air pockets make the ice less dense and increase its buoyancy, helping it float. The bubbles occupy space without adding significant mass, effectively lowering the ice’s overall density. Sometimes, an ice cube might appear to sink temporarily if it gets stuck to the bottom or side of a glass, perhaps due to a thin layer of water freezing it in place, but it will typically float once dislodged or as melting occurs.