Whether water floats on alcohol or not is determined by the density of the two liquids. Density describes how tightly molecules are packed together in a substance. Substances that are more dense than water will sink in water, while substances that are less dense than water will float.
The Densities of Water and Alcohol
At room temperature, water has a density of about 1 g/cm3. This means that one cubic centimeter (or milliliter) of water has a mass of 1 gram. The density of pure ethanol alcohol is 0.789 g/cm3 at room temperature, which is lower than that of water. This means ethanol molecules are less tightly packed than water molecules. Other types of alcohol have different densities, but all are lower than the density of water:
| Liquid | Density (g/cm3) |
|---|---|
| Water | 1 |
| Ethanol | 0.789 |
| Methanol | 0.791 |
| Propanol | 0.803 |
| Butanol | 0.810 |
Since alcohol is less dense than water, we expect it to float on top of water. The lower density liquid (alcohol) will be buoyed by the higher density liquid (water). Next we’ll look at some examples to demonstrate this.
Alcohol Floating on Water
A good way to visualize alcohol floating on water is to pour alcohol into a glass of water. For example, if you pour a small amount of isopropyl rubbing alcohol into a glass of water, the alcohol will quickly float to the top and form a separate layer on top of the water. This occurs because the alcohol is less dense than the water, so it is buoyed upward until it forms its own layer.
The same thing happens with ethanol alcohol. If you pour some vodka, rum, or other ethanol-based liquor into a glass of water, it will float on top of the water. The ethanol alcohol molecules are less densely packed than the water molecules, causing the ethanol to be pushed upward.
In fact, this will occur with any water-alcohol mixture, regardless of the proportions. Even if there is more alcohol than water, the water will still sink below the alcohol. For example, if you mix 2 cups vodka (95% ethanol) and 1 cup water, the water will sink to the bottom and the ethanol will float on top. This demonstrates conclusively that alcohol is less dense than water.
Surface Tension Effects
In some cases, you may observe a phenomenon where the alcohol appears to be suspended within the water rather than sitting on top. This can occur due to surface tension effects. The molecules at the surface of a liquid exhibit a stronger attraction than those in the interior, creating an elastic “skin” on the liquid surface. This surface tension can temporarily hold up small amounts of alcohol within water.
However, given enough time, the lower density alcohol will always migrate upward and form a separate layer on top of water. Surface tension just slows down the separation but cannot prevent it, as gravity works to pull the lower density alcohol upward.
Why Does Alcohol Density Matter?
The fact that alcohol floats on water is hugely important for aquatic life. Bodies of water contain microorganisms, plants, insects, fish and other creatures. If alcohol easily mixed with water, it could poison aquatic habitats. Fortunately, the lower density of alcohol causes it to float as a separate layer on top of water.
This has ecological benefits, as it prevents severe alcohol contamination in the lower portions of lakes, rivers and ponds. Any alcohol pollution tends to only affect surface-level organisms rather than penetrating deep into an aquatic ecosystem. The density discrepancy between alcohol and water creates a natural protective barrier.
The importance of alcohol’s low density also extends to alcohol-based hand sanitizers. Many hand sanitizers contain 60-80% ethanol or isopropanol alcohol. However, despite their high alcohol content, they still contain enough water to ensure the alcohol floats on top rather than sinking into skin. This enables effective hand sanitization without excessive skin absorption.
Alcohol Density Impacts Food and Beverages
The fact that alcohol floats on water is also extremely relevant for food and beverage production. Ethanol’s low density allows it to mix in varying proportions with water while still separating into distinct layers. This enables drinks like cocktails and wine spritzers that gently blend alcohol with other liquids.
Alcohol’s lower density than water also explains why ice floats in cocktails and other beverages. Since ice is frozen water, it remains higher density than the ethanol components in a drink. This keeps ice cubes buoyed upward rather than sinking.
In cooking, alcohol’s partial miscibility with water lets it integrate into sauces and extracts while still retaining a lower density. This allows flavors to dissolve out of herbs and spices into the small amount of added alcohol, even in largely aqueous mixtures.
Can You Make Alcohol Sink in Water?
Since alcohol floats on water due to its low density, is there a way to force it to sink? One possibility is to dissolve salts or other dense compounds into the alcohol to increase its density. For example, dissolving table salt (which has a density of 2.17 g/cm3) into ethanol can increase the overall liquid density.
However, it requires extremely high salt concentrations to make the alcohol salty enough to sink below water. This results in very caustic, unpalatable alcohol solutions. It also does not work well for pure water – the salt merely makes the alcohol drop through the water to the bottom of the container. A better technique is to use brines or salt-water mixtures.
Using Brines to Make Alcohol Sink
By dissolving salt in water to make a brine, you can increase the density of the water beyond that of alcohol. The salt ions increase the overall mass per volume. If the brine is dense enough, alcohol added to it will sink below the brine surface rather than floating.
For example, you can stir 350 g of table salt into 1 liter of water to make a concentrated brine. This will increase the density of the water to around 1.25 g/cm3. If you then pour ethanol (0.789 g/cm3) into the brine, the alcohol will sink through the water rather than floating! The higher density brine pushes the lower density ethanol downward.
This method illustrates that it is possible to force alcohol to sink, given a sufficiently dense water mixture. However, extremely concentrated brines are needed. It does not change the fact that in normal water, the lower density of alcohol causes it to float on the surface.
Conclusion
In summary, alcohol does float on water, due to its lower density compared to water. At room temperature, ethanol alcohol has a density of 0.789 g/cm3 compared to 1 g/cm3 for water. This density discrepancy causes the alcohol molecules to be buoyed upward when mixed with water, forming a separate floating layer.
Experiments with ethanol, methanol, propanol and other alcohols confirm they all float on water. Even in extreme cases with more alcohol than water, the water component will sink while the alcohol floats on top. This density difference has ecological benefits, prevents over-absorption of alcohol, and enables mixing of alcohol in beverages.
While it is possible to force alcohol to sink by using concentrated salt brines, this does not change the fundamental density relationships. Under normal circumstances, alcohol will always float on water rather than sinking through it.