You must have come across a simple thought, why ice floats in water? This question is very simple that may be termed as common sense, but has a very complex scientific explanation. One of the most favorite buoyancy experiments for young school children, that explains the difference between which things float and which sink.
Ice Floating on Water
Density plays an important role in ice floating on water. Density is defined as the qualitative property of a material that is calculated as mass per unit volume. Ice is the solid state of liquid water, that is frozen water. Ice is denser than water. Ice does not follow the basic rules of thermodynamics that is, a solid mass heavier than water will sink in water, but ice instead of sinking floats on water.
Liquids contract on cooling. The individual, free molecules of liquid slow down and stay till the matter turns solid. Water too contracts but up to a certain point. As the temperature drops, molecules slow down and contract. Once the temperature reaches 4ºC , water begins to expand. This unique property of ice lies in its molecular structure. The following explanation on the hydrogen bonds of water and ice will explain why it floats on water.
Molecular Structure of Water
The chemical structure of water is H2O. Thus, there are two atoms of hydrogen(H) and one atom of oxygen (O) in water. These atoms are bound to each other by sharing a 'covalent bond', that is, sharing electrons with one another. The oxygen atoms are slightly negatively charged than hydrogen atoms as oxygen tends to pull the electrons strongly. This pull of oxygen gives hydrogen atoms a slightly positive charge. Thus, adjacent H2O molecules are attracted to each other by the negatively charged oxygen atoms and the slightly positively charged hydrogen atoms. This interaction between the molecules of water is known as 'hydrogen bonding'. The covalent bonds are stronger than the hydrogen bonding. The H2O molecule is said to be polar due to the slightly negative end and slightly positive end.
Why Ice Floats in a Glass of Water?
Just imagine, in a glass of water, all the water molecules are running helter-skelter in the glass. There are a few atoms with slightly negative charge and a few with slightly positive charge. This leads to some amount of attraction between the molecules. There is some amount of density of water, that is, 2 pounds per square inch (psi). When you add ice in the glass of water, the density of the ice cube is less than 2 psi, therefore ice floats in a glass of water.
Why Ice Floats in Water but Sink in Alcohol?
The answer is just as simple, the density of alcohol is lighter than water and has a specific gravity that is less than 1. The molecular density of ice is denser than alcohol and therefore it sinks to the bottom. Whereas, in case of water, as seen above, ice is lighter than water and floats in the glass of water.
Ice is less dense than water and instead of contracting, it expands on cooling. Thus, the volume of ice is greater than the mass of liquid water and therefore ice floats. According to the principle of thermodynamics, the density decreases as the molecules spread out from each other and decrease in temperature increases the density. In case of ice, the opposite occurs. The ice takes up more volume than the liquid water and hence is less dense than water.
Ice is 9% less dense than liquid water and takes up 9% more volume than water. The denser water displaces ice and ice floats on top. Mother nature is very intelligent. In winters, when all the rivers and lakes freeze, only the top layer is frozen and the water underneath the ice is liquid. This helps in survival of all the plants and aquatic animals living in water. If ice sank to the bottom, it would lead to water begin displaced to the top. As more water will be exposed, the lakes and rivers will turn into ice, leading to the death of aquatic life. Thus, this is nature's way of protecting its prized possessions.
Ice Floating on Water
Density plays an important role in ice floating on water. Density is defined as the qualitative property of a material that is calculated as mass per unit volume. Ice is the solid state of liquid water, that is frozen water. Ice is denser than water. Ice does not follow the basic rules of thermodynamics that is, a solid mass heavier than water will sink in water, but ice instead of sinking floats on water.
Liquids contract on cooling. The individual, free molecules of liquid slow down and stay till the matter turns solid. Water too contracts but up to a certain point. As the temperature drops, molecules slow down and contract. Once the temperature reaches 4ºC , water begins to expand. This unique property of ice lies in its molecular structure. The following explanation on the hydrogen bonds of water and ice will explain why it floats on water.
Molecular Structure of Water
The chemical structure of water is H2O. Thus, there are two atoms of hydrogen(H) and one atom of oxygen (O) in water. These atoms are bound to each other by sharing a 'covalent bond', that is, sharing electrons with one another. The oxygen atoms are slightly negatively charged than hydrogen atoms as oxygen tends to pull the electrons strongly. This pull of oxygen gives hydrogen atoms a slightly positive charge. Thus, adjacent H2O molecules are attracted to each other by the negatively charged oxygen atoms and the slightly positively charged hydrogen atoms. This interaction between the molecules of water is known as 'hydrogen bonding'. The covalent bonds are stronger than the hydrogen bonding. The H2O molecule is said to be polar due to the slightly negative end and slightly positive end.
Why Ice Floats in a Glass of Water?
Just imagine, in a glass of water, all the water molecules are running helter-skelter in the glass. There are a few atoms with slightly negative charge and a few with slightly positive charge. This leads to some amount of attraction between the molecules. There is some amount of density of water, that is, 2 pounds per square inch (psi). When you add ice in the glass of water, the density of the ice cube is less than 2 psi, therefore ice floats in a glass of water.
Why Ice Floats in Water but Sink in Alcohol?
The answer is just as simple, the density of alcohol is lighter than water and has a specific gravity that is less than 1. The molecular density of ice is denser than alcohol and therefore it sinks to the bottom. Whereas, in case of water, as seen above, ice is lighter than water and floats in the glass of water.
Ice is less dense than water and instead of contracting, it expands on cooling. Thus, the volume of ice is greater than the mass of liquid water and therefore ice floats. According to the principle of thermodynamics, the density decreases as the molecules spread out from each other and decrease in temperature increases the density. In case of ice, the opposite occurs. The ice takes up more volume than the liquid water and hence is less dense than water.
Ice is 9% less dense than liquid water and takes up 9% more volume than water. The denser water displaces ice and ice floats on top. Mother nature is very intelligent. In winters, when all the rivers and lakes freeze, only the top layer is frozen and the water underneath the ice is liquid. This helps in survival of all the plants and aquatic animals living in water. If ice sank to the bottom, it would lead to water begin displaced to the top. As more water will be exposed, the lakes and rivers will turn into ice, leading to the death of aquatic life. Thus, this is nature's way of protecting its prized possessions.
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