Mason Delacroix
Water droplets can maintain their shape and stay on a penny longer than alcohol due to their higher surface tension. Surface tension is the force that holds the molecules of a liquid together at its surface, and it is influenced by the intermolecular forces present in the liquid. Water has strong hydrogen bonds between its molecules, creating a cohesive force that pulls the molecules at the surface tightly together, increasing surface tension. This allows water droplets to maintain a compact shape, while alcohol droplets, with weaker intermolecular forces, spread out more, taking up additional space.
| Characteristics | Values |
|---|---|
| Water's surface tension | 72.8 mN/m |
| Alcohol's surface tension | 20 mN/m |
| Water's intermolecular forces | Strong hydrogen bonds |
| Alcohol's intermolecular forces | Weaker hydrogen bonds |
| Water's droplets shape | More rounded |
| Alcohol's droplets shape | Flatter |
| Water's droplets size | Smaller |
| Alcohol's droplets size | Larger |
| Water's droplets cohesiveness | Higher |
| Alcohol's droplets cohesiveness | Lower |
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What You'll Learn
Water has a higher surface tension than alcohol
The cohesive force between water molecules creates a "skin" on the surface of the water, known as surface tension. This surface tension allows water to form a nearly spherical "bead" or dome shape when a single drop is placed on a flat, nonporous surface like a penny. The water molecules cling to one another, forming an optimal shape, similar to the bonds on the surface of a blown bubble.
In contrast, alcohol has weaker intermolecular forces and lower surface tension. The structure of alcohol molecules, such as ethyl alcohol (ethanol), leads to less cohesive interaction among them. As a result, alcohol droplets spread out more and take on a flatter shape, occupying more space on the penny's surface.
The difference in surface tension between water and alcohol explains why more water drops can fit on a penny. Water's higher surface tension allows the droplets to maintain their shape and sit closely together, enabling a greater number of drops to fit within the same area. Therefore, the higher surface tension of water, resulting from its strong hydrogen bonds, is the key factor that determines its behaviour on a penny compared to alcohol.
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Water has stronger hydrogen bonds
Water has a higher surface tension than rubbing alcohol, allowing more water drops to be placed on a penny. This is due to the strong hydrogen bonds between water molecules. These hydrogen bonds create a cohesive force that pulls the molecules at the surface tightly together, increasing surface tension.
Water molecules are held together by strong hydrogen bonds. Although hydrogen bonds are the weakest of the chemical bond types, they are of sufficient strength to make water unusually cohesive. This cohesiveness gives water its high degree of surface tension, which is visible in the small indentations made by the legs of certain insects that can walk on water.
The polarity of the water molecule can also cause it to be attracted to molecules of other polar substances. There are two types of "stickiness" in this phenomenon: cohesion and adhesion. The attraction between water molecules is called cohesion. The cohesive force that occurs between water molecules is so strong that, at the water's surface, it creates a "skin", which is known as surface tension.
When water is dropped carefully onto the surface of a penny, it can pile up into a dome shape before spilling over the small lip around the penny's perimeter. This is due to the strong surface tension of water, which allows the droplets to bulge slightly and maintain their shape, enabling more droplets to fit closely together on the penny’s surface.
In contrast, rubbing alcohol has a lower surface tension, causing its drops to spread out and take up more space on the penny. The structure of alcohol molecules leads to less cohesive interaction among them, resulting in lower surface tension. Consequently, fewer drops of alcohol can be held on the penny compared to water.
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Alcohol has weaker intermolecular forces
Water has a higher surface tension than alcohol, which allows more water drops to be placed on a penny. This is due to the strong hydrogen bonds between water molecules, which create a cohesive force that pulls the molecules at the surface tightly together, increasing surface tension.
Alcohol, such as ethyl alcohol (ethanol), has weaker hydrogen bonding compared to water. This results in less cohesive interaction among its molecules, leading to lower surface tension. Consequently, alcohol droplets spread out more and take on a flatter shape, occupying more space and leaving less room for additional droplets.
The concept of surface tension is influenced by the intermolecular forces present in a liquid. Water's strong hydrogen bonds create a high surface tension, resulting in a strong cohesive force that attracts water molecules to each other. This force allows water to form small and compact droplets, enabling more drops to be placed on a penny.
In contrast, alcohol's weaker intermolecular forces result in lower surface tension. This means that alcohol cannot hold its shape as effectively as water, leading to fewer drops being able to fit on the penny's surface. Thus, the difference in surface tension between water and alcohol is due to the varying strengths of their intermolecular forces.
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Water forms a dome shape, alcohol does not
Water forms a dome shape on a penny, whereas alcohol does not. This phenomenon can be explained by the concept of surface tension, which is influenced by the intermolecular forces present in a liquid. Water has a higher surface tension than rubbing alcohol due to the presence of strong hydrogen bonds within its molecular structure. These hydrogen bonds create a cohesive force that pulls the molecules at the surface tightly together, increasing surface tension. The polarity of the water molecule, with its positive and negative ends, also plays a role in its high surface tension.
In contrast, alcohol has weaker intermolecular forces, resulting in lower surface tension. The molecules in alcohol are less attracted to each other and do not form such compact droplets. When dropped onto a penny, alcohol spreads out more and takes on a flatter shape, occupying more space and leaving less room for additional droplets.
The cohesive forces between the polar molecules in water are stronger than those in non-polar molecules, such as those in oil or syrup. This is why water can form a dome shape on a penny, while oil and syrup would quickly spread out and form a thin layer. The same principle applies to alcohol, which has weaker intermolecular forces than water, resulting in its lower surface tension.
The shape of the droplets on a penny is not solely due to surface tension, but also the cohesive and adhesive properties of the liquid. The attraction between water molecules is called cohesion, and it is this force that allows water to form a dome shape. Adhesion, on the other hand, is the attraction between the liquid and the surface it is dropped on. In the case of the penny experiment, adhesion is the force between the water molecules and the penny's surface.
The dome shape of the water droplet on a penny is a result of the balance between cohesive and adhesive forces. As the droplets build up, the cohesive forces between the water molecules pull them together, forming a dome. However, eventually, the force of gravity overcomes the cohesive forces, causing the "'skin'" of the dome to burst, and all the water to spill off.
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Water molecules are more cohesive
In contrast, rubbing alcohol has weaker intermolecular forces, resulting in lower surface tension. This means that when alcohol is dropped onto a penny, its droplets spread out more and take on a flatter shape, occupying more space and leaving less room for additional droplets. The lower surface tension of alcohol causes its drops to spread out and take up more space on the penny, resulting in fewer droplets fitting on the surface compared to water.
The cohesive forces between polar molecules in water are stronger than those between non-polar molecules, such as those found in oil or syrup. This is why a bigger "pile" of water can be formed on a penny compared to these other liquids. The polarity of the water molecule also contributes to its cohesiveness, as the slightly negative oxygen atom can be attracted to the slightly positive hydrogen atoms of neighboring water molecules, forming a weak link known as a hydrogen bond.
The concept of surface tension is influenced by the intermolecular forces present in a liquid, such as hydrogen bonding. Water's high surface tension, due to its strong hydrogen bonds, allows it to form smaller and more cohesive droplets that can fit closely together without spilling over. This results in a higher number of water droplets being able to fit on a penny compared to rubbing alcohol or other liquids with lower surface tension.
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Frequently asked questions
Water has a higher surface tension than alcohol, allowing more water drops to fit on a penny.
Surface tension is the force that holds the molecules of a liquid together at its surface.
Water has strong hydrogen bonds between its molecules, which creates a cohesive force that pulls the molecules at the surface tightly together, increasing surface tension.
You can place a penny on a flat surface and carefully add one drop of water at a time to the penny. The water will form a dome shape before spilling over. Now, repeat the experiment with rubbing alcohol. You will notice that it spills over before forming a well-rounded dome.
Other examples of liquids with varying surface tension include oil, syrup, and soapy water.
