Problem 91 - Chemistry The Science in Context

Question

Explain why a needle floats on the surface of water but sinks in a container of methanol \(\left(\mathrm{CH}_{3} \mathrm{OH}\right)\).

Step-by-Step Solution

Answer

Answer: A needle floats on the surface of water due to the high surface tension of water and the upward buoyant force. However, in methanol, the surface tension is weaker, and the combination of weaker surface tension and buoyant force is not enough to keep the needle from sinking. This is why a needle floats on water but sinks in methanol.

Step 1: Understanding Surface Tension

Surface tension is a result of the cohesive forces between liquid molecules. In water, the hydrogen bonding between water molecules gives it a high surface tension. This high surface tension allows a needle, which has a greater density than water, to float on the surface of the water without breaking the surface. In methanol, the intermolecular forces are weaker compared to water, resulting in a lower surface tension. Therefore, when a needle is placed on the surface of methanol, the surface tension is not strong enough to support the needle.

Step 2: Understanding Buoyancy

Buoyancy is the upward force exerted on an object submerged in a fluid, and it is related to the density of the fluid and the object. When an object is placed in a fluid, the fluid exerts an upward force on the object equal to the weight of the fluid displaced by the object. This is called the buoyant force. The needle, having higher density than water, would normally sink. However, the strong surface tension of the water supports the needle, and due to the distribution of pressure on the surface of the water, the needle does not sink.

Step 3: Comparing Surface Tension and Buoyancy in Water and Methanol

The needle can float on water due to the combination of high surface tension and upward buoyant force. The high surface tension of water supports the needle, and the buoyant force helps to counteract the needle's weight. In methanol, the weaker surface tension cannot support the needle, and the upward buoyant force is not enough to keep the needle from sinking. Due to the weaker intermolecular forces in methanol compared to water, the needle sinks into the methanol.

Step 4: Conclusion

A needle floats on the surface of water due to the high surface tension of water and the upward buoyant force. However, in methanol, the surface tension is weaker, and the combination of weaker surface tension and buoyant force is not enough to keep the needle from sinking. This is why a needle floats on water but sinks in methanol.

Key Concepts

Buoyancy

Buoyancy is the ability of an object to float in a fluid, which is determined by the relationship between the object and the fluid's density. When you place an object in a fluid, the fluid exerts an upward force known as buoyant force. This force is equal to the weight of the fluid that the object displaces. Essentially:

• If an object is denser than the fluid, it tends to sink because the buoyant force is less than the object's weight.
• If an object is less dense, it will float as the buoyant force is greater than or equal to the weight of the object.

Therefore, if a needle were simply subject to buoyancy, it would sink in both water and methanol, as a needle is denser than both fluids. However, in the case of water, other forces come into play, helping illustrate why buoyancy alone doesn't determine whether a needle floats.

Hydrogen Bonding

Hydrogen bonding is a special type of intermolecular force that occurs when hydrogen is directly bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine. In water (H₂O), each water molecule can form multiple hydrogen bonds due to the presence of oxygen:

• These hydrogen bonds between water molecules create a strong attraction that pulls the molecules close together.
• This tight network of bonds gives water its characteristic high surface tension.

The high surface tension is significant because it acts like a 'skin' on the water’s surface. When a needle is placed on the surface of the water, it can float due to the hydrogen bonds holding the water molecules tightly together, preventing the needle from breaking through.

Intermolecular Forces

Intermolecular forces are forces of attraction or repulsion between neighboring particles (molecules, atoms, or ions). They are weaker than the bonds holding atoms together within a molecule but are crucial in determining the properties of substances:

• In water, as mentioned, strong hydrogen bonds form due to oxygen's electronegativity.
• In methanol (\( \mathrm{CH}_{3} \mathrm{OH} \)), the intermolecular forces are weaker than in water. Although methanol can form hydrogen bonds, they are less extensive and robust than those in water.

These weaker forces mean methanol has a lower surface tension. As a result, when a needle is placed on methanol, these insufficient intermolecular forces fail to support the needle, causing it to sink. Thus, while buoyancy and surface tension play a role, the nature and strength of the intermolecular forces in the fluid directly affect an object's ability to float.