Question

If one is very careful, it is possible to float a needle on the surface of water. (If the needle is magnetized, it will turn to point north and south and become a makeshift compass.) What would happen to the needle if a drop of liquid soap is added to the solution? Explain the observation.

Short answer

The needle sinks because soap reduces water's surface tension, breaking the 'film' that allows the needle to float.

Step-by-step solution

Understanding Surface Tension

The needle floats on the water because the surface tension of the water creates a sort of 'film' that keeps the needle from sinking, despite being denser than water. This is a demonstration of how surface tension can support light objects.

Effect of Surface Tension on a Magnetized Needle

When a needle is magnetized, it aligns with Earth's magnetic field, and because it is floating due to surface tension, it will act like a compass, pointing towards the magnetic north and south.

Impact of Adding Soap

Adding a drop of liquid soap to the water decreases the water's surface tension. Soap molecules disrupt the hydrogen bonds between water molecules, which weakens the surface tension that was keeping the needle afloat.

Resulting Observation after Adding Soap

Once the surface tension is weakened by the soap, the 'film' supporting the needle breaks, and the needle sinks instead of floating. This occurs because the reduced surface tension can no longer hold the weight of the needle.

Key concepts

Magnetic Field

A magnetic field is an invisible force that surrounds magnetic materials and electric currents. When a needle is magnetized, it becomes a tiny magnet itself. This means it can align with Earth's magnetic field. If you gently place a magnetized needle on water, surface tension allows it to float, essentially turning it into a simple compass.

This phenomenon happens because the needle's north-seeking pole aligns with the Earth's magnetic north, while the opposite pole points south. It's the same principle that allows a compass to work. As long as the surface tension is intact, the magnetic properties of the needle are observable.

Hydrogen Bonds

Hydrogen bonds are special connections that occur between water molecules. These bonds are the reason water has a high surface tension. Imagine them as little hooks linking the molecules, forming a tight surface. This tightness allows objects like a lightweight needle to rest on the surface without sinking.

When you add soap to water, it disrupts these hydrogen bonds. Soap molecules have a hydrophilic (water-attracting) head and a hydrophobic (water-repelling) tail. This structure interferes with the bond between water molecules, lowering the surface tension.

Water Molecules

Water molecules are fascinating due to their unique shape. Each molecule is made up of two hydrogen atoms bonded to one oxygen atom, forming an angular shape. This geometry is key to water's ability to form hydrogen bonds with neighboring molecules.

The cohesive forces between water molecules create a sort of 'skin' on the surface, known as surface tension. This property of water can hold small, dense objects that wouldn't usually float. The arrangement and interaction of water molecules are central to understanding many phenomena in chemistry and physics.

Soap and Detergents

Soap and detergents are agents that help reduce surface tension. They work by inserting themselves between water molecules with their unique molecular structure. The hydrophilic head attaches to the water, while the hydrophobic tail seeks non-water environments.

When soap is introduced to water, it spreads out across the surface, breaking up the arrangement of water molecules and weakening the hydrogen bonds. This reduction in surface tension means that water can no longer hold up the needle, causing it to sink. Soap also helps dissolve grease and oils, making it an essential cleaning agent.