Question

A cotton ball is dipped in alcohol and wiped across a tabletop. Explain what happens to the alcohol molecules deposited on the tabletop? Is this a physical or chemical change?

Short answer

The alcohol evaporates, and this is a physical change.

Step-by-step solution

Alcohol Evaporation

When the cotton ball wipes alcohol onto the tabletop, the alcohol molecules spread out over the surface. Since alcohol has a low boiling point and is volatile, it evaporates quickly into the air. The energy required for this evaporation comes from the tabletop surface, which often feels cooler as a result.

Molecule Transition

As alcohol evaporates, its molecules transition from the liquid phase to the gas phase. This involves the alcohol molecules gaining enough kinetic energy to overcome intermolecular forces and disperse into the surrounding air.

Classifying the Change

Evaporation is considered a physical change because it involves a change in the state of matter (from liquid to gas) without altering the chemical composition of the alcohol. The molecules remain the same, they are just in a different physical state.

Key concepts

Evaporation

Evaporation is a process where a liquid changes into a gas without needing to get hot. It's kind of like when a puddle of water disappears on a sunny day. This happens because some of the molecules in a liquid move fast enough to break away from the surface and become gas molecules.

For alcohol spread on the tabletop, its molecules are already moving a lot because alcohol has a low boiling point. This means it's easier for alcohol molecules to escape into the air even at room temperature. You might notice that the surface where the alcohol was wiped feels cooler. This is because the process of evaporation absorbs heat from the surroundings, which is called the cooling effect. So, when the alcohol evaporates, you get a cooler tabletop!

Molecular Transition

In molecular transition, molecules shift from one physical state to another. When alcohol is first applied to the tabletop, it's in a liquid state. Here, the molecules are closely packed but can still move around, making it flow.

As the alcohol evaporates, its molecules gain kinetic energy. This is energy that things have when they're moving. More energy allows the molecules to break free from the liquid and scatter into the air as a gas.

This transition from liquid to gas involves energy change but no chemical change. The molecules of alcohol are still alcohol; they've just moved to a different state of matter.

Intermolecular Forces

Intermolecular forces are the forces that hold molecules together. In a liquid, these forces are strong enough to keep molecules close but not so strong that they can't move.

In alcohol, these forces need to be overcome for the molecules to leave the liquid and become gas. The energy from the surface of the tabletop helps the alcohol molecules gain enough movement to break these forces. Imagine it's like a tug-of-war; the stronger the intermolecular forces, the more energy you need to break free.

When molecules succeed in overcoming these forces, they evaporate. This is why alcohol, with its relatively weak intermolecular forces, evaporates so quickly.

State of Matter

The state of matter is basically what form a substance is in—solid, liquid, or gas. When alcohol is on the tabletop as a liquid, it's in the liquid state. Here, the molecules are mobile and flow over surfaces.

During evaporation, alcohol transitions into a gas. This involves changing the arrangement and energy of its molecules. In a gas, molecules move freely and are spread far apart.

Evaporation is a perfect example of a physical change. During this change, only the state rather than the chemical identity of alcohol changes. The chemical formula stays the same, but the way molecules are organized differs as the process advances.