Question

Describe and illustrate the concept of boiling point using water as an example.

Short answer

Water boils at 100°C at 1 atmosphere due to its kinetic energy overcoming intermolecular forces.

Step-by-step solution

Understanding the Boiling Point Concept

The boiling point of a substance is the temperature at which its vapor pressure equals the external pressure surrounding the liquid. For water, the normal boiling point is 100°C at 1 atmosphere of pressure.

Examining Water's Unique Properties

Water has hydrogen bonds between its molecules, which are relatively strong intermolecular attractions. These bonds require energy to break, and thus, water has a higher boiling point compared to many other substances of similar molecular weight.

Heating Water to Boil

As heat is applied to water, its temperature rises, causing an increase in the kinetic energy of the water molecules. At 100°C and 1 atm, the kinetic energy is sufficient to overcome intermolecular forces, allowing molecules to escape as vapor.

Illustration of Phase Transition

When water is heated and reaches its boiling point, there is a phase transition from liquid to gas (steam). Bubbles form within the liquid and rise to the surface, demonstrating the change from liquid to gas throughout the entire liquid volume.

Key concepts

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor when it is in equilibrium with its liquid phase at a given temperature.
As the temperature of a liquid increases, so does its vapor pressure. This is because the increased heat gives more molecules the energy needed to escape into the vapor phase.

• For example, water at room temperature has a low vapor pressure because fewer molecules have sufficient energy to escape into the air as vapor.
• When water is heated to 100°C, its vapor pressure equals atmospheric pressure, allowing it to boil.

At this point, the temperature is known as the boiling point, and this is when the liquid transitions into gas.
This concept is crucial in understanding how and why water turns into steam during boiling.

Hydrogen Bonds

Hydrogen bonds are a type of intermolecular force that occurs between molecules containing a hydrogen atom bonded to a highly electronegative atom like oxygen.
In water, the oxygen atom of one molecule is attracted to the hydrogen atom of a neighboring molecule.

• This attraction results in relatively strong bonds that hold the water molecules together.
• These bonds are the reason for water's relatively high boiling point compared to other substances of similar size and weight.

Breaking hydrogen bonds requires energy, which explains why water needs more heat energy to reach its boiling point.
Without this energy input, water molecules remain in the liquid phase due to these strong connections.

Phase Transition

A phase transition is the change from one state of matter to another, such as from liquid to gas.
Water undergoing a phase transition from liquid to gas at its boiling point is a classic example.

• As water reaches 100°C at 1 atm, it undergoes this transition in which molecules throughout the liquid gain enough energy to enter the gaseous state.
• Bubbles form within the liquid, which signifies this transformation, and they rise to the surface to release steam.

This transition is called boiling, and it involves breaking intermolecular forces, notably hydrogen bonds, allowing water molecules to disperse widely into the gas phase.

Kinetic Energy

Kinetic energy refers to the energy an object has due to its motion. In the context of boiling, it applies to the water molecules within the liquid.
As heat energy is applied, the water molecules move more vigorously, increasing their kinetic energy.

• At the boiling point, this kinetic energy is sufficient to overcome the intermolecular forces, specifically hydrogen bonds, between the molecules.
• This allows for the transition from liquid to gas to occur, with molecules having enough energy to escape into the vapor phase.

Thus, a higher temperature provides more kinetic energy, ultimately leading to the phase transition of boiling.