Question 1

What are the three phases (states) of water?

Accepted Answer

The three phases of water are solid (ice), liquid (water), and gas (water vapor/steam).

Question 2

At what temperature does water freeze at sea level?

Accepted Answer

Water freezes at 0°C (32°F) at sea level (standard atmospheric pressure).

Question 3

At what temperature does water boil at sea level?

Accepted Answer

Water boils at 100°C (212°F) at sea level (standard atmospheric pressure).

Question 4

Describe the shape of a heating curve for water.

Accepted Answer

The heating curve shows temperature rising, then a flat plateau at 0°C (melting), rising again, then another flat plateau at 100°C (boiling), then rising again. The pattern is: up → flat → up → flat → up.

Question 5

Why does the temperature stop rising during a phase change?

Accepted Answer

During a phase change, the added energy (heat) is used to break intermolecular bonds rather than increase the temperature. This energy is called latent heat.

Question 6

What are the two flat sections on the heating curve called?

Accepted Answer

The first flat section at 0°C is the melting point (solid → liquid). The second flat section at 100°C is the boiling point (liquid → gas). Both represent phase changes where latent heat is absorbed.

Question 7

What is latent heat?

Accepted Answer

Latent heat is the energy absorbed or released during a phase change without a change in temperature. It is used to break or form intermolecular bonds.

Question 8

What is happening at the molecular level when ice melts?

Accepted Answer

When ice melts, the water molecules absorb energy and break free from their rigid crystal lattice structure. They gain enough energy to move past each other, becoming liquid water.

Question 9

What is happening at the molecular level when water boils?

Accepted Answer

When water boils, molecules absorb enough energy to completely overcome intermolecular attractions and escape into the gas phase, moving rapidly and independently as water vapor.

Question 10

How do the molecules in ice differ from molecules in liquid water?

Accepted Answer

In ice, molecules are locked in a fixed hexagonal crystal lattice and can only vibrate. In liquid water, molecules are still close together but can slide past each other and move more freely.

Question 11

Why does ice float on liquid water?

Accepted Answer

Ice floats because its crystal lattice structure spaces molecules farther apart than in liquid water, making ice less dense (≈0.92 g/cm³) than liquid water (1.0 g/cm³).

Question 12

How does a cooling curve differ from a heating curve?

Accepted Answer

A cooling curve is the reverse of a heating curve. Temperature decreases, then plateaus at 100°C (condensation) and again at 0°C (freezing), with energy being released rather than absorbed during phase changes.

Question 13

What is the difference between evaporation and boiling?

Accepted Answer

Evaporation occurs at the surface of a liquid at any temperature, while boiling occurs throughout the entire liquid at the boiling point (100°C at sea level). Both convert liquid to gas.

Question 14

What is condensation?

Accepted Answer

Condensation is the phase change from gas to liquid. Water vapor loses energy, slows down, and intermolecular attractions pull molecules together to form liquid water.

Question 15

A student heats ice from -20°C to 120°C. Describe what happens at each stage.

Accepted Answer

-20°C to 0°C: Ice temperature rises as molecules vibrate faster. At 0°C: Temperature plateaus — latent heat of fusion absorbed, ice melts to liquid. 0°C to 100°C: Liquid water temperature rises. At 100°C: Temperature plateaus — latent heat of vaporization absorbed, water boils to steam. Above 100°C: Steam temperature rises.

Question 16

Which requires more energy — melting ice or boiling water? Why?

Accepted Answer

Boiling water requires more energy. The latent heat of vaporization (2260 J/g) is much greater than the latent heat of fusion (334 J/g) because all intermolecular bonds must be completely broken to convert liquid to gas.

Question 17

If you add heat to water at 100°C, why doesn't the temperature increase immediately?

Accepted Answer

At 100°C, all added energy goes into breaking intermolecular bonds to convert liquid water to steam (latent heat of vaporization). The temperature only rises again once all the liquid has vaporized.

Phases of Water Answer Key

Prior Knowledge Questions

Activity A – Heating Curve

Activity B – Molecular Level Changes

Activity C – Cooling Curve

Assessment Questions

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