Particulate Nature of Matter · Lesson 3 of 6
Liquids, Gases, and Flow
“Compare how particles behave in liquids and gases, and connect that motion to boiling and flow.”
• Use shape and volume observations to distinguish liquids from gases. • Explain why liquids flow yet retain a definite volume. • Distinguish boiling at the boiling point from evaporation below it. • Explain how moving gas particles fill an available space and why smoke is a visual tracer. • Identify liquids and gases as fluids.
The changing shape of water
Pour water from a bottle into a broad bowl and then a narrow cup. It takes a new shape each time. Yet, if none spills or stays stuck to the walls, the amount of water is the same. In the textbook activity, the 200 mL level is marked in three differently shaped containers to make that comparison measurable.
Liquid particles are close together and still attract one another, though a little less strongly than particles in a solid. They are able to move past neighbours within a limited space. The liquid can therefore change shape and flow, while keeping a definite volume.
| State | Shape | Volume | Particle movement |
|---|---|---|---|
| Solid | Definite | Definite | Vibration near positions |
| Liquid | Takes container shape | Definite | Moves past nearby particles |
| Gas | Takes container shape | Fills available space | Moves freely in all directions |
Problem
A student moves 200 mL of water through three containers and reads slightly less in the last one. Does this prove that liquid has no fixed volume?
- 1.First consider whether any water spilled or remained on the earlier containers.
- 2.Water clinging to a wall can make the amount transferred smaller.
- 3.For the same sample without loss, its volume remains definite even as its shape changes.
Why a finger passes through water
Move a finger through a shallow vessel of water. You temporarily push water aside; once the finger is removed, water fills the space again. You cannot move through a stone in the same way. The particles in water remain close but rearrange readily, so the liquid flows without being permanently cut into separate pieces.
Problem
Why does water conform to a jug while a wooden block does not?
- 1.Water particles can move past neighbours and the liquid flows around the jug.
- 2.The block’s particles remain near their positions, so the block keeps its own shape.
- 3.Both have a definite volume, but their shapes behave differently.
Heating a liquid
As a liquid is heated, its particles move more vigorously. At its boiling point under atmospheric pressure, vapour forms quickly throughout the liquid as well as at its surface. The bubbles you see during boiling contain vapour. Particles move far enough apart to enter the gaseous state.
The temperature at which a liquid boils and turns into vapour at atmospheric pressure.
A liquid can become vapour without reaching its boiling point. A small water spill can disappear at ordinary temperature: this slower process is evaporation, which occurs at the liquid’s surface. Boiling is fast and produces vapour within the liquid as well as at its surface.
| Process | Where vapour forms | Temperature condition |
|---|---|---|
| Evaporation | At the surface | Can happen below the boiling point |
| Boiling | At the surface and within the liquid | At the boiling point for the stated pressure |
The disappearance of a spill at room temperature does not show that the water reached its boiling point. Surface evaporation explains it.
A gas occupies the available space
Collect incense smoke in one inverted gas jar, cover it, place a second jar above it, and carefully remove the glass plate between the jars. Smoke spreads into the second jar. It does not remain confined to its original portion of the available space. This illustrates the movement of the surrounding gas.
Gas particles move freely in all directions. Their attraction is negligible for this comparison, so a gas has neither a fixed shape nor a fixed volume: it takes the shape and fills the space of its container. Iodine vapour spreading in a closed jar is another illustrated example, but iodine can irritate and must be handled with care.
Smoke contains tiny visible particles suspended in air. Collisions with moving invisible gas particles help make the spreading visible. Smoke particles are not themselves the constituent gas particles shown by the particle model.
Problem
Why does smoke eventually appear in an initially clear jar placed above the smoke jar?
- 1.Removing the plate opens an available path between the jars.
- 2.Gas particles move in many directions and continually collide with suspended smoke particles.
- 3.The suspended smoke spreads into the newly available space.
What is a fluid?
Liquids and gases both flow and do not retain a fixed shape. They are therefore called fluids. The word does not mean they have identical properties: a liquid retains a definite volume, while a gas spreads to occupy the available space.
A substance that can flow. In this chapter, both liquids and gases are fluids.
Quiz
What happens to the volume of a carefully transferred 200 mL water sample?
Where does vapour form during boiling?
What is true of evaporation?
Why does a gas fill its container?
Which two states are called fluids here?
Practice Problems
- A 200 mL sample changes from a tall vessel to a wide one. Predict its shape and volume in each, assuming no loss.
- Explain how moving a finger through water shows that liquid particles can rearrange.
- Compare the formation of vapour in boiling water with the slow disappearance of a small spill.
- Draw particle diagrams for a liquid in the lower part of a sealed container and a gas occupying the entire container.
- Explain why smoke in a jar helps us infer gas movement without equating smoke particles with gas particles.
- Milk spreads over a table when spilled, but the glass tumbler holds its shape. Explain using the states of matter.
- Give one property shared by a liquid and a gas and one important difference.
Key Takeaways
• A liquid has a definite volume but takes the shape of its container. • Boiling makes vapour rapidly within a liquid and at its surface; evaporation can occur slowly at the surface below the boiling point. • Gas particles move freely, so gases fill the available space and have no fixed shape or volume. • Smoke can reveal movement caused by gas particles but is not a collection of gas constituent particles. • Liquids and gases are fluids because both can flow.