Properties and Behavior of Matter
Matter exists in three main states: solid, liquid, and gas. Each state behaves differently because of how its particles are arranged and how strongly they interact. Since fluids (liquids and gases) are the focus of this unit, you need a solid grasp of what separates them from solids and from each other.
States of Matter and Properties
- Solid – Maintains a definite shape and volume because its particles are closely packed in fixed positions. The particles vibrate in place but don't move freely (think of ice).
- Liquid – Maintains a definite volume but conforms to the shape of its container. Particles are still close together, but they can slide past one another, which is why liquids flow (think of water).
- Gas – Has no definite shape or volume and expands to fill whatever container it's in. Particles are far apart and move randomly, making gases easy to compress and expand (think of air).
Fluids vs. Solids
The word "fluid" covers both liquids and gases. What they share is the ability to flow and change shape because their particles aren't locked in place.
- Fluids flow and conform to the shape of their container. Pour water into a glass and it takes the glass's shape; a gas released into a room spreads to fill the room.
- Solids maintain a fixed shape and volume. A brick sitting on a table doesn't reshape itself to match the table.
One important distinction: fluids cannot resist shear stress (a force applied parallel to a surface) the way solids can. If you push sideways on a block of wood, it holds its shape. If you push sideways on a layer of water, it simply flows. This inability to resist shear stress is actually the defining property of a fluid in physics.
Molecular Arrangement in Fluids
Liquids
- Molecules are close together but not locked into a rigid structure like in a solid. This gives liquids a definite volume but no fixed shape.
- Intermolecular forces are weaker than in solids, so molecules can move past each other (liquid mercury flowing across a surface, for example).
- Liquids are nearly incompressible because the molecules are already packed close together. Trying to squeeze water into a smaller volume takes an enormous amount of force.
Gases
- Molecules are far apart and move randomly, giving gases no definite shape or volume.
- Intermolecular forces are negligible, which is why gases expand to fill any container and compress easily.
- The large spaces between molecules make gases highly compressible. Squeezing a balloon or compressing air in a piston are everyday examples.
Fluid Behavior and Forces
A few key terms you'll see throughout this unit:
- Hydrostatics – The study of fluids at rest and the forces they exert. This is the main focus of Unit 11.
- Fluid dynamics – The study of fluids in motion and how they interact with their surroundings.
- Cohesion – The attraction between molecules of the same substance. Cohesion is what creates surface tension in liquids (water beading up on a waxed car, for instance).
- Adhesion – The attraction between molecules of different substances. Adhesion explains why water climbs slightly up the walls of a glass tube or wets a paper towel.
Both cohesion and adhesion come up again when you study capillary action and surface tension later in the course, so it's worth getting comfortable with the distinction now.