Describe solids, liquids, and gases in terms of how they fill a container. Use your descriptions to identify the physical state (at room temperature) of the following: (a) helium in a toy balloon; (b) mercury in a thermometer; (c) soup in a bowl.

Solids are one of the basic states of matter. They have a definite shape and volume, meaning they maintain their form regardless of the container you put them in. For example, if you place a rock in a bowl, the rock will not change its shape to conform to the bowl's shape. It will simply sit in the bowl, taking up a fixed amount of space. The particles in solids are tightly packed together, which makes them maintain their rigid structure. This is why solids are often hard and not easily compressible.

Liquids are another fundamental state of matter. Unlike solids, liquids do not have a fixed shape, but they do have a fixed volume. This means they will take the shape of the container they are placed in, but they won't expand to fill the container completely if there isn't enough liquid. For instance, if you pour water into a glass, the water will spread out to fit the shape of the glass but won't fill the glass to the top if there's not enough water. This happens because the particles in a liquid are close together but can move past each other, allowing the liquid to flow and take the shape of its container.

Gases are unique among the states of matter because they have neither a fixed shape nor a fixed volume. Instead, gases will expand to fill the entire space available to them. For example, if you release helium into a balloon, the helium will spread throughout the balloon, filling it entirely. This property occurs because the particles in a gas are far apart and move freely, which allows the gas to expand and occupy any available space. Due to this, gases are easily compressible and can fill containers of any shape and size.

Understanding how different states of matter interact with containers is crucial for many practical applications.
- **Solids:** They retain their shape and only occupy the space they are placed in.
- **Liquids:** They take the shape of the container but do not expand to fill it if there isn't enough liquid.
- **Gases:** They will expand to fill any container completely, no matter the amount of gas present.
For example, **helium** in a toy balloon is a gas and will fill the balloon entirely, **mercury** in a thermometer is a liquid and will take the shape of the thermometer's reservoir but not fill it entirely, and **soup** in a bowl, being primarily liquid, will take the shape of the bowl but might not fill it to the top if there's not enough soup.