Final answer:
Gas is the matter with definite weight but no fixed volume or shape. Gases expand to fill their container, demonstrating that they have mass and occupy space. The inflatable nature of a balloon with air exemplifies these properties.
Step-by-step explanation:
Matter with a definite weight but an indefinite volume and shape is called gas. Gases are one of the three common phases of matter, alongside solids and liquids. While solids have both a definite shape and volume, and liquids have a definite volume but take the shape of their container, gases neither have a definite shape nor a specific volume. Instead, gas particles move freely to fill the entire space available to them, which is why their volume is indefinite.
The mass of gases, such as the air in a balloon, can be detected by weighing the balloon when it is deflated and again after it is inflated, demonstrating that gases have mass and take up space. This property also explains why a balloon inflates: the gas inside occupies space and exerts pressure on the walls of the balloon.