Gases:
* Helium (He): The most famous example. It's why helium balloons float!
* Hydrogen (H2): Even lighter than helium, but highly flammable.
* Neon (Ne): Used in bright signs.
* Methane (CH4): A major component of natural gas.
* Ammonia (NH3): Used in fertilizers and as a refrigerant.
Liquids:
* Liquid Hydrogen (LH2): Used as rocket fuel.
* Liquid Helium (LHe): Used in scientific research.
* Liquid Nitrogen (LN2): Used in cryogenics (very low temperatures).
Solids:
* Aerogels: Extremely porous materials with densities as low as 0.001 g/cm³. They can be made from silica, carbon, or other materials.
* Expanded Polystyrene (EPS): The familiar "Styrofoam" used in packaging.
* Balsa Wood: Lightweight wood used in model airplanes.
* Some plastics: Certain types of plastic foams can have very low densities.
Important Notes:
* Density is dependent on temperature and pressure: Changing these factors can alter the density of a substance, making it denser or less dense than air.
* Air itself is a mixture: Its density is determined by the gases it contains (primarily nitrogen and oxygen) and can vary depending on altitude, humidity, and temperature.
Let me know if you want more detail about specific substances or their uses!
