An opaque gas is a gas that does not allow light to pass through it. This is in contrast to a transparent gas, which allows light to pass through it without being absorbed or scattered. Opaque gases are often used to create a barrier between two spaces, such as in a window or a greenhouse.

Some examples of opaque gases include:

* Carbon dioxide (CO2)

* Water vapor (H2O)

* Nitrogen dioxide (NO2)

* Sulfur dioxide (SO2)

These gases are all composed of molecules that are large and complex, which makes it difficult for light to pass through them. In addition, these gases often have a high density, which further reduces the amount of light that can pass through them.

Opaque gases can be used for a variety of purposes. For example, they can be used to:

* Create a barrier between two spaces

* Block out light

* Create a greenhouse effect

* Absorb heat

Opaque gases are an important part of our environment. They play a role in regulating the Earth's temperature and in creating the conditions that allow life to exist on Earth.