Fossil fuels release carbon dioxide when they burn, which adds to the greenhouse effect and increases global warming. Of the three fossil fuels, for a given amount of energy released, coal produces the most carbon dioxide and natural gas produces the least.

The fossil fuels are coal, oil and natural gas. When fossil fuels are burned, they release nitrogen oxides into the atmosphere, which contribute to the formation of smog and acid rain. Burning of natural gas produces nitrogen oxides and carbon dioxide, but in lower quantities than burning coal or oil.

Fossil fuels are primarily composed of carbon and hydrogen molecules. When these fuels are burned, a chemical reaction takes place, which produces various gases as byproducts. The most common gas produced on burning of fossil fuels is carbon dioxide.

Explanation:
- Fossil fuels such as coal, oil, and natural gas are made up of hydrocarbons, which are molecules composed of carbon and hydrogen atoms.
- When these fuels are burned, the molecules react with oxygen in the air to produce energy and various byproducts.
- The primary byproduct of burning fossil fuels is carbon dioxide (CO2), which is released into the atmosphere as a gas.
- Carbon dioxide is a greenhouse gas that contributes to global warming and climate change.
- Other gases produced on burning fossil fuels include water vapor, nitrogen oxides, sulfur dioxide, and carbon monoxide.
- These gases can also have negative impacts on the environment and human health.

In conclusion, the burning of fossil fuels produces various gases as byproducts, with carbon dioxide being the most common. It is important to reduce our reliance on fossil fuels and transition to cleaner energy sources to mitigate the negative impacts of greenhouse gas emissions on the environment and human health.