CO2 is a colorless gas at room temperature. It is slightly soluble in water. Its melting point is -56.6 ̊C (-69.88 ̊F). CO2 is the main substrate of photosynthesis which is essential for plants and some microorganisms like algae. It is also the metabolism product of animals etc. CO2 is one of the main greenhouse gases that cause global warming. The concentration of CO2 in the atmosphere has been constantly rising, due to human activities such as usage of fossil fuels. Global warming caused by greenhouse gas including CO2 may lead to more extreme weathers and disasters and is a threat to human lives on earth.