Chemical reactions, which can be classified into various categories, are processes in which substances change to form other substances with different properties. Some chemical reactions are fast and some are slow. The study of energy changes in chemical reactions is known as thermochemistry.

All chemical reactions involve energy transformations. For example, consider the carbon cycle, shown below.

It is important to realize that solar energy is a major source of stored energy on Earth.

• Through the process of photosynthesis, plants store the sun's energy within the bonds of glucose molecules.
• Through the process of cellular respiration, the energy stored within these glucose molecules is utilized to power the daily life processes of both plants and animals. When these plants and animals die, the plant and/or animal matter decays and, over time, may be converted into fossil fuels.
• Through the process of combustion, energy stored in wood and fossil fuels is released. This allows us to heat our homes, drive our vehicles, and power our industries.

As you examined the diagram illustrating the carbon cycle, you probably noted the abundance of arrows indicating a surplus of carbon dioxide being released into the atmosphere. You may recall from Module 2 that an increase in atmospheric greenhouse gases, such as carbon dioxide, has been linked to the greenhouse effect and global climate changes.

Unit B consists of two modules: