Synthesis reactions, also called combination reactions, are a basic type of chemical reaction. In these reactions, two or more substances come together to make one new substance.

Here’s a simple way to understand it:

General Equation: $A + B \rightarrow AB$

Key Features:

1. Making Complex Substances: Synthesis reactions usually take simple substances and combine them to form more complicated ones.

   For example, when hydrogen gas ($H_2$) combines with oxygen gas ($O_2$), they create water ($H_2O$): $2H_2 + O_2 \rightarrow 2H_2O$

2. Energy Changes: Many synthesis reactions release energy. This is called being exothermic.

   For instance, when nitrogen and hydrogen combine to form ammonia ($NH_3$), energy is given off: $N_2 + 3H_2 \rightarrow 2NH_3 + \text{energy}$

3. Importance in Industry and Nature: These reactions are important in many areas. For example, they happen in factories that make chemicals, and they also occur in nature.

   A cool example is photosynthesis. This is when plants take carbon dioxide ($CO_2$) and water and turn them into glucose ($C_6H_{12}O_6$) and oxygen. The equation looks like this: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

In short, synthesis reactions are essential for making new substances. They are important both in labs and in nature, showing how energy and matter work together in chemical changes.