The phrase "simple chemicals" refers to basic, fundamental substances that make up matter. These are typically small molecules that can be composed of a single element or a combination of elements, and they are the building blocks of more complex compounds. In chemistry, simple chemicals are often used to represent the simplest forms of elements and compounds, such as hydrogen (H2) and oxygen (O2). They are essential for understanding the behavior of more complex substances and are used in a wide range of industrial, scientific and domestic applications.