Here are some key characteristics of substances:
* Homogeneous: A substance is uniform throughout, meaning its properties are the same everywhere in the sample.
* Pure: It is made up of only one type of atom or molecule.
* Fixed composition: The ratio of elements in a substance remains constant.
* Specific properties: Substances have unique physical and chemical properties that can be used to identify them.
Examples of substances:
* Elements: Iron (Fe), Oxygen (O2), Gold (Au)
* Compounds: Water (H2O), Salt (NaCl), Sugar (C12H22O11)
Things that are NOT substances:
* Mixtures: Mixtures are combinations of two or more substances that are not chemically bonded, and their composition can vary. Examples include air (a mixture of gases), saltwater (a mixture of salt and water), or a salad (a mixture of vegetables).
* Solutions: Solutions are special types of mixtures where one substance is dissolved in another. While they are homogeneous, they are still mixtures.
In summary: A substance is a pure form of matter with a definite composition and consistent properties. It is a fundamental building block of all matter.