18. What is matter?

Matter is anything that has mass and occupies space. It is composed of particles, such as atoms and molecules, and can exist in various states, including solid, liquid, gas, and plasma. Matter can be classified into two main categories: 1. **Pure substances**: These have a uniform and definite composition, such as elements (like oxygen or gold) and compounds (like water or carbon dioxide). 2. **Mixtures**: These consist of two or more substances that are physically combined but not chemically bonded, such as air (a mixture of gases) or salad (a mixture of various ingredients). Matter is fundamental to the study of physics and chemistry, as it interacts with energy and undergoes various physical and chemical changes.