A mixture is a combination of two or more substances that retain their individual properties. These substances can be solids, liquids, or gases, and they can be mixed in varying proportions. Common examples of mixtures include salad, which combines different vegetables, and air, which is a mixture of gases like nitrogen and oxygen. Mixtures can be classified into two main types: homogeneous and heterogeneous. A homogeneous mixture has a uniform composition throughout, such as saltwater, where the salt dissolves evenly. In contrast, a heterogeneous mixture has distinct, separate components, like sand and gravel, which can be easily identified and separated.