Which of the following would you characterize as a pure or nearly pure substance? (a) baking powder; (b) lemon juice; (c) propane gas, used in outdoor gas grills; (d) aluminum foil; (e) ibuprofen; (f) bourbon whiskey; (g) helium gas; (h) clear water pumped from a deep aquifer.

A chemical compound is a substance that is made up of two or more different types of atoms that are chemically bonded together in a fixed ratio. This means that a chemical compound will always have the same composition and structure throughout, making them distinct entities with unique properties. For example, ibuprofen is considered a pure chemical compound with the molecular formula \( C13H18O2 \). The specific arrangement and proportion of carbon (C), hydrogen (H), and oxygen (O) atoms in ibuprofen ensure that regardless of the sample size, its intrinsic properties remain consistent.

In the context of our original exercise, ibuprofen qualifies as a pure substance due to its specific chemical makeup that does not vary. This consistency is a hallmark of chemical compounds, which can be contrasted with mixtures, where the ratio of components may differ. Having a clear understanding of chemical compounds is crucial for students as it lays the groundwork for analyzing the purity and composition of substances in chemistry.

Elements are the simplest form of pure substances that cannot be broken down into simpler substances by chemical means. They consist entirely of one type of atom, such as aluminum (Al) in aluminum foil or helium (He) in helium gas. These elements are found on the periodic table and have unique properties.

A mixture, on the other hand, involves the physical blending of two or more components. The proportions of the components in a mixture are not fixed and can vary without changing the identity of the mixture. For instance, bourbon whiskey and lemon juice are both mixtures. They are composed of various compounds like water, ethanol, citric acid, and other substances that can be physically separated.

To help students better differentiate between elements and mixtures, considering context such as aluminum foil, which is almost exclusively aluminum atoms, hence a near-pure element, versus baking powder, which contains distinct compounds that retain their individual characteristics, is crucial. Understanding elements and mixtures is fundamental to the study of matter and its interactions.

The composition of a substance refers to what it is made of, including the type and the arrangement of atoms or molecules within that substance. A pure substance has a uniform and definite composition. In contrast, the composition of mixtures can vary. Water from a deep aquifer is a practical example: it's primarily made up of water molecules (H2O), but it may contain varying amounts of dissolved minerals, which makes it a complex mixture rather than a pure substance.

Grasping the concept of composition is essential when determining whether a substance is pure or a mixture. Propane gas is mostly composed of propane molecules (C3H8) and considered pure because its composition is consistent. Exploring the composition enables students to understand the atomic or molecular level of matter, predict physical and chemical properties, and grasp the methods used in separating mixtures.