Which one of the following is a compound: ozone, \(\mathrm{O}_{3}\); stainless steel, a combination of iron and other metals; sulfuric acid, \(\mathrm{H}_{2} \mathrm{SO}_{4} ;\) brewed coffee.

Ozone, often symbolized as O3, plays a critical role in protecting our planet from harmful ultraviolet radiation. It's made up of three oxygen atoms bound together, forming a molecule that's significantly less stable than the more common diatomic oxygen (O2) found in the air we breathe.

Think of ozone as a protective blanket high up in the Earth's stratosphere, absorbing the majority of the sun's high-energy ultraviolet rays. However, ozone is not a compound. Compounds consist of atoms of two or more different elements chemically combined in a fixed ratio. Ozone only contains oxygen atoms, so it's categorized as a molecule of a single element, not a compound.

Alloys like stainless steel are fascinating materials that blend the best properties of their component metals. An alloy is made by melting and mixing two or more metals (or a metal and a non-metal) together, resulting in an entirely new material with distinct physical and chemical characteristics.

The composition of an alloy plays a vital role in determining its durability, resistance to corrosion, magnetism, and even its color. For example, the addition of chromium to iron in the production of stainless steel creates a shiny, rust-resistant material ideal for cookware, medical instruments, and building materials. Despite their mixed nature, alloys are not chemical compounds because their constituent elements are not bound in a fixed ratio.

Sulfuric acid stands out as an essential compound with the chemical formula H2SO4. Its structure consists of two hydrogen atoms, one sulfur atom, and four oxygen atoms, and it is known for being a highly corrosive strong acid.

In industry and laboratories, sulfuric acid has a myriad of applications due to its ability to react with other substances, such as metals, and its importance in the production of fertilizers, dyes, and explosives. This compound's key characteristic is the specific ratio in which these different elements are chemically combined, a defining trait that unambiguously qualifies it as a compound.

Mixtures are all around us, from the air we inhale to the beverages we enjoy, like brewed coffee. These are combinations of two or more substances that are not chemically bonded and can vary in composition.

The components in a mixture maintain their own chemical identities and properties, and they can be separated by physical means such as filtration or evaporation. A mixture can be homogeneous with a uniform composition throughout, like salt dissolved in water, or heterogeneous with distinct parts, like a salad. Because the elements or compounds in a mixture are not fixed in a precise ratio, mixtures differ fundamentally from compounds.