Question

How does the composition of a mixture compare to the composition of a pure substance? Which material has a fixed composition and which has a variable composition?

Short answer

A mixture has a variable composition as the substances are combined in different proportions, without being chemically combined, resulting in varying properties. In contrast, a pure substance has a fixed composition with constant properties throughout, formed when atoms or molecules chemically bond together.

Step-by-step solution

Define a mixture and a pure substance

A mixture is a combination of two or more substances, where each substance retains its own chemical properties and can be separated by physical means, such as filtration, evaporation, or distillation. A mixture can have variable composition, depending on the proportions of the mixed substances. A pure substance is a form of matter with a uniform composition and distinct properties. It cannot be separated into its components by physical means and has a fixed composition. Examples of pure substances include elements and compounds.

Compare the composition of a mixture to the composition of a pure substance

The composition in a mixture varies, as the substances are combined in different proportions, without being chemically combined. Consequently, the properties of a mixture depend on the relative amounts of the substances present and can vary. On the other hand, a pure substance has a fixed composition. It is formed when atoms or molecules chemically bond together and has constant properties throughout at a given temperature and pressure.

Determine which material has a fixed composition and which has a variable composition

A pure substance has a fixed composition, as its properties remain constant throughout with unchanging proportions of its components. Examples of a pure substance include water (H2O), carbon dioxide (CO2), and gold (Au). A mixture has a variable composition, as the substances in a mixture can be combined in varying proportions, changing the properties of the mixture. Examples of mixtures include air, saltwater, and salad dressing. In conclusion, a mixture has a variable composition depending on the proportions of the combined substances, while a pure substance has a fixed composition with distinct, unchanging properties.

Key concepts

mixture

A mixture is when two or more substances are combined. Each substance keeps its own unique properties. This means, for instance, that if you mix salt and water to make saltwater, both the salt and the water can still be separated. They do not change into something else. Examples of mixtures include

• Air (a mix of gases like nitrogen and oxygen)
• Fruit salad (various fruits combined)
• Sand and sugar mixed together

A mixture can easily be separated by physical methods such as filtration, sifting, or boiling. Because the substances in a mixture are not chemically bonded, the mixture can have a variable composition. This means the proportions of the substances can change. Think about adding more fruit to your salad. It is still a salad, but its composition is different!

pure substance

A pure substance consists of only one type of particle. It has a uniform and unchanging composition. This makes its properties unique and constant. Pure substances are not mixtures and cannot be broken down physically. They are either elements or compounds.

• Elements are pure substances that consist of a single type of atom, like gold (Au) or oxygen (O₂).
• Compounds are pure substances formed when two or more elements chemically bond together. An example is water (H₂O), which is made from hydrogen and oxygen.

Since a pure substance has only one type of particle throughout, it is said to have a fixed composition. It does not change no matter how much you have or where you find it.

fixed composition

The term 'fixed composition' refers to substances that maintain the same makeup no matter where you find them. This is typical of pure substances. The ratio of components in a pure substance never changes. For example, water always has two hydrogen atoms bonded to one oxygen atom, no matter if it's from a river or a bottle.
Fixed composition is a key identifying feature of substances that are either elements or compounds, not mixtures. That's because in mixtures, the proportions of their components can vary widely, unlike with pure substances.

variable composition

In contrast to a fixed composition, a variable composition means that the proportions of the components can change. This is characteristic of mixtures. Mixtures don't have a specific ratio of these components, which means they can be adjusted. For example, if you're making lemonade, you can have more or less sugar depending on how sweet you want it.
Because the substances in a mixture are combined physically (not chemically bonded), changing their proportions is easy. This flexibility allows for a wide variety of combinations, making mixtures versatile.