Question

How does the composition of a pure compound differ from that of a mixture?

Short answer

A pure compound has a fixed ratio and uniform composition, while a mixture has variable composition and its components retain individual properties.

Step-by-step solution

Identify the Definitions

Start by understanding and defining the terms 'pure compound' and 'mixture'.

Define Pure Compound

A pure compound is a substance that is composed of two or more elements chemically combined in a fixed ratio. It has consistent properties and a homogeneous composition.

Define Mixture

A mixture consists of two or more substances that are physically combined. These substances do not have a fixed ratio and retain their individual properties. A mixture can be homogeneous (uniform) or heterogeneous (non-uniform).

Compare Composition

In a pure compound, the composition is uniform and definite, meaning every sample of the compound has the same proportion of elements. In contrast, the composition of a mixture can vary because there is no fixed ratio and the components can be present in any proportion.

Illustrate Examples

For example, water (H2O) is a pure compound with a fixed ratio of 2 hydrogen atoms for every 1 oxygen atom. Saltwater, on the other hand, is a mixture where the amount of salt can vary. It can be more salty or less salty depending on the amount of salt dissolved in water.

Key concepts

Pure Compound

A pure compound is a substance composed of two or more elements that are chemically combined in a fixed ratio. This means the elements are bonded together in specific proportions and cannot easily be separated by physical means.
For instance, water (H2O) is a pure compound. It always contains hydrogen and oxygen in a 2:1 ratio. Regardless of where you find water, this ratio remains constant.
Pure compounds have uniform properties and a consistent composition throughout. They are always homogeneous, meaning they have the same structure and composition wherever you look.

Chemical Combination

A chemical combination occurs when two or more elements join together through chemical bonds to form a new substance with new properties. This process involves the sharing or transferring of electrons.
For example, when hydrogen and oxygen combine chemically, they form water (H2O). The new substance, water, has properties distinct from those of its constituent elements.
Through chemical combination, the elements lose their individual characteristics and form a new compound with unique properties.

Fixed Ratio

In a pure compound, elements combine in a fixed ratio by mass. This ratio determines the chemical formula of the compound.
For instance, in carbon dioxide (CO2), there is always one carbon atom for every two oxygen atoms. This fixed ratio is crucial because it defines the compound's identity and properties.
Fixed ratios ensure that pure compounds are always the same, with consistent chemical and physical properties.

Mixture

A mixture consists of two or more substances that are physically combined, not chemically bonded. This means each component retains its individual properties.
Mixtures do not have a fixed ratio. The composition can vary. A salad, for instance, is a mixture. You can add as much or as little of each ingredient as you like without changing the nature of the salad.
Mixtures can be divided into two types: homogeneous and heterogeneous. Let's explore these types further.

Physical Combination

A physical combination is when two or more substances are mixed without any chemical bonding between them. This means no new substances are formed.
Because physical combinations do not alter the substances' chemical identities, the components can often be separated by simple physical means like filtration, evaporation, or decantation.
For example, when you mix sand and salt, you can separate them again because no chemical reaction has taken place.

Homogeneous Mixture

A homogeneous mixture is a mixture in which the components are uniformly distributed. This means that any sample you take from the mixture will have the same composition.
Common examples of homogeneous mixtures include solutions like saltwater, where the salt is evenly dissolved throughout the water.
Homogeneous mixtures have consistent properties and appearance throughout, making them look like a single substance even though they are not.

Heterogeneous Mixture

A heterogeneous mixture is a mixture where the components are not uniformly distributed. This means that different parts of the mixture have different compositions.
Examples include a mixture of oil and water, or a salad, where you can see and pick out different components.
Heterogeneous mixtures have visibly different parts, and the individual substances are typically easy to distinguish from one another.