Question

Explain the difference between a pure substance and a mixture.

Short answer

A pure substance has a constant composition and properties throughout the sample and consists of a single type of particle, while a mixture is a combination of two or more substances that retain their individual properties and can be separated by physical means.

Step-by-step solution

Define a Pure Substance

A pure substance is a form of matter that has a constant composition and properties that are constant throughout the sample. It consists of a single type of particle, which can be an element or a compound. An element is a substance that cannot be separated into simpler substances by chemical means, while a compound is a substance formed when two or more chemical elements are chemically bonded together. Examples include water, gold, and oxygen.

Define a Mixture

A mixture is a combination of two or more substances that are not chemically bonded together and can be separated by physical means. The components of a mixture maintain their own properties and can be present in variable proportions. Mixtures can be homogeneous, where the composition is uniform throughout the sample, or heterogeneous, where the composition varies within the sample. Examples include salt water, air, and salad.

Contrast Pure Substances and Mixtures

The main difference between a pure substance and a mixture is in their composition and properties. A pure substance has a uniform and definite composition, while a mixture is composed of different substances that can vary in proportion. The components of a mixture retain their individual properties and can be physically separated, unlike the elements in a compound which are chemically bonded.

Key concepts

Chemical Composition

Understanding the chemical composition of substances is crucial when differentiating between pure substances and mixtures. A pure substance has a specific chemical composition, which means it contains only one type of particle, either atoms of a single element or molecules of a single compound. These particles are consistent throughout the substance, giving it a definite proportion of elements that constitute its makeup.

For instance, water (H₂O) always consists of two hydrogen atoms bonded to one oxygen atom, and table salt (NaCl) is always comprised of sodium and chloride ions in a 1:1 ratio. In contrast, mixtures have a variable chemical composition. Air, for example, is a mixture containing nitrogen, oxygen, carbon dioxide, and other gases in varying amounts. Since mixtures are not chemically bonded, the proportion of their components can differ from one sample to another. This fundamental difference in chemical composition is what separates pure substances from mixtures.

Physical Separation

The concept of physical separation is pivotal in understanding mixtures. Unlike pure substances, the components of a mixture can be separated using physical methods because they do not entail changing the chemical identities of the substances involved. Separation techniques rely on physical properties like boiling point, density, solubility, or magnetic properties. For example, salt can be recovered from saltwater through the process of evaporation, which is possible because the water has a lower boiling point than salt.

Filtration and Distillation

These are two common methods of physical separation. Filtration is used when particles of different sizes need to be separated, while distillation separates substances based on differences in their boiling points. Both processes exemplify how mixtures can be taken apart to yield their original components, which is a distinct characteristic that differentiates them from pure substances.

Homogeneous and Heterogeneous Mixtures

Mixtures can be classified into two broad categories: homogeneous and heterogeneous. Homogeneous mixtures have a uniform composition throughout, meaning that their components are evenly distributed on a molecular level. Solutions like saltwater or vinegar are prime examples of homogeneous mixtures where the solute is completely dissolved in the solvent, creating a single-phase system that appears the same throughout.

Conversely, heterogeneous mixtures have a non-uniform composition where the individual substances remain distinct and can often be seen as separate phases. Examples include oil and water, which do not mix evenly, or a salad where the different ingredients can be visually and physically separated. Understanding these two types of mixtures helps in comprehending the diversity of matter and provides insight into the processes necessary for their separation and identification, reinforcing the distinction between mixtures and pure substances.