Question

Give definitions for the terms pure substance and mixture. Give five examples of each type of material.

Short answer

A pure substance is a matter with a fixed composition, consisting of only one kind of element or compound with distinct properties. Examples include: \(H_2O\) (water), \(O_2\) (oxygen), \(NaCl\) (table salt), \(C\) (diamond), and \(Au\) (gold). A mixture is a matter composed of two or more substances combined physically, where each substance retains its properties. Examples include air, sand, lemonade, soil, and brass.

Step-by-step solution

Definition of Pure Substance

A pure substance is a type of matter with a fixed composition, which consists of only one kind of element or compound with distinct chemical and physical properties. It cannot be separated into its components by physical means, such as filtration or evaporation.

Definition of Mixture

A mixture is a type of matter composed of two or more different substances that are combined physically, not chemically. The substances in a mixture retain their individual properties, and the mixture can be separated into its components by physical means, such as evaporation, filtration, or distillation.

Pure Substance Examples

Here are five examples of pure substances: 1. \(H_2O\) (water): Water is a compound composed of two elements, hydrogen and oxygen, in a fixed proportion. 2. \(O_2\) (oxygen): Oxygen is an element, and pure oxygen gas is a substance made up of only oxygen molecules. 3. \(NaCl\) (table salt): Table salt is a compound made up of sodium and chlorine elements combined in a fixed ratio. 4. \(C\) (diamond): Diamond is a form of carbon, an element; pure diamond consists only of carbon atoms arranged in a specific crystalline structure. 5. \(Au\) (gold): Gold is a metallic element, and pure gold consists only of gold atoms.

Mixture Examples

Here are five examples of mixtures: 1. Air: Air is a mixture of multiple gases (primarily nitrogen, oxygen, and small amounts of other gases), water vapor, and particulates. 2. Sand: Sand is a mixture of various minerals and rock particles. 3. Lemonade: Lemonade is a mixture of water, sugar, and lemon juice, which can be separated by physical means such as evaporation and filtration. 4. Soil: Soil is a mixture of organic material, minerals, water, and air. 5. Brass: Brass is an alloy, a mixture of metals (primarily copper and zinc) that retains the properties of its individual components and can be separated by physical means such as distillation.

Key concepts

Chemical Composition

Understanding the chemical composition of substances is fundamental to distinguishing between pure substances and mixtures. It refers to the identity and proportion of the elements that make up a substance. For a pure substance, the chemical composition is uniform and invariant. It means that anywhere you sample the substance, you'll find the exact same types of atoms or molecules in the same proportions.

For example, the chemical composition of pure water, denoted as \(H_2O\), consists of two hydrogen atoms bonded to one oxygen atom. Even if you collect water from different sources, as long as it's pure, its composition remains the same. However, when substances are physically combined to create a mixture, they retain their original chemical composition. So, in the air, oxygen (\(O_2\)) and nitrogen (\(N_2\)) exist together, but their atoms do not bond to form a new substance; their chemical identities remain unchanged.

Physical Separation Methods

When it comes to separating mixtures into their individual components, physical separation methods are employed because these components retain their physical properties. Unlike chemical reactions, physical methods don't alter the chemical composition of the substances. Some common techniques include:

• Filtration: Used to separate solids from liquids or gases, often employed when the particles of one phase are larger than the other.
• Evaporation: Useful for separating a dissolved solid from a liquid by heating the mixture until the liquid becomes gas and leaves behind the solid.
• Distillation: This method separates substances based on differences in their boiling points. It is commonly used for liquid mixtures.

Each method leverages different physical properties, such as boiling point, particle size, or solubility, to achieve separation without changing the materials' nature.

Examples of Pure Substances

To further elucidate pure substances, consider these examples in greater detail:

• Water (\(H_2O\)): Regardless of the quantity, pure water always has the same molecular structure of two hydrogen atoms bonded to one oxygen atom.
• Oxygen gas (\(O_2\)): A pure sample of oxygen gas contains only molecules made up of two oxygen atoms.
• Table salt (\(NaCl\)): This is a crystalline compound formed from a one-to-one ratio of sodium to chloride ions.
• Diamond (\(C\)): An allotrope of carbon, consisting purely of carbon atoms arranged in a lattice structure.
• Gold (\(Au\)): In its pure form, gold is composed only of gold atoms and is a highly valuable precious metal.

Pure substances have consistent properties and cannot be separated into simpler substances by physical means.

Examples of Mixtures

To contrast with pure substances, here are clearer explanations of the given mixtures examples:

• Air: A physical blend of various gases, each retaining its own properties despite being mixed.
• Sand: Composed of tiny particles of different minerals that are physically bonded but not chemically combined.
• Lemonade: A combination of water, sugar, and lemon juice that can be separated back into its original components.
• Soil: An often complex mixture of organic material, minerals, water, and gases, variable depending on the sample location.
• Brass: An alloy of copper and zinc which, unlike chemical compounds, can be separated into its pure metals.

Mixtures can exhibit a wide range of physical properties, as their composition is not fixed and can vary between samples.