Chapter 1-6 Concepts: Elements, Pure Substances, Molecules, and Mixtures

Substances come in two broad categories: pure substances and mixtures.
A pure substance has only one type of particle in a given sample.
A mixture contains two or more substances and the components are not chemically bound to each other.
Elements are pure substances consisting of only one type of atom.
Compounds (a type of pure substance) consist of two or more elements in a definite, fixed ratio.
Molecules are groups of atoms bonded together; a pure substance can be made of atoms (element) or molecules (compound).
In many cases, elements in nature occur as molecules (e.g., O₂ for oxygen gas), but the substance is still an element because all constituent atoms are of the same element.
The distinction between pure substances and mixtures is observable by looking at the types of particles present.

A pure substance equals either an element or a compound.
Elements are pure substances that contain only one type of material (one kind of atom).
- Examples: copper, lead, aluminum, iron, hydrogen, oxygen.
- Note: Oxygen in nature often exists as O₂ molecules; this does not make it a non-element, since all particles are still oxygen atoms.
Compounds are pure substances that contain two or more elements in a definite ratio.
- Examples: carbon dioxide, CO₂; water, H₂O.
- In CO₂, there is one carbon atom covalently bonded to two oxygen atoms in a fixed arrangement.
- In H₂O, there are two hydrogen atoms covalently bonded to one oxygen atom.
A pure substance can be a pure element or a pure molecule (compound). A molecule is a specific arrangement of atoms bonded together.
If you have a cup of water (H₂O), you have a sample consisting of the H₂O molecules only, so it is a pure substance.

If you examine a piece of iron with a powerful microscope, you would find it composed of iron atoms only (no other elements’ atoms involved in the sample).
This makes the iron piece a pure substance (an element).
Some substances exist as molecules made of different atoms, but the whole sample is still a pure substance if every particle is that same molecule.
- Example: carbon dioxide (CO₂) molecules in the sample; every particle is CO₂ molecule.
- Example: water (H₂O) molecules in the sample; every particle is H₂O molecule.
When a sample contains only one type of molecule or one type of atom, it is a pure substance.

Elements (pure substances, single type of atom):
- Copper, Lead, Aluminum, Iron, Hydrogen, Oxygen (in pure form).
Compounds (pure substances with two or more elements in a fixed ratio):
- Carbon dioxide: ext{CO}_2
- Water: ext{H}_2 ext{O}
Key point: A pure substance can be either an element or a compound.

A mixture contains two or more substances that are not chemically bonded to each other.
The components in a mixture retain their own identities and can be elements or compounds.
Examples:
- A sample of lemon juice in water (two different substances/molecules present).
- River or lake water containing multiple dissolved substances and various molecules.
In mixtures, the sample does not have a single type of particle; you can find multiple kinds of molecules or atoms.
Distinguishing point: separation by physical means (filtration, distillation, etc.) is often possible for mixtures, whereas pure substances require chemical change to separate into different substances.

A cup of water contains only H₂O molecules if it is pure water; that’s a pure substance.
If you add another liquid (or another solute) to water, the sample becomes a mixture because it contains more than one type of molecule.
Natural waters (e.g., river water) are mixtures containing minerals and various dissolved substances, not pure substances.

Pure elements: single type of atom (no fixed molecular formula in general, as many exist as atoms in a sample).
Pure compounds (molecules composed of multiple elements in fixed ratio):
- Carbon dioxide: ext{CO}_2
- Water: ext{H}_2 ext{O}
Fixed-ratio examples to remember:
- Carbon to Oxygen in CO₂: ext{C:O} = 1:2
- Hydrogen to Oxygen in H₂O: ext{H:O} = 2:1

Pure Substance
- Definition: contains only one type of particle.
- Types: elements (one type of atom) or compounds (two or more elements in a definite ratio).
- Examples: ext{Fe} ext{ (iron)}, ext{Cu} ext{ (copper)}, ext{CO}2, ext{H}2 ext{O}.
Mixture
- Definition: contains two or more substances not chemically combined.
- Examples: lemon juice in water; river water with multiple dissolved substances.
- Key feature: components can be separated by physical methods; not a single particle type.

Understanding purity helps in chemical reactions, material science, and quality control in industry.
Many real-world samples are mixtures (water from natural sources, beverages with additives, air with various gases).
The idea of fixed ratios in compounds underpins chemical formulas and the predictability of reactions.

The line between purity and impurity is context-dependent and often depends on the level of analysis (microscopic vs macroscopic).
In practice, truly pure samples are idealizations; natural materials are typically mixtures to some extent.

Identify whether a given sample is a pure substance or a mixture:
- A bottle of distilled water: Pure substance (compound: ext{H}_2 ext{O}).
- Ice cream with vanilla, sugar, and chocolate: Mixture.
- A rod of pure copper: Pure substance (element).
For each compound, state its constituent elements and their fixed ratio:
- ext{CO}_2: ext{C:O} = 1:2
- ext{H}_2 ext{O}: ext{H:O} = 2:1