Chapter 2 Section 1 Classifying Matter

Objectives Classify a pure substance as elements or compounds. Describe the characteristics of an element and the symbols used to identify elements. Describe the characteristics of a compound. Distinguish pure substances from mixtures. Classify mixtures as heterozygous or homozygous. Classify mixtures as solutions, suspensions, or colloids.

Key Concepts Why are elements and compounds classified as pure substances? How do mixtures differ from pure substances? What is the main difference among solutions, suspensions, and colloids?

Composition Materials have different properties because they have different compositions. Composition = “a putting together” The combining of parts into a whole. Based on their composition materials can be divided into pure substances and mixtures.

Pure Substances Pure substance – matter that always has exactly the same composition. Every sample of a given substance has the same properties because a substance has a fixed, uniform composition. Ex: Aluminum, carbon dioxide

Pure Substances Pure Substance Elements Compounds

Elements Element – A substance that cannot be broken down into simpler substances. Approximately 115 known elements. An element has a fixed composition because it contains only one type of atom

Elements Elements contain only one type of atom. Atom – smallest particle of an element No two elements contain the same type of atom.

Elements At room temperature: Examples of Elements Most elements are solid (aluminum, carbon) Some are gases (oxygen, nitrogen) Only 2 are liquid (bromine, mercury) Examples of Elements Gold (Au) Aluminum (Al) Iodine (I) Carbon (C) Copper (Cu)

Symbols for Elements Symbols are used to represent elements. Each symbol has one or two letters. The first letter is always capitalized. If there is a second letter it is not capitalized.

Symbols for Elements Symbols come from the Latin names of the elements. Symbols allow scientists to communicate without confusion. Some symbols give clues to an elements properties. Hydrogen – comes from the Greek words hydro and genes, meaning “water” and “forming.”

Compounds Compound – a substance that is made from two or more simpler substances and can be broken down into those simpler substances. Example: Water Water (H2O) Oxygen Hydrogen

Compounds The simpler substances are elements or other compounds. H2O  H2 + O2 Properties of the compound can be different than properties of the elements that make it up Hydrogen and oxygen are gases at room temperature but water is a liquid at room temperature!

Compounds Another example: Silicon dioxide (quartz) – found commonly in sand, a colorless, transparent solid. Silicon – gray solid Oxygen – colorless gas

Compounds A compound always contains two or more elements joined in a fixed proportion Example: Silicon dioxide – there are always 2 oxygen atoms for each silicon atom Di- means “two” In water, there are always 2 hydrogen atoms for each oxygen atom

Mixtures Retain some of the properties of their individual substances. Properties of a mixture are less constant than the properties of a substance. The properties of a mixture can vary because the composition varies Mixtures can be classified by how well the parts of the mixture are distributed throughout the mixture.

Mixtures Hetero = “different” Heterogeneous Homogenous Hetero = “different” The parts of the mixture are noticeably different from one another. Example: Sand Homo = “same” Mixture appears to contain only one substance Substances so evenly distributed that they are difficult to distinguish Example: Sweet Tea