Lesson 1.2 Classifying Matter
Vocabulary:
Element –
Atom –
Chemical bond –
Molecule –
Compound –
Chemical formula –
Mixture –
What is matter made of? In 450BC, a Greek philosopher named Empedocles suggested that all matter was made of four “elements” – air, earth, fire, and water. He thought that all other matter was a combination of these elements. People believed this idea for 2000 years!
In the late 1600s, experiments by early chemists began to show that matter was made up of many more than four elements. Scientists now know that all matter in the universe is made of more than 100 different substances, called elements.
An element is a substance that cannot be broken down into any other substances by chemical nor physical means.
Elements are the simplest substances. Each element can be identified by its specific physical and chemical properties. Elements are represented by one or two-letter symbols, such as C for carbon, O for oxygen, and Ca for calcium.
Atoms – the basic particle from which all elements are made. An atom has a positively charged center (nucleus with protons and neutrons), and is surrounded by a “cloud” of negative charge (electrons). The elements have different properties because their atoms are different
.
Molecules – a group of two or more atoms held together by chemical bonds.
Atoms of most elements are able to combine with other atoms. When atoms combine, they form a chemical bond. In many cases, atoms combine to form larger particles called molecules.
A chemical bond is a force of attraction between two atoms.
A molecule of water, for example, is made up of an oxygen atom chemically bonded to two hydrogen atoms. Two atoms of the same element can also combine to form a molecule. Oxygen gas is made up of two oxygen atoms.
Water molecule:Ammonia molecule:
Oxygen gas molecule:Nitrogen gas molecule:
A compound is a substance made of two or more elements that are chemically combined in a set ratio.
Water, ammonia, and carbon dioxide are all compounds.
A compound is represented by a chemical formula, which shows the elements in the compound and the ratio of the atoms.
For example, the chemical formula for carbon dioxide is CO₂. The 2 below the O for oxygen tells you that the ratio of carbon atoms to oxygen atoms is 1 to 2. If there is no number after an element’s symbol, it is understood that the number is 1.
Carbon dioxide molecule:
A different number of atoms in a formula represents a different compound. For example CO is carbon monoxide, a toxic gas. The ratio of carbon to oxygen is 1:1.
When elements chemically combine, they form compounds with properties different from those of the elements.
Example: Sulfur is a yellow solid and copper is a shiny metal. When copper and sulfur combine, they form a compound called copper sulfide. The new compound has different properties from both copper and sulfur.
Copper:Sulfur:
Elements and compounds are substances, but most materials are mixtures.
Mixtures – two or more substances that are together in the same place, but their atoms are not chemically bonded.
Each substance keeps its own properties, and they are not combined in a set ratio.
Two types of mixtures:
Heterogenous mixtures – you can usually see the different parts and they can easily be separated out (like a salad)
Homogenous Mixtures – substances so evenly mixed that you can’t see the different parts. Difficult to separate the parts. Air is a homogeneous mixture of gases. You know oxygen is present in the air because you are able to breathe, but you cannot identify where the oxygen is in the air.
Separating mixtures – Since the substances in a mixture keep their properties, you can separate a mixture back into its individual parts. Methods used to separate the parts of a mixture include distillation, evaporation, filtration, and magnetic attraction.
Magnetic attraction – Iron objects can be separated from a mixture using a magnet.
Evaporation – When left in the open air, liquid solutions can change to gas, leaving solid components behind.
Filtration – Solids can be separated from liquids by pouring the mixture through a filter.
Distillation – Liquids can be separated from each other by heating them up to the temperature at which one of them boils. The liquid boils into a gas, then cools, forming the separated liquid.