Transcript element
What is matter made of? About 450 B.C. a Greek philosopher named Empedocles proposed that all matter was made of 4 “elements” – air, earth, fire, and water He thought all matter was a combination of two or more of these 4 elements People believed this for more than 2,000 years What is an element? In the late 1600’s, experiments by early chemists began to show that matter is made up of more than 4 elements Scientists know that all matter is made up of slightly more than 110 different substances called elements What is an element? An element is a pure substance that cannot be broken down into any other substances by either chemical or physical means Elements are the simplest substances Each element can be identified by its specific physical and chemical properties What are examples of elements? Aluminum which is used to make f0il and outdoor furniture, is an element Zinc is an element which makes up pennies Copper is another element and is used to coat pennies Oxygen and nitrogen which makes up nearly 99% of Earth’s atmosphere are also elements What is an atom? Since the early 1800’s scientists have known that all matter is made up of atoms An atom is the basic particle from which all elements are made Different elements have different properties because their atoms are different The atom has a positively charged center, or nucleus surrounded by a “cloud” of negative charge The Atom e le c tr o n n e u tr o n p r o to n What happens when atoms combine? Atoms of most elements have the ability to combine with other atoms When atoms combine they form a chemical bond – a force of attraction between 2 atoms Atoms can combine to form larger particles called molecules- groups of 2 or more atoms held together by chemical bonds What is a compound? In nature, most elements are found combined with other elements A compound is a pure substance made up of 2 or more elements chemically combined in a set ratio For example, water is a compound because it consists of the elements hydrogen and oxygen H2O How is a compound represented? A compound may be represented by a chemical formula, which shows the elements in the compound and the ratio of atoms For example, the chemical formula for carbon dioxide (the gas you exhale) is CO2 The number 2 below the symbol for oxygen tells you that the ratio of carbon atoms to oxygen atoms is 1 to 2 If there is no number after the element’s symbol, the number 1 is understood How is a compound represented? If a different ratio of carbon atoms and oxygen atoms are seen in a formula you have a different compound For example, carbon monoxide – a gas produced in car engines has the formula CO The ratio of carbon atoms to oxygen atoms is 1 to 1 What happens to the properties of elements when they are chemically combined? When elements are chemically combined they form compounds having properties that are different from those of the uncombined elements For example, table sugar, C12H22O11, is a compound made of the elements carbon, hydrogen, and oxygen The sugar crystals do not resemble the gases oxygen and hydrogen or the black carbon you see in charcoal What is a mixture? Elements and compounds are pure substances Most of the materials we see everyday are not pure – they are mixtures A mixture is made up of 2 or more substances – elements, compounds, or both – that are together in the same place but are not chemically combined How do mixtures differ from compounds? Mixtures differ from compounds in 2 ways: Each substance in a mixture keeps its individual properties The parts of a mixture are not combined in a set ratio An example would be soil: If you look at soil through a magnifying glass you would find particles of sand, clay, or even pieces of decaying plants What is a heterogeneous mixture? A mixture can be heterogeneous or homogenous In a heterogeneous mixture you can see the different parts A heterogeneous mixture has larger parts that are different from each other Examples of heterogeneous mixtures are soil, a salad, tacos, vegetable soup, a toy box full of toys, or a toolbox full of nuts and bolts What is a homogeneous mixture? Homogeneous means “the same throughout” In a homogeneous mixture, the substances are so evenly mixed that you can’t see the different parts You might not always know that homogeneous mixtures are mixtures because you can’t tell by looking Homogeneous mixtures can be solids, liquids, or gases Examples are solutions such as air, sugar dissolved in water, brass, juice, milk How can a mixture be separated? Compounds and mixtures differ also in their ability to be separated A compound can be difficult to separate into its elements because it is held together by chemical bonds A mixture is easy to separate into its components because each component keeps its own property Examples of How To Separate Mixtures Examples of methods used to separate mixtures include magnetic attraction, filtration, distillation, and evaporation