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Fall 2011 8th grade Science Chemistry Study Guide Matter Matter is anything that has mass and takes up space (has volume). It is typically classified into three categories: Solid Liquid Gas Particles are held in place and vibrate Particle are able to slide past each other but not move apart Particles are able to move away from each other and spread Volume Definite volume Definite Volume No definite volume Shape Definite Shape Takes shape of container No definite shape Density Tend to be more dense Tend to be in between Tend to be least dense Energy Least amount of energy Mid energy Highest energy How the particle or atoms behave: Diagram of particles Physical changes in matter: Solid Liquid Melting Solid Freezing Gas boiling Liquid Gas condensation Thermal energy must be added to a system for a substance to change from a solid to a liquid to a gas Or Thermal energy must be removed from a system for a substance to change from a gas to a liquid to a solid. Fall 2011 8th grade Science Compounds, Elements, Mixtures Atom: the smallest unit of matter. Composed of protons, electrons and usually neutrons Element: a substance that cannot be broken down into smaller substance with different properties than the original substance. All elements are named on the Periodic Table of the Elements Compound: A set of two or more different elements that cannot be separated by physical means. They are bonded together. Ex: water, salt, a cake after being baked Mixture: A set of two or more substances that CAN be separated by physical means. They are NOT bonded together Ex: soda and carbon dioxide, salad, sand and iron fillings, chocolate milk, kool-aid Some physical means to separate mixtures: Evaporation, sorting, sifting, using magnets, running a current through the solution, centrifuging Chemical Changes A chemical change occurs when a reaction produces a new substance. The new substance can be in the form of solid, liquid, or gas. Indicators that a chemical change has occurred: Change in temperature Flame or light created Rusting Tarnishing Color change formation of a precipitate (solid) Formation of a gas formation of a liquid Physical changes DO NOT form a new substance. These are changes such as: Change in size Change in shape Boiling point Melting point Freezing point Chemical and Physical Properties -Density and Specific Heat Density: a physical property of matter that describes how much mass a substance has per unit volume. It is basically a measure of how compact the particle are in a substance. The equation to find density is D=m/v Liquid water has a density of 1g/ml Any substance that has a lower density than 1g/ml will float in water, any substance with a higher density will sink. Ice is less dense than water, therefore it floats. Fall 2011 8th grade Science Atomic Structure Element: A substance that cannot be broken down into smaller substances. All elements are listed on the periodic table of the elements. Atom: the smallest unit of an element that retains the properties of that element. Atoms are composed of three parts (subatomic particles) Protons- positive charge found in the nucleus Neutrons- neutral (no charge) found in the nucleus Electrons- negative charge found outside the nucleus in a “cloud” Information about the subatomic particles can be found on the Periodic Table of the Elements. Atomic Number (number of protons) Atomic Symbol (First letter is capital, all others are lowercase) Atomic Mass amu ( atomic mass units) Atomic mass is equal to the number of proton plus the number of neutrons because both protons and neutrons have a mass about equal to one amu. Electrons are so small they do not factor into the mass of an atom. In summary: # protons = atomic number # electrons = number of protons # neutrons = atomic mass - # protons Each element has an atom that is slightly different. We draw models of them to show the numbers of protons, neutrons and electron. Fall 2011 8th grade Science Called Bohr models: We show the protons and neutrons in the nucleus and electrons in circular “clouds” outside the nucleus. Electron clouds can hold , 2, 8, then 18 electrons from the inside ring out. We fill the inside ring first. He Li P Periodic Table The first version of the modern periodic table was created by Dmitri Mendeleev. He was Russian chemist that classified matter based on physical and chemical properties. He organized the known elements of the time by increasing atomic mass. He left gaps in his table where he believed new elements that had yet to be discovered would one day fit in the table. Years later E. Moseley created the modern version of the Periodic table. The current one is arranged from left to right (a period) by increasing atomic number. Each element increases by one proton. There are still gaps where chemists believe new elements will be discovered. The Periodic Table of the Elements can tell us: (incomplete list) Atomic number Size Atomic mass Family Chemcial symbol Reactivity Metal., nonmetal, metalloid Conductivity (most conductive to the left, least to the right) State of Matter (solids to the left, gases to the right) # valence electrons # of electron shells Fall 2011 8th grade Science The jagged line that goes from Boron to Antimony is called the staircase line. This indicates the division between Metals and nonmetals. The elements that boarder the line are called the metalloids. They share properties with both metals and nonmetals. Chemical Formulas, Equations/Reactions Chemical compounds are represented by a system of letters and number to indicate the “recipe” to make the compound. These are called chemical formulas. Examples: CO2 – 1 carbon atom and 2 oxygen atoms (carbon dioxide) H2O – 2 hydrogen atoms and 1 oxygen atom (water) C6H12O6 – 6 carbon atoms, 12 hydrogen atoms, 6 oxygen atoms (glucose) NaCl- 1 sodium atom and 1 chlorine atom (table salt) The formula describes the elements that are involved and the numbers indicate the amount of the particular element. Subscript: the little number to the lower right of a chemical symbol. It tells the number of atoms of the chemical to it’s left Chemical Equations 6CO2 + 6H2O Reactants C6H12O6 + 6O2 yield Products Coefficiant: the large number to the left of some of the chemical formulas indicates the number of that molecule in the equation. Ex: 6CO2 means that there are six molecules of CO 2 So… there are a total of 18 atoms in the formula CO2. Six carbons and twelve oxygen. You can find this by multiplying the coefficient by the subscript. If an element does not have a subscript it is understood that it is one. Law of Conservation of Mass (Matter or Energy): this law states that matter, mass or energy CANNOT be created or destroyed only transformed. In other words, before, during and after a reaction you must maintain the same amount of mass, matter and energy. So… 1. The mass or the reactants = mass or the products 2. The number of atoms in the reactants = number of atoms in the products Fall 2011 3. What ever amount of energy you begin with you must end with. All chemical equations must show a balanced relationship. 8th grade Science