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Copy of Chemical and Physical Changes

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Amy Boettger

on 26 March 2013

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Transcript of Copy of Chemical and Physical Changes

Chemical and Physical Changes Chemical Reactions produce
chemical changes. Reactants - These are the substances that exist before the reaction. They are the substances that react. Products - These are the substances that form as a result of the chemical reaction. Chemical Equation - This shows the reactants, products, and the proportion of each in a reaction. The reaction is shown with a chemical equation, with reactants on the left, products on the right and an arrow separating them. Chemical Formula - Shorthand used in place of the names of chemicals Subscripts are the small numbers written to the right and below the symbol of an element. It tells how many atoms of that element there are. No subscript means there is only one atom. This shows there are 3 atoms of hydrogen in sodium acetate. Conservation of Mass - In a chemical reaction the mass of the reactants must equal the mass of the products. The number of atoms on each side is also the same. Balancing Equations Balance equations using coefficients, numbers in front of the formulas. All atoms in the formula are multiplied by the coefficient. The 2 is a coefficient. Energy in Chemical Reactions Sometimes, energy is released from a chemical reaction. This is called an exothermic reaction. In exothermic reactions, energy is released when bonds are broken in the reactants. It is released as heat or light. When energy is absorbed in a chemical reaction, it is called an endothermic reaction. An example of an exothermic reaction is combustion. That is when a substance burns and produces a flame. During endothermic reactions, more energy is needed to make the bonds of the products than is produced by breaking bonds in the reactants. A cold pack that you put on an injured muscle is an example of an endothermic reaction. It absorbs heat from your body and makes your skin feel cold.
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