Formulas, Equations & the Conservation of Mass

Symbols One or two letters used to represent an element. The symbols are found on the periodic table. Each element is assigned a chemical symbol of one or two letters. The first letter is capitalized and the second letter, if there is one, is not capitalized.

Chemical Formula A combination of symbols and numbers that represent the number and types of elements (atoms) present in a compound. An example of a chemical formula is NaCl. This is the chemical formula for sodium chloride (salt).

Subscript A small number to the right of a symbol that is written below the normal line of letters to show how many ATOMS of an element are present in a substance. No subscript means there is only one atom present. One example of a chemical formula with subscripts is CO2. This is the chemical formula for carbon dioxide. There is 1 carbon atom and 2 oxygen atoms in one molecule of dioxide.

Chemical Reactions When substances interact to form one or more new substances with different properties than the original substances.

Chemical Equation A combination of chemical formulas used to describe what happens in a chemical reaction. The equation identifies the reactants and resulting products. An example of a chemical equation for the formation of water is: 2H2 + O2  2H2O.

Coefficient A number written in front of a chemical formula to show how many MOLECULES of that substance are present An example of coefficients in an equation is: 2Na + Cl2  2 NaCl. As is the case with subscripts, no number in front of a chemical formula indicates that there is only 1 of those molecules involved in the chemical reaction.

Reactants In a chemical equation, the formulas written on the left side of the arrow symbol () are the starting substances in a chemical reaction. A plus sign (+) between them indicates that there is more than one substance reacting in the reaction.

Product In a chemical equation, the formulas written on the right side of the arrow symbol () are the new substances being formed in a chemical reaction. A plus sign (+) between them indicates that there is more than one new substance being produced in the reaction

Yields An arrow symbol that shows a reaction has taken place. The reactants have reacted to produce a new substance (the product).

Compound In a chemical equation, elements and compounds may be displayed. You have learned that an element is one kind of atom. A compound is a substance composed of two or more elements chemically combined and in a definite proportion.

Chemical Change A change in which a substance (or substances) is changed into one or more new substances with different properties than the original substances. These changes are chemical: iron rusting - Iron oxide forms through the reaction of iron and oxygen in the presence of water or air moisture. gasoline burning – When gasoline is burned in car engines, water vapor and carbon dioxide form. eggs cooking - Fluid protein molecules uncoil and crosslink to form a network. bread rising - Yeast changes carbohydrates into carbon dioxide gas. milk souring – When bacteria use the lactose sugars in milk they change the sugar and sour-tasting lactic acid is produced.

Physical Change A change in which the characteristics of a substance are only changed physically and the original properties stay the same. Some examples of physical change are: whipping egg whites - Air is forced into the fluid, but no new substance is produced. magnetizing a compass needle - There is realignment of groups of iron atoms but no real change within the iron atoms themselves. boiling water - Water molecules are forced away from each other when the liquid changes to vapor, but the molecules are still H2O. dissolving sugar in water - Sugar molecules are spread within the water, but the individual sugar molecules are unchanged. dicing potatoes - Cutting usually separates molecules without changing them.

Balancing Equations

To balance an equation, we need to make sure that the total number of each element on the reactant side is equal to the total number of each element on the product side. We can only put a coefficient (number) before a single element or a compound to help balance the equation. We cannot put a number after an element, add a subscript, or place parenthesis around elements to balance the equation.

Copy these rules for balancing chemical equations in your notebook: Write out the Chemical Equation and create a T chart below it. In the T chart, write the element symbols you see in the equation on each side. Count the number of atoms of each element on both sides of the equation and write that next to the symbol. Add coefficients to balance the equation. Recount the number of atoms of each element to make sure the number is the same of both sides.

___H2 + ___O2  ___H2O

___H2 + ___O2  ___H2O Look at the equation, and see where the number of atoms on the left does not equal the number on the right. In this case, there are 2 oxygen on the left and only 1 on the right. Place the coefficient needed to make the right side oxygen equal to the left side. In this case a 2 is correct. ___H2 + ___O2  2 H2O Look at the number of atoms again. There are 2 hydrogen on the left and 4 on the right side of the equation. Place the coefficient needed to make the right side hydrogen equal to the left side. In this case a 2 is correct. 2 H2 + ___O2  2 H2O Count the atoms again. The numbers should be equal on both sides now. A coefficient of 1 before oxygen gas is not added because the written symbol indicates 1 molecule of the gas.