 #### Significant Figures

Lovingly dubbed "sig figs" by many chemists, significant figures are a way to round long numbers. There are two rules to remember when counting how many significant figures appear in a number. If there is a decimal point present, start counting sig figs from the first non-zero number on the left. For example, 56.90 has four sig figs, while 59.0 has three. If there is no decimal point present, start counting sig figs from the right-most non-zero number. For example, 3200 has 2 sig figs, while 1643 has four. To add and subtract, the final answer should have the same number of decimal points as the number with the fewest decimal points in the problem. For example, 42.3-10.00=32.3 (since 42.3 has one number after the decimal, the final answer also only has one). To multiply and divide, the final answer should have the same number of total significant figures as the number with the fewest sig figs. For example, 4x12.0=50 (since 4 only has one sig fig, the answer should only have one as well).

#### Density

Density is a  way to measure how compact a substance is, or how much matter is present within a given space. To solve for the density of an object used the equation: Density=mass/volume

#### Scientific Notation

Scientific notation is a way to make extremely large or extremely small numbers more manageable. To write a number in scientific notation, first move the decimal point in the number to a value between 1 and 10, such as 1.4 or 8.5. Then count the number of spaces that you moved the decimal point and multiply the number between 1 and 10 by 10 raised to the power of the number of decimal points counted. If the decimal was moved to the left, the exponent should be negative; if the decimal was moved to the right, the exponent should be positive. For example, 45,000 becomes 4.5 x 10^4. Or, 0.0076 becomes 7.6 x 10^-3

#### States of Matter

Matter can be classified into solid, liquid, and gas states. In a solid, molecules are very compact and are unable to move very much, causing the solid to maintain its shape. In a liquid, the molecules are more free to move around and can conform to the shape of a vessel. In a gas, molecules are completely free to move and thus spread out in all directions, diffusing to fill any space.

#### Separation of Mixtures

Separation of mixtures is an extremely important process in chemistry. To start, some mixtures can be separated by hand if there are large, solid pieces of a certain substance. Another method is with magnetism: if there is iron present in the mixture it can be separated with a magnet. Next, filtration with a filter paper and funnel can be used to separate water soluble substances from non-water soluble substances. Finally, evaporation removes water from a solution and leaves behind and dissolved compounds

#### Chemical and Physical Changes

Chemical and physical changes constantly occur in the world around us. Chemical changes occur when there is a change at the atomic level. Usually, these changes are hard to reverse. For example, a reaction between baking soda (sodium bicarbonate) and vinegar (acetic acid) produces bubbles (a release of gas). This reaction is difficult to reverse and takes place at the atomic level, thus making it a chemical change. Other common examples of chemical changes include a nail rusting or baking a cake. Contrastingly, physical changes occur when state of matter changes or when shape or size change. For example, crushing a can is a physical change because only the size/shape have changed and the molecular composition remain the same. Other common examples of physical changes are boiling water, breaking glass, or dissolving sugar in water.

#### Precision/Accuracy Both precision and accuracy are important in the study of chemistry, but each word has a slightly different meaning. As shown in the image, precision means obtaining repetitive and consistent results, even if they are not necessarily correct. Contrastingly, accuracy denotes a more "correct" set of data, even if the points are more spread out.

(Image Courtesy of: https://wp.stolaf.edu/it/gis-precision-accuracy/)

#### SI Units of Measurement The basic units of SI (Système Internationale/International System) are the meter, gram, and second. The beauty of SI measurement lies in the ease of transfer between orders of magnitude of measurements. Each prefix in the SI system represents a multiple of 10 from the basic meter/gram.

(Image Courtesy of: https://www.craftsmanspace.com/knowledge/international-system-of-units.html)