Chem I Lecture

Chemistry

•is the study of matter, its properties, and the changes it undergoes.

Matter

anything that has mass and takes up space

Composition of Matter

pure substances vs mixtures

substance

has distinct properties and a composition that does not vary from sample to sample

Types of substances

Elements and compounds

element

–An element is a substance which can not be decomposed to simpler substances. Ex: elements shown in the periodic table

compound

–A is a substance which can be decomposed to simpler substances because it is made up of more than one element. Ex: Nacl, MgCl₂

•A compound is made of atoms from two or more different elements.

Atoms

•are the building blocks of matter.

Most important element in the earth’s crust and in the human body

Oxygen

The Law of Constant Composition (or The Law of Definite Proportions).

•Compounds have a definite composition. That means that the relative number of atoms of each element in the compound is the same in any sample.

Mixtures

•Mixtures exhibit the properties of the substances that make them.

Heterogenous

•Mixtures can vary in composition throughout a sample (heterogeneous)

Homogenous

Mixtures can have the same composition throughout the sample (homogeneous).

Also called a solution

Physical properties

Physical properties. Examples: density, solubility, hardness, conductivity, state of matter.

•Physical properties can be observed without changing a substance into another substance. Matter remains the same substance after observation or measurement.

•changes are changes in matter that do not change the composition of a substance.

–Examples include changes of state, temperature, and volume.

Chemical properties.

Chemical properties. Examples: reactivity with acids, tendency to rust, oxidation states, toxicity, ability to decompose.

•Chemical properties can only be observed when a substance is changed into another substance. Measuring a chemical property always involves a change in the substance’s composition.

•Chemical changes result in new substances.

–One common chemical property is flammability, or the ability to burn in oxygen.

Intensive properties.

Intensive properties. Examples: melting point, temperature

•Intensive properties are independent of the amount of the substance that is present.

–Examples include density, boiling point, or color.

–These are important for identifying a substance.

Extensive properties.

Extensive properties. Examples: Length.

•Extensive properties depend upon the amount of the substance present. Change with the sample size.

–Examples include mass, volume, or energy.

•Mixtures can be separated based on physical properties of the components of the mixture. Some methods used are

1.filtration

2.distillation

3.chromatography

Filtration

•In filtration, solid substances are separated from liquids and solutions.

•Filtration is a mechanical or physical process used to separate solid particles from a liquid or gas by passing the mixture through a medium (filter paper as shown) that allows only the fluid to pass through while retaining the solid particles (yellow solid).

Distillation

•Distillation uses differences in the boiling points of substances to separate a homogeneous mixture into its components.

•It involves heating a liquid mixture to form vapor and then condensing the vapor back into liquid to collect it separately.

Chromatography

•This technique separates substances on the basis of differences in the ability of substances to adhere to the solid surface, in this case, dyes to paper.

Work

•is the energy transferred when a force exerted on an object causes a displacement of that object.

Heat

•Heat is the energy used to cause the temperature of an object to increase.

Force

•Force is any push or pull on an object.

•Kinetic energy (KE)

•Kinetic energy (KE) is the energy of motion.

Its magnitude depends on the object’s mass and its velocity

•Potential energy

•Potential energy of an object depends on its relative position compared to other objects. There are several types and hence different formulas.

Mass

•a measure of the amount of material in an object. S I uses the kilogram as the base unit. The metric system uses the gram as the base unit.

Length

a measure of distance. The meter is the base unit.

derived unit

•Note that volume is not a base unit for S I; it is a derived unit from

Density

•Density is a physical property of a substance.

Exact

•Exact numbers are counted or given by definition. For example, there are 12 eggs in 1 dozen. Numbers that have no uncertainty; they are known with absolute certainty.

Inexact

•Inexact (or measured) numbers depend on how they were determined. Scientific instruments have limitations (equipment errors) and individuals can read some instrumentation differently (human errors).

Precision

•Precision is a measure of how closely individual measurements agree with one another.

Accuracy

•Accuracy refers to how closely individual measurements agree with the correct, or “true,” value.

Significant Figures

1.All nonzero digits are significant.

2.Zeroes between nonzero digits are significant.

3.Zeroes at the beginning of a number are never significant.

4.Zeroes at the end of a number are significant if it contains a decimal point.

•The least certain measurement limits the number of significant figures in the answer.

•When addition or subtraction is performed, answers are rounded to the least significant decimal place.

•When multiplication or division is performed, answers are rounded to the same number of digits as the measurement with the fewest number of significant figures.

•Know the number of appropriate digits throughout, but round off at the end only!

addition & subtraction

•the result is reported to the least number of decimal places among the numbers.

multiplication & division

•In multiplication & division → the result is reported to the least number of significant figures among the numbers.

Dimensional analysis

•Dimensional analysis is used to change units.

•We apply conversion factors (e.g., 1 inch = 2.54 centimeter), which are equalities.

We can set up a ratio of comparison for the equality: