Chemistry: Composition and Classification of Matter

• Chemistry studies the composition, properties, and changes of matter.
• It provides a fundamental understanding of various science-related fields.
• Matter is defined as anything that has mass and occupies space.

Methods of Classification

• Matter can be classified by its state (solid, liquid, gas) or its composition.

States of matter

• The three states of matter include solid, liquid, and gas
• Ice, liquid water, and water vapor are examples of states of matter

Classification of Matter by Composition

• If a substance is uniform throughout, it is homogeneous.
• If it is not uniform throughout, it is a heterogeneous mixture.
• Homogeneous mixtures are also called solutions.
• Substances can exist as elements or compounds.
• Elements are composed of only one kind of atom.
• Compounds have at least two kinds of atoms.

Substances

• A substance possesses distinct properties therefore composition does not vary from one sample to another.
• Elements cannot be broken down into simpler substances.
• Compounds can be broken down since they are made up of more than one element.

Atoms

• Atoms constitute the building blocks of matter
• Each element consists of unique kinds of atoms but can contain more than one atom of that kind.
• Compounds involve atoms derived from two or more distinct elements.
• Groups of atoms are referred to as molecules.

Elements and Composition

• There are currently 118 named elements.
• Five elements make up 90% of Earth's crust by mass.
• Three elements make up 90% of the human body by mass.

Representing Elements Using Symbols

• Chemists use symbols to represent elements
• Symbols consist of one or two letters; the first letter is always capitalized.
• Some element symbols come from their Latin, Greek, or other foreign language names.

Compounds and Composition

• Compounds possess a definite composition and relative numbers of atoms of each element in the compound will always be the same within any sample.
• The Law of Constant Composition is also known as The Law of Definite Proportions

Mixtures

• Mixtures exhibit the properties of the substances that compose them and can be heterogeneous or homogeneous.
• A homogeneous mixture is called a solution.

Types of properties

• Physical properties can be observed without changing a substance into another substance.
• Chemical properties can only be observed when a substance is changed into another substance.
• Intensive properties are independent of the amount of the substance; these include color, odor, density, melting point, boiling point and hardness.
• Extensive properties depend on the amount of the substance present; these include mass, volume, or energy.

Types of changes

• Physical changes do not alter the composition of a substance, like changes of state, temperature, and volume.
• Chemical changes result in new substances, such as oxidation, combustion, and decomposition

States

• Converting between the three states of matter is a physical change
• When ice melts or water evaporates, each molecule still contains 2 hydrogen and 1 oxygen atom.

Chemical reactions

• Reacting substances are converted to new substances during a chemical reaction.
• Physical properties can help identify when a chemical change has occurred like a change in color

Separating Mixtures

• Filtration can separate solid substances from liquids and solutions
• Distillation can separate a homogeneous mixture into its components based on differences in boiling points.
• Chromatography separates substances based on the differences they have in being able to adhere to a solid surface.

Scientific Method

• The Scientific Method is a process for understanding the world that involves these steps in order:
  • Collect information through observations and experiments
  • Formulate a hypothesis
  • Test the hypothesis via experiments
  • Formulate a theory based on the most successful hypotheses
  • Repeatedly test the theory and modify if needed
  • Reject is necessary

Numbers and Chemistry

• Chemistry depends heavily on numbers, with many topics that are quantitative.
• The concepts of numbers in science include units of measurement, quantities that are measured and calculated, uncertainty in measurement, significant figures, and dimensional analysis.

Units of Measurements

• The Système International d'Unités (SI) is the "International System of Units"
• A unique base unit is used for each quantity.

SI Base Units:

• Length is measured in meters (m)
• Mass is measured in kilograms (kg)
• Temperature is measured in kelvin (K)
• Time is measured in seconds (s)
• Amount of substance is measured in moles (mol)
• Electric current is measured in ampere (A or amp)
• Luminous intensity is measured in candela (cd)

Metric System Prefixes

• Prefixes convert base units into units that are appropriate for different situations.

Mass and length

• Mass and length are basic units of science that can be measure
• Mass measures quantity of material of an object.
• The SI unit for mass is kilogram (kg)
• The metric system uses the gram as a base unit.
• Length is a measure of distance.
• The meter is the length base unit

Volume

• Volume is a derived unit for SI using length (m x m x m = m³)
• The most commonly used metric units for volume are the liter (L) and the milliliter (mL).
• A liter is a cube 1 decimeter (dm) long on each side.
• A milliliter is also called 1 cubic centimeter (cm x cm x cm = cm³).

Glassware for measuring volume

• Glassware for measuring volume includes graduated cylinders, syringes, burettes, pipettes, and volumetric flasks

Density

• Density is a physical property of a substance with units derived from mass and volume (g/mL or g/cm³).
• Density (D) = mass (m) / Volume (V)

Temperature

• In general temperature determines the direction of heat flow.

Temperature Scales:

• The Celsius and Kelvin scales are most often used in scientific measurements
• The Celsius scale is based on the properties of water with 0 °C as water's freezing point and 100 °C as water's boiling point.
• The Kelvin is the SI unit, based on the properties of gasses there are no negative kelvin temperatures, 0 K represents absolute zero

Temperature Conversions:

• K = °C + 273.15
• °F = 9/5(°C) + 32
• °C = 5/9(°F – 32)

Energy and Work:

• Energy can be defined at the capacity to perform work or transfer heat.
• Work is energy transferred when a force exerted on an object.
• Heat is the energy used to cause the temperature of an object to increase. Any push or pull on an object is force

Measuring Energy:

• The unit of energy is the joule (J), which is a derived unit
• The kilojoule (kJ) is commonly used for chemical change.
• 1 cal = 4.184 J
• 1 nutritional Calorie = 1 Cal = 1000 cal

Forms of Energy

• Kinetic energy is the energy of motion
• Potential energy is the energy an object has due to its relative position.

Numbers that determine accuracy encountered in Science

• Exact numbers are those that are counted or defined like 12 eggs in 1 dozen.
• Inexact or measured numbers depend on how they were determined due to limitations of equipment or human interpretation.

Uncertainty in Measurements

• Measuring equipment can have different uses and accuracy
• All measured numbers have some degree of inaccuracy
• The last digit measured is always reliable, rather than exact

Significant Figures

• All digits of an amount measured that include uncertain ones are significant figures
• When numbers are calculated and rounded off, significant figures are considered

Rules for Significant Figures:

• All nonzero digits will be significant
• Zero's between nonzero digits will be significant
• Zeros found at the beginning are never significant
• Zeros found at the end are significant if it contains a decimal point.

Significant Figures in Calculations

• The least certain measurement limits the number of significant figures in the answer.
• When adding and subtracting, answers should be rounded to the least accurate decimal place.
• When multiplying or dividing, answers should be rounded to the same number of the digits of the measurement with the fewest significant figures.

Accuracy and Precision

• Precision measures how closely individual measurements agree.
• Accuracy indicates how closely individual measurements agree with the correct or true value.

Dimensional Analysis

• Analysis is applied change change units
• Apply conversion factors (equalities)
• A ratio of comparison for the equality can be set up
• Use the ratio (puts the units we have in the denominator to cancel).
• Multiple conversions can be applied so long as each one is an equality.