Classification of Matter

Atoms, Molecules, Compounds, and Mixtures

Matter can be classified based on its composition and the nature of its constituent particles. Understanding these classifications is foundational in chemistry.

• Atom: The basic building block of matter, consisting of protons, neutrons, and electrons.

• Molecular Element: A substance made up of atoms with different numbers of protons, but all atoms are of the same element (e.g., O2, N2).

• Molecular Compound: Substances composed of two or more atoms, typically from different elements, joined by chemical bonds (e.g., H2O, CO2).

• Molecule: An electrically neutral entity consisting of at least two atoms joined by covalent bonds.

  • Homonuclar: Molecules comprised of two or more identical atoms (e.g., O2, N2).

  • Heteronuclear: Molecules comprised of at least two different elements (e.g., CO, H2O).

• Chemical Mixture: Two or more substances (compounds or elements) combined without chemical bonding.

  • Homogeneous Mixture: Uniform composition throughout (e.g., salt water).

  • Heterogeneous Mixture: Non-uniform composition (e.g., oil and vinegar salad dressing).

Properties of Matter

Physical and Chemical Properties

Matter is described by its physical and chemical properties, which help identify and classify substances.

• Physical Properties: Characteristics that can be observed without changing the substance's identity (e.g., color, temperature, mass).

• Chemical Properties: Characteristics that describe a substance's ability to undergo chemical changes (e.g., toxicity, combustibility, heat of formation).

Physical and Chemical Changes

• Physical Change: Involves a change in physical properties without altering the chemical composition (e.g., phase changes).

• Chemical Change: Involves a change in chemical properties, resulting in the formation of new substances (e.g., oxidation, combustion).

Phases of Matter: Gas, Solid, Liquid

• Solid → Liquid: Fusion

• Liquid → Solid: Solidification

• Liquid → Gas: Evaporation

• Gas → Liquid: Condensation

• Solid → Gas: Sublimation

• Gas → Solid: Deposition

Measurement in Chemistry

Qualitative vs. Quantitative Measurement

• Qualitative: Describes qualities (e.g., heavy, light) without numerical values.

• Quantitative: Involves numerical measurements (e.g., 10.0 kg, 23 °F).

SI Units of Measurement

Chemistry relies on the International System of Units (SI) for standardized measurements.

Chem 152 Prefixes

Example: To convert 800 nm to Mm:

Temperature Scales and Conversions

Celsius, Fahrenheit, and Kelvin

Temperature can be measured in Celsius (°C), Fahrenheit (°F), or Kelvin (K). Each scale has different reference points for the freezing and boiling points of water.

Conversion Equations:

• From Celsius to Fahrenheit:

• From Celsius to Kelvin:

Example: 350 °C = 623.15 K

Density and Volume Measurement

Density

Density is the ratio of mass to volume and is an intensive property (independent of quantity).

• Formula:

• Units: g/mL, kg/L, etc.

Example: If a stone displaces 10.3 mL of water and has a mass of 28.4 g, its density is:

Volume Measurement

• Liquids: Measured with graduated cylinders, pipettes (not beakers).

• Solids: Measured by geometric methods or water displacement.

Dimensional Analysis and Unit Conversions

Dimensional analysis is used to convert between units using conversion factors.

Example: Convert 3.4 hours to seconds:

Example: Convert to :

Percent Composition and Concentration

Percent composition expresses the mass of a part relative to the mass of the whole.

• Formula:

• Other units: ppm (parts per million), ppb (parts per billion), ppt (parts per trillion).

Example: If southern sweet tea is 10% sugar by mass, there are 10 g of sugar per 100 g of tea.

Example: How many kilograms of ethanol are present in 25.0 L of a gasohol solution that is 90% gasoline and 10% ethanol by mass? The density of gasohol is 0.71 g/mL.

Significant Figures and Scientific Notation

Significant Figures (Sig Figs)

Significant figures reflect the precision of a measurement.

• All non-zero digits are significant.

• Trailing zeros are significant (e.g., 24.070 has 5 sig figs).

• Leading zeros are not significant (e.g., 0.000278 has 3 sig figs).

Rules for Calculations:

• Addition/Subtraction: The result should have the same number of decimal places as the measurement with the fewest decimal places.

• Multiplication/Division: The result should have the same number of significant figures as the measurement with the fewest significant figures.

Rounding: If the last digit is 5, round so the final answer ends in an even number (e.g., 1.245 becomes 1.24; 1.255 becomes 1.26).

Scientific Notation

• Used for very large or very small numbers, or for numbers with more than 3 significant figures.

• Example: 0.00254 should be written as ; 4578.1 as .

Precision and Accuracy

Precision refers to the repeatability of measurements, while accuracy refers to how close a measurement is to the true value.

These values are precise (close to each other) but not accurate (not close to the true value of 1.000 g/mL).

These values are both precise and accurate.

Summary

This guide covers the foundational concepts of matter classification, properties, measurement, unit conversions, significant figures, and the distinction between precision and accuracy. Mastery of these topics is essential for success in General Chemistry.