Measurement, Physical and Chemical Change: Foundations of General Chemistry

Chapter 1: Measurement, Physical and Chemical Change

Chemical Changes and Physical and Chemical Change

Chemistry is the scientific discipline that seeks to understand the properties and behavior of matter by studying atoms and molecules. Matter can undergo both physical and chemical changes, each with distinct characteristics.

• Physical change: A change that alters the state or appearance of a substance without changing its chemical composition. Example: Boiling wance undergoes. Example: Rusting of iron (Fe reacts with O2 to form Fe2O3).

Energy: A Fundamental Part of Physical and Chemical Change

Physical and chemical changes are usually accompanied by energy changes. The total energy of an object is the sum of its kinetic and potential energy.

• Potential energy: The energy associated with an object's position or composition.

  • Gravitational

  • Elastic

  • Chemical (stored in chemical bonds)

• Kinetic energy: The energy associated with an object's motion.

  • Movement of atoms/molecules/electrons

  • Heat

  • Mechanical

• Law of Conservation of Energy: Energy is neither created nor destroyed, but transferred from one form to another.

• Substances with high potential energy tend to change in a way that lowers their potential energy, often releasing energy in the process (e.g., combustion of hydrocarbons).

Measurement in Chemistry

The Units of Measurement

Scientific measurements use the International System of Units (SI units). The fundamental SI units include:

• Length: meter (m)

• Mass: kilogram (kg)

• Time: second (s)

• Temperature: kelvin (K)

• Amount of substance: mole (mol)

• Electric current: ampere (A)

• Luminous intensity: candela (cd)

SI prefixes are used to express very large or very small numbers. For example, kilo- (k) means 103, milli- (m) means 10-3, and nano- (n) means 10-9.

Derived Units

Derived units are combinations of SI base units used to express other quantities:

• Volume: cubic meter (m3), often expressed as liters (L), where 1 L = 1 dm3

• Density: kilogram per cubic meter (kg/m3)

• Speed: meter per second (m/s)

• Acceleration: meter per second squared (m/s2)

Significant Figures

Significant figures reflect the precision of a measured or calculated quantity. The more significant figures, the greater the certainty.

• All nonzero digits are significant (e.g., 28.03 has 4 significant figures).

• Zeros between nonzero digits are significant (e.g., 408 has 3 significant figures).

• Leading zeros are not significant (e.g., 0.0032 has 2 significant figures).

• Trailing zeros are significant only if there is a decimal point (e.g., 140.00 has 5 significant figures).

• Scientific notation is used to avoid ambiguity (e.g., 1.20 × 103 has 3 significant figures).

Rules for calculations:

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

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

Accuracy and Precision

• Accuracy: How close a measured value is to the actual value.

• Precision: How close a series of measurements are to one another.

Solving Chemical Problems: Dimensional Analysis

Dimensional analysis is a method for converting between units using conversion factors. It ensures that units cancel appropriately, leaving the desired unit.

• Example: To convert 1 meter to inches, use the conversion factor 1 inch = 2.54 cm.

General problem-solving steps:

1. Identify the given information.

2. Identify what you need to find.

3. Plan a solution using equations and conversion factors.

4. Carry out the calculation, ensuring units cancel correctly.