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 changes that are classified as either physical or chemical, each with distinct characteristics and implications.

• Physical properties: Describe a substance without changing its chemical composition (e.g., melting point, boiling point, density).

• Chemical properties: Describe the characteristic chemical reactions a substance undergoes (e.g., reaction with oxygen).

Physical Change

• Involves changes in the state or appearance of matter without altering its chemical identity.

• Examples: Melting of ice, boiling of water, dissolving sugar in water.

Chemical Change

• Involves a change in chemical composition, resulting in the formation of new substances.

• Examples: Rusting of iron, combustion of gasoline.

Example

• Physical change: Water molecules changing from liquid to gas (boiling water).

• Chemical change: Iron atoms reacting with oxygen to form iron(III) oxide (rust).

Energy: A Fundamental Part of Physical and Chemical Change

Energy Changes in Chemical Processes

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

• Potential energy: Energy associated with position or composition.

  • Gravitational

  • Elastic

  • Chemical (stored in chemical bonds)

• Kinetic energy: Energy associated with 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.

Example

• Combustion of hydrocarbons: High potential energy in chemical bonds is converted to kinetic energy and heat, lowering the potential energy of the products.

Measurement in Chemistry

Units of Measurement

Scientific measurements use the International System of Units (SI). Accurate measurement is fundamental to chemical analysis and problem-solving.

• Base SI Units:

  • Length: metre (m)

  • Mass: kilogram (kg)

  • Time: second (s)

  • Temperature: kelvin (K)

  • Amount of substance: mole (mol)

  • Electric current: ampere (A)

  • Luminous intensity: candela (cd)

Derived Units

• Volume: cubic meter (m3)

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

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

SI Prefixes

• Prefixes are used to express very large or very small quantities (e.g., kilo-, milli-, micro-).

Accuracy, Precision, and Significant Figures

Reporting Measurements

The certainty of a measurement is reflected in the number of significant figures reported. More significant figures indicate greater certainty.

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

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

Significant Figures Rules

• All nonzero digits are significant.

• Zeros between nonzero digits are significant.

• Leading zeros are not significant.

• Trailing zeros are significant only if there is a decimal point.

Example Table: Significant Figures

Number	Significant Figures
28.03	4
0.0540	3
408	3
0.0032	2
45000	2 (ambiguous, use scientific notation)
1.200 x 103	4

Calculations with Significant Figures

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

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

Solving Chemical Problems

Dimensional Analysis

Dimensional analysis is a method for converting units and solving problems in chemistry. It ensures that calculations are accurate and units are consistent.

• Identify the given information and the quantity to be found.

• Write out the relevant equations and conversion factors.

• Plan the steps needed to solve the problem, using unit conversions as necessary.

Example

• Convert cm to inches: $1~ ext{inch} = 2.54~ ext{cm}$

Additional info: These notes cover foundational concepts in general chemistry, including the nature of chemical and physical changes, energy, measurement, and problem-solving strategies. The content is suitable for introductory college-level chemistry courses.