Measurement, Physical and Chemical Change: Foundations of General Chemistry

Measurement, Physical and Chemical Change

Introduction to Chemistry

Chemistry is the scientific discipline that seeks to understand the properties and behavior of matter by studying atoms and molecules. It involves describing substances and their transformations, both physical and chemical.

• Physical properties: Characteristics that can be observed or measured without changing the substance's chemical composition (e.g., boiling point, melting point).

• Chemical properties: Characteristics that describe a substance's ability to undergo chemical changes or reactions (e.g., reactivity with oxygen).

Example: Water boiling is a physical change; iron rusting is a chemical change.

Physical vs. Chemical Changes

Physical and chemical changes are fundamental concepts in chemistry, distinguishing between changes that alter only the state or appearance of matter and those that change its chemical identity.

• Physical change: A change that does not alter the chemical composition of a substance. Examples include phase changes (melting, boiling, freezing).

• Chemical change: A change that results in the formation of one or more new substances with different chemical properties. Examples include rusting of iron, combustion of hydrocarbons.

Example: Boiling water (H2O(l) to H2O(g)) is a physical change; iron reacting with oxygen to form rust (Fe2O3) is a chemical change.

Energy in Physical and Chemical Changes

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

• Kinetic energy: Energy associated with motion. Examples: movement of atoms, molecules, electrons; heat; mechanical energy.

• Potential energy: Energy associated with position or condition. Examples: gravitational, elastic, and chemical (energy stored in chemical bonds).

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

Units of Measurement

Scientific measurements use the International System of Units (SI units) to ensure consistency and accuracy.

• 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)

• SI prefixes: Used to express very large or very small quantities (e.g., kilo-, milli-, micro-, nano-).

Derived Units

Derived units are combinations of SI base units used to measure quantities such as volume, density, and speed.

• Volume: $\text{Volume} = \text{length} \times \text{width} \times \text{height}$ (e.g., cubic meter, liter)

• Density: $\text{Density} = \frac{\text{mass}}{\text{volume}}$

Significant Figures

Significant figures reflect the precision of a measurement. The more significant figures, the greater the certainty.

• 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: 0.0540 (3 significant figures), 408 (3 significant figures), 0.0032 (2 significant figures).

In calculations:

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

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

Accuracy and Precision

Accuracy refers to how close a measured value is to the true value. Precision refers to how close repeated measurements are to each other.

• Accuracy: Closeness to the actual value.

• Precision: Closeness of repeated measurements to one another.

Solving Chemical Problems

Problem-solving in chemistry often involves unit conversions and dimensional analysis.

• Identify the given information and what you need to find.

• Write out the relevant equations and conversion factors.

• Plan the steps from the starting point to the end point.

• Use dimensional analysis to ensure units cancel appropriately.

Example: Converting meters to inches using $1 \text{ inch} = 2.54 \text{ cm}$.

Physical Change	Chemical Change
Boiling water (H2O(l) to H2O(g))	Rusting iron (Fe to Fe2O3)
No new substance formed	New substance formed
Reversible	Often irreversible

Additional info: These notes are based on textbook slides and introductory chapters, suitable for foundational General Chemistry study.