Chapter 1 Study Guide

Classification of Changes: Physical vs. Chemical

Understanding Physical and Chemical Changes

In chemistry, it is important to distinguish between physical and chemical changes. This distinction helps in understanding how substances interact and transform.

• Physical Change: A change that affects the form of a chemical substance, but not its chemical composition. Examples include changes in state (melting, freezing, etc.).

• Chemical Change: A change that results in the formation of one or more new substances with new chemical properties. Examples include rusting, burning, and tarnishing.

Examples:

• Iron rusting: Chemical change (formation of iron oxide)

• Gold melting: Physical change (change of state from solid to liquid)

• Silver tarnishing: Chemical change (reaction with sulfur compounds in air)

• Propane burning: Chemical change (combustion reaction producing CO2 and H2O)

Significant Figures and Calculations

Rules for Significant Figures

Significant figures (sig figs) are the digits in a measurement that are known with certainty plus one digit that is estimated. Proper use of significant figures is essential in scientific calculations to reflect the precision of measurements.

• Nonzero digits are always significant.

• Any zeros between significant digits are significant.

• Leading zeros are not significant.

• Trailing zeros in a decimal number are significant.

Sample 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.

Examples:

• Addition: $\frac{\left(6.5+8.550\right)}{3.975}=3.7786183522$ (rounded to 2 sig figs : 3.78)

• Multiplication: $(4.06 \times 10^2) \times (2.10 \times 10^{-2}) = 8.526$ (rounded to 3 significant figures: 8.53)

Unit Conversions

Dimensional Analysis

Unit conversion is a fundamental skill in chemistry, allowing you to express measurements in different units. Dimensional analysis (factor-label method) uses conversion factors to systematically convert units.

• Set up the problem so that units cancel appropriately.

• Multiply by conversion factors until you reach the desired unit.

Examples:

• Convert 6.55 × 107 mm to km:

$6.55 \times 10^7 \text{ mm} \times \frac{1 \text{ m}}{1000 \text{ mm}} \times \frac{1 \text{ km}}{1000 \text{ m}} = 6.55 \times 10^4 \text{ km}$

• Convert 4.570 × 104 kg to μg:

$4.570 \times 10^4 \text{ kg} \times \frac{10^6 \text{ g}}{1 \text{ kg}} \times \frac{10^6 \mu\text{g}}{1 \text{ g}} = 4.570 \times 10^{16} \mu\text{g}$

Density and Volume Calculations

Density Formula and Applications

Density is a physical property defined as mass per unit volume. It is commonly used to identify substances and solve for unknown masses or volumes.

Formula: $\text{Density} = \frac{\text{Mass}}{\text{Volume}}$

To find volume when mass and density are known:

$\text{Volume} = \frac{\text{Mass}}{\text{Density}}$

Example: If a student obtains a mass of 17.0 g of ethanol (density = 0.789 g/mL), the volume is:

$\text{Volume} = \frac{17.0 \text{ g}}{0.789 \text{ g/mL}} = 21.5 \text{ mL}$

Accuracy and Precision in Measurements

Definitions and Comparison

• Accuracy: How close a measured value is to the true or accepted value.

• Precision: How close repeated measurements are to each other, regardless of their closeness to the true value.

Example: In a lab, Student A's measurements are close to the true value (accurate), while Student B's measurements are very close to each other (precise).

Common Unit Conversions

Frequently Used Conversion Factors

From	To	Conversion Factor
mm	m	1 m = 1000 mm
m	km	1 km = 1000 m
kg	g	1 kg = 1000 g
g	μg	1 g = 1,000,000 μg
cm	in	1 in = 2.54 cm

Summary Table: Physical vs. Chemical Changes

Process	Type of Change	Description
Iron rusting	Chemical	Formation of iron oxide (new substance)
Gold melting	Physical	Change of state, no new substance formed
Silver tarnishing	Chemical	Reaction with sulfur compounds, new substance formed
Propane burning	Chemical	Combustion, new substances (CO2, H2O) formed