Chemistry Unit 1

Measurement & Uncertainty

Measurement

  • Measurement involves estimating between the smallest marks on a scale.

  • More graduations increase measurement accuracy.

Understanding Rulers

  • The smallest marks on a ruler are known as graduations.

  • Each ruler has a minimum graduation; e.g., one ruler may count in 0.1 increments and another in 0.01.

  • The precision of measurements depends on the smallest graduation.

Recording Measurements

  • Each recorded measurement includes an estimated digit one decimal place beyond the smallest graduation.

  • For example, if using a ruler with a graduation of 0.1 cm, you can estimate to the hundredths place (0.01).

  • Important: Do not add estimated digits to digital measurements.

Measurement Examples

  1. Reading a ruler shows a measurement of 4.5 units, which should be recorded as 4.50 units to reflect precision.

  2. For measurements with thermometers, identify the smallest graduation first before reading.

Measuring Volume

  • Volume of liquid is measured from the meniscus, which is the curve seen at the surface of the liquid in a graduated cylinder.

Uncertainty in Measurements

Overview of Uncertainty

  • All measuring devices have a level of uncertainty based on their design.

  • Uncertainty is denoted with ± after the measurement value.

Types of Equipment

  • Digital Equipment: The uncertainty (±) is equal to the smallest increment (e.g., 2.31 g ± 0.01).

  • Non-Digital Equipment: The uncertainty (±) is typically half of the smallest increment (e.g., volume of 43.0 mL ± 0.5).

Adding Uncertainty to Measurements

  • After measuring values with rulers, thermometers, and volume measurements, include uncertainty.

  • Estimated digits and uncertainty must be recorded to the same decimal place.

The Scientific Method

Steps in the Scientific Method

  1. Question/Observation/Problem

  2. Research (gathering information)

  3. Hypothesis

  4. Experiment/Test

  5. Results/Analysis

  6. Conclusions

Identifying Problems and Making Observations

  • Types of Observations:

    • General observation uses senses.

    • Quantitative includes numerical data.

    • Qualitative describes observable changes.

Research and Hypothesis

  • Research existing knowledge about the problem using books and valid sources.

  • A hypothesis is an educated guess, often framed in an if...then format.

Experiment Phase

  • Design experiments that allow repetition. Identify variables:

    • Constants: Unchanged conditions.

    • Control: Standard comparison.

    • Independent Variable (IV): Factor being changed.

    • Dependent Variable (DV): Factor being measured.

Collecting Results

  • Data represented in tables/graphs. Ensure clarity and accuracy.

Density

Definition of Density

  • Density is the mass of a substance divided by its volume.

Units of Density

  • Base Units: meter, gram, liter, etc.

  • Derived Units: density, which uses mass (g) and volume (mL).

  • Common unit of density: g/mL (1 cm³ = 1 mL).

Density Formula

  • Formula: D (g/mL) = m (g) / V (mL)

Solving Density Problems

  1. Write out formula with known values.

  2. Substitute known values.

  3. Solve for missing value while ensuring to include units.

Precision vs. Accuracy

Definitions

  • Precision: Agreement between measurements.

  • Accuracy: Closeness of a measurement to the accepted value.

Visual Representation

  • High precision and high accuracy quadrants.

  • Poor precision illustrated with examples of measurement error.

Percent Error

  • Percent error formula: % error = |measured - accepted| / accepted x 100%.

Properties of Matter

Physical Properties

  • Characteristics observable without altering composition, e.g., color, size, shape.

Intensive vs. Extensive Properties

  • Intensive: Independent of amount (e.g., density).

  • Extensive: Dependent on amount (e.g., mass).

Chemical Properties

  • Properties determined by changing chemical identity (e.g., flammability, corrosion).

Changes in Matter

  • Physical Change: Alters form without changing chemical composition (e.g., melting ice).

  • Chemical Change: Forms new substances (e.g., burning wood).

  • Signs of chemical change include gas production, heat/light emission, color change, and precipitate formation.

Matter Classification

Definitions

  • Matter has mass and occupies space.

  • Pure substances cannot be separated physically.

Types of Matter

  1. Elements: Made of one type of atom.

  2. Compounds: Combine of different elements.

  3. Mixtures: Can be physically separated.

Classifications of Mixtures

  • Homogeneous: Uniform composition throughout.

  • Heterogeneous: Varying composition parts.

Separation Techniques

Methods of Separation

  1. Manual Separation: Sort components visually.

  2. Magnetic Attraction: Use magnets to remove iron.

  3. Centrifugation: Separate substances based on density.

  4. Filtration: Using barriers to separate solids and liquids.

  5. Evaporation: Convert liquid to gas.

  6. Distillation: Separate based on boiling point.

  7. Chromatography: Separate based on movement rates in a medium.