CHEMISTRY

Experiment 2: Identification of Substances by Physical Properties

Objective

  • Analyze and determine the identity of unknowns by their physical properties.

Materials

  • Isopropanol

  • Ethanol

  • NaCl (Sodium Chloride)

  • Unknown liquid and solid

  • 10 mL graduated cylinder

  • Capillary tubes for melting point determination

  • Melting point apparatus

  • 3 medium-sized test tubes and test tube rack

  • Ring stand and utility clamp

  • Boiling point apparatus

  • Thermometer

  • Hot plate

  • Ceramic plate and thermometer clamp

Safety

  • Consult your instructor for the best disposal methods for unknown substances.

  • Never dump unknowns down the drain; place them in the appropriate waste container.

Introduction

  • Observations show that salt (NaCl) dissolves in water, but oil does not, and floats on top instead.

  • The differences in behavior between substances like oil and water can be attributed to their physical properties.

  • This lab will involve studying and identifying various substances based on their physical properties including:

    • Solubility

    • Density

    • Boiling Point

    • Melting Point

Definitions of Key Concepts

Solubility

  • Solubility refers to the maximum amount of solute that can dissolve in a solvent to form a saturated solution.

  • A solution consists of:

    • Solute: The substance being dissolved.

    • Solvent: The substance doing the dissolving, commonly water.

  • Different solutes behave differently in response to various solvents (e.g., acetone, cyclohexane, methanol).

Density

  • Density is a physical property unique to each substance, calculated using the formula:
    D = \frac{Mass}{Volume}

Boiling Point (BP)

  • Boiling point is related to the molecular structure and bonding properties of substances.

  • The procedure for measuring boiling points is specific and will be demonstrated during the lab.

Melting Point

  • Melting Point is defined as the temperature at which a solid turns into a liquid, indicating the energy required to convert the phases.

  • At the melting point, solid and liquid states coexist in equilibrium.

  • Example: Solid water (ice) has a melting point of 0 \, °C under 1 atm, while sodium chloride melts at approximately 1500 \, °C.

Procedure Overview

  • The experiment is divided into parts A, B, C, and D, which must be completed sequentially.

Part A: Solubility

  1. Use three separate solvents:

    • Isopropanol

    • Ethanol

    • Water

  2. Determine solubility of:

    • NaCl

    • Unknown Solid

    • Unknown Liquid

Steps

  1. Obtain three clean, dry test tubes labeled a, b, and c.

  2. Add one dropper full of each solvent to the test tubes.

  3. Add a small pinch of NaCl and the unknown substances to the respective tubes, avoiding saturation.

Boiling Point Correction

  • Correct boiling point measurement for atmospheric pressure:
    (760 \, mmHg - \text{today's pressure in } mmHg) imes (0.037 \, °C/mmHg) = \text{corrected factor (cf)}

  • Depending on pressure, adjust boiling point:

    • If pressure > 760 mmHg: add cf to 100 \, °C.

    • If pressure < 760 mmHg: subtract cf from 100 \, °C.

Part B: Density of Unknown Solid and Liquid

Unknown Solid

  1. Select an insoluble solvent from Part A.

  2. Obtain a 10 mL graduated cylinder and measure approximately 4-5 mL of the solvent.

  3. Add 2 grams of the unknown solid and record the new volume.

Unknown Liquid

  1. Tare the balance with a clean graduated cylinder.

  2. Pour approximately 5 mL of the unknown liquid.

  3. Measure and record volume and mass precisely.

Part C: Melting Point of Unknown Solid

  1. Obtain a capillary tube and insert a small amount of the unknown solid.

  2. Ensure the melting point apparatus is room temperature before beginning measurements.

  3. Gradually heat the substance and record the temperature range at which melting occurs.

Part D: Boiling Point of Unknown Liquid

  1. Set up boiling apparatus according to instructor’s instructions (example setup includes:

    • Ring stand

    • Hot plate

    • Thermometer clamp

    • Beaker filled with water).

  2. Gradually heat the unknown liquid and record temperatures until evaporation is observed.

Disposal

  • Dispose of cyclohexane and toluene in designated containers, not down the drain.

  • Unknowns should be sent to the proper waste containers.

  • Capillary tubes are disposed of in glass waste.

Data Analysis Sections

Part A: Solubility Data

  • Record observations for NaCl, Unknown Solid, and Unknown Liquid under each solvent category (soluble, insoluble, sparingly soluble).

Part B: Density Measurements

  • Record:

    • Mass of unknown solid

    • Initial and final volume in graduated cylinder

    • Density calculations.

Part C: Melting Point of Unknown Solid

  • Record melting range and median melting point.

Part D: Boiling Point of Unknown Liquid

  • Record boiling point and adjust for atmospheric pressure as necessary.

Identification of Unknowns

  • Utilize a table of compounds and their physical properties for identifying the unknown substances studied in the lab.

Post-lab Questions

  1. Discuss colors of CoCl2 and CoCl2•6H2O (use CRC handbook).

  2. Analyze how incomplete measuring affects density results.

  3. Examine physical states at room temperature for hydroquinone.

  4. Determine layering for ethyl acetate in a mixed solvent of water and cyclohexane.

  5. Identify a liquid with a density of 0.86 ± 0.01 g/mL, soluble in cyclohexane.