Name Date Class

Distinguishing Ionic and Covalent Compounds

When ionically bonded substances dissolve in water, the ions separate from each other enabling electric current to flow through the solution. An ionic compound can usually be identified by its high melting point. As a result of this high melting point, ionic compounds are crystalline solids at room temperature.

A covalent compound is made of molecules. Weak intermolecular forces hold molecules of covalent materials together. The weak forces are responsible for their relatively low melting points. Covalent compounds are usually liquids or gases at room temperature. With a few important exceptions, covalent compounds do not conduct electricity when dissolved in water, when they dissolve in water at all.

In this laboratory activity you will:

  1. build a simple conductivity tester to determine which substances produce ions in aqueous solution;
  2. examine various compounds to determine their state;
  3. research the literature to determine the melting points of these compounds;
  4. predict whether the compounds are more likely to be ionic or covalent.

OBJECTIVES

  1. Construct a conductivity tester.
  2. Measure conductivity of solutions.
  3. Observe the states of compounds.
  4. Research the melting temperatures of compounds.
  5. Relate properties to bond type.

MATERIALS

Inline Figure

general purpose LED

9 V battery

battery clip

1k-ohm resistor

connecting wire

alligator clips (3)

96-well microplate

96-well template

soda straw

thin-stem pipet

wire stripper

apron

goggles

Handbook of Chemistry and Physics

PROCEDURE

Part 1: Building a Conductivity Tester

  1. Refer to Figure A to assemble the LED, resistor, battery, and wires together.

    figure A

  2. Connect the red lead of the battery to one end of the resistor with an alligator clip.
  3. Using an alligator clip, connect the other end of the resistor to the positive lead of the LED (usually the longer lead).
  4. Strip the insulation from both ends of two pieces of connecting wire, each 3–4 cm long.
  5. Connect one piece of wire to the other lead of the LED with an alligator clip. Connect the second piece of wire to the lead of the battery clip.
  6. Push the end of one of the connecting wires crosswise through a soda straw.
  7. Push the end of the other wire through the straw about 0.5 cm from the first wire. (See Figure B.) The ends of the connecting wires will be the electrodes of the conductivity tester.

    figure B

Part 2: Testing Solutions for Conductivity

CAUTION: Many of the chemicals you will use are toxic. Both hydrochloric acid (HCl) and sulfuric acid (H2SO4)are very corrosive. Avoid contact with skin and eyes. Follow proper chemical hygiene procedures. Wash your hands thoroughly after completing this laboratory activity.

  1. Place 10 drops of solution 1 in well A1 of your microplate. Record the name and formula of the solute on your template and Microplate Data Form. Place 10 drops of solution 2 in well A2 of your microplate. Record the name and formula of this solute on your template and Microplate Data Form. Continue in like manner until each solution available to you has been dispensed into your microplate. (Rinse the pipet both inside and outside after each use).
  2. Insert the electrodes of the tester into a well that contains a solution. The electrodes must not touch each other.
  3. Note the relative conductivity of the solution by looking at the brightness of the LED.
  4. Record your observation on the Microplate Data Form. Use the following code: C = Conduction; NC = No Conduction; PC = Partial Conduction
  5. Test all the solutions. Wash the leads with distilled water after each test. Record the results.

Part 3: Observing the Physical State

  1. Inspect the flasks containing each of the undissolved substances.
  2. In the corresponding box of your Microplate Data Form, record for each compound "s" if the compound is a crystalline solid, "1" if the material is liquid, and "g" if the substance is a gas.

Part 4: Finding the Melting Point

  1. Use a reference source, such as the Handbook of Chemistry and Physics, to find the melting temperatures of all the substances used in this activity.
  2. In the corresponding box of your Micro-plate Data Form record the melting temperature (in degrees Celsius) for each compound.

DATA AND OBSERVATIONS

Refer to the instructions in step 4 of Part 2, step 2 of Part 3, and step 2 of Part 4 to record your observations and information about each substance in the following table. Also, indicate your predictions about bond type.