Chemistry 130 Laboratory

Properties of Ionic Compounds

 
GOALS:
1) Build an instrument to measure conductivity (a conductivity detector).
2) Explore the properties of ionic compounds found in the laboratory including water solubility
and electrical conductivity.
3) Explore ordinary household materials to determine if they contain ionic compounds.

  Pre Laboratory

Laboratory

INTRODUCTION: Ionic compounds are a combination of positively charged ions (cation) and negatively charged ions (anions). Ionic compounds can form when a metal loses its valence electrons, becoming a cation, and these electrons are transferred to a nonmetal which becomes an anion. The charge on each ion is determined by the number of electrons gained or lost so that the ion contains an octet of electrons in its outer shell. Once cations and anions are formed, they are strongly attracted to each other due to the difference in charge (opposites attract). This attraction between oppositely charged ions is called ionic bonding. In this laboratory you will be exploring the properties of ions and the nature of ionic bonding.

One property you will be exploring is the electrical conductivity of ionic compounds. In order for a material to conduct electricity it must contain charged particles that can move throughout the material. You are already know that metal wires conduct electricity from your everyday experience. In metals the charged particles are electrons that can freely move throughout the wire. In this experiment you will test the ability of ionic compounds to conduct electricity. Ionic compounds certainly contain charged particles (ions) but can they move throughout the material?

Instructions

SECTION A. OBSERVATION OF PHYSICAL PROPERTIES

As you did for the elements in the last experiment, record the appearance and properties of the ionic compounds you are working with.

  1. Physical state - Is the ionic compound a solid, liquid or gas?
  2. Appearance, Luster, Color - Is the compound shiny or dull? Does it have a distinctive color?
  3. Hardness, Malleability - Is the ionic compound brittle or pliable when pressed with a spatula?
  4. In your conclusions indicate what properties ionic compounds have in common and what properties differ.
SECTION B. TESTING WATER SOLUBILITY OF IONIC COMPOUNDS

For each ionic compound place about 0.5 g of the solid in a clean test tube and try to dissolve it in 10. ml, of water by stirring. You only need to weigh the first compound, after this use about the same amount for the other compounds. Estimate how much of the compound dissolved in the water (all, some, half, none) and record your observations in your notebook. Label the tubes and keep these solutions for later tests.
SECTION C. CONDUCTIVITY

Building a conductivity detector: Scientists often need to build an instrument to conduct novel experiments. An entire branch of chemistry, instrumental analysis, is devoted to building and modifying instruments to make chemical measurements. In this laboratory you get to experience the flavor of this field of chemistry by building your own instrument, a conductivity tester. The last two attached pages describe the construction of the conductivity detector in detail. If you have never soldered before, practice this on two pieces of wire as suggested. Once you can solder with confidence, assemble a conductivity detector as described. Attach a paper clip to each alligator clip to use for leads and make sure the LED lights when the paper clips are touched together.

Testing the conductivity of ionic compounds: Pour a small amount of NaCl into a small dry beaker. Test the conductivity of the NaCl using your conductivity detector. Be sure that both paperclips touch the NaCl but don't touch each other. Record your observations. Do you think ions are moving around in solid NaCl? Test the conductivity of the ionic solutions you made earlier in the laboratory. Be sure to clean off the paper clips between each test. Do any of these solutions contain moving ions? In your conclusions discuss any relationship between solubility of ionic compounds and conductivity of their solutions.

Testing the conductivity of household materials: For each of the liquids provided, pour a small amount into a clean test tube and test its conductivity. For each of the solids, dissolve a small amount in 10.0 mL of water and test the conductivity of the solution. Indicate which household products conduct electricity and which do not. By reading the ingredients, try to identify compounds that might be responsible for conductivity. Discuss your findings, especially anything surprising, in your conclusions.

 
 

BUILDING A CONDUCTIVITY DETECTOR
BACKGROUND:

The modem chemical laboratory usually has an array of devices that are designed to assist chemists in gathering the information needed to solve specific problems. The first pieces of equipment were undoubtedly hand-built by the individual scientist. In this exercise, you will construct a simple conductivity detector.
In your study of water purity and properties of materials, it would be useful to have a simple, safe device to test the conductivity of solutions, pure liquid, and solids because the conductivity of solutions is directly proportional to the amount of ionic solid that is dissolved in the water. The device you will construct will simply indicate whether a material or solution is a conductor. When the leads are immersed in a conducting liquid the light-emitting diode (LED) will light. (Light emitting diodes are solid state devices that emit light when electric current flows through them.) The detector is designed so that a conducting material completes the circuit and current flow causes the LED to emit visible light.
OVERVIEW
  1. Gather together all the parts for the detector.
  2. Practice soldering if necessary.
  3. Prepare the film canister case for the detector.
  4. Assemble and solder together the detector.
  5. Use the detector to test various liquids, solutions, and materials.
GETTING ORGANIZED
Tools
 Parts list for conductivity tester
Scissors 
Wire stripper 
Tape solder 
1 nail 
Pliers 
Matches and/or Candle		 9-V battery 
Battery snap connectors 
2-12 inch lengths of wire (black and red) 
1- light emitting diode 
2-alligator clips 
1 kilo-ohm 1/4 watt resistor 
12 inches of black electrical tape 
1 black plastic film canister
 
I.  SOLDERING  Solder is a low-melting lead alloy that is melted onto wires, which are then joined together as the solder solidifies, making an electrical connection. In this lab you will use tape solder, which is a strip of metal that is wrapped around the wires to be joined and then heated with a match or candle flame. The tape solder melts at a low temperature that makes a soldering iron or blowtorch unnecessary. The

Procedure Used to Join Two Wires

  1. Practice soldering by soldering two extra pieces of wire together.
  2. Using a wire stripper, strip off about I cm of the plastic coating from the ends of two wires. (There are 

    3.  several types of wire strippers. Your instructor will demonstrate the type that is available in your 
     laboratory.)
  3. Twist the bare ends of the wires together to make a mechanical connection.
  4. Cut a piece of solder tape that is 0.5-1.0 cm long.
  5. Wrap the solder tape around the connected wires and then heat the tape solder gently with a match or candle. The tape solder should melt and flow over the wires, joining them together.
  6. Repeat steps 1-5 until you can make a simple, lasting solder joint. 
II. ASSEMBLING THE DETECTOR. 
  1. Gather the parts for the conductivity detector and be sure you can correctly identify each part.


  2. Prepare the film canister by punching four holes in the cap arranged as shown below. Punch the holes from the inside of the cap to avoid damaging the lip of the cap. 
    3. If a suitable hole puncher is available use it. Otherwise, use the pliers to hold a nail in a flame until it is hot (not red hot). Then use the nail to melt suitable holes in the top of the plastic film container
  3. Flush the leads of the LED through the two closely spaced holes in the cap of the film canister.
  4. Locate the longer lead of the LED. If thin plastic tubing is available, cut a length of it so that it covers all of the lead except for about I cm. This tubing, or "spaghetti," insulates the bare wires.
  5. Twist the long lead of the LED and a lead from the resistor together. (Note: If the wrong lead is attached to the resistor, the LED will be permanently damaged when current flows through the circuit.)
  6. Using a piece of tape solder, solder the resistor and the LED together.
  7. If it is available, cut a length of thin tubing so that it covers all of the other resistor lead except for about 1 cm.
  8. Solder the remaining resistor lead to the red lead from the battery clip.
  9. Thread a lead from an alligator clip through one of the remaining holes in the cap. Locate the other LED lead. If thin tubing is available, cut a length of it so that it covers all of the LED lead except for about 1 cm.
  10. Solder the shorter lead of the LED to the lead from the alligator clip.
  11. Thread the lead from a second alligator clip through the cap. Locate the black lead from the battery clip and solder it to the lead from this second alligator clip.
  12. Connect the battery clip to a nine volt battery and test the detector by touching the alligator clips together. The LED should light.
  13. If the tester does not work, check the entire system against the diagram on page 11-4 to see if you have made a soldering error. Also check for loose connections. If you cannot find an error, check with your instructor.
  14. If the detector works, package it in a black plastic film canister and tape the top on with black tape. Be careful not to pull apart any connections or have any bare wire connection touching any other bare connection. 
    15. Diagram of the finished conductivity detector a) film canister, b) 9-V battery, c) battery clip, d) canister cap, e) resister, f) LED, and g) alligator clips
III. USING THE CONDUCTIVITY TESTER

To use the detector, do not insert the metal alligator clips directly into the solutions. Instead, clip a short piece of wire (or unfolded paper clip) in each alligator clip. Dip these wires into the solutions to be tested, Using wires keeps the alligator clips from corroding. It is important to rinse off the wires with distilled water each time they are inserted into a new solution so there is no carry over contamination from one solution to the next.

Pre-lab Questions

PRELAB QUESTIONS: None
 Department of Chemistry
Capital University

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