Elementary, Middle School, High School
There are many ways that elements can join together to form compounds, depending on the nature of the chemical bonds that form between them.
For example, when a nonmetal (elements in the upper right of the periodic table, such as nitrogen or oxygen) combines with another nonmetal, electrons in the chemical bond are shared between the atoms in the bond. This type of interaction is called a covalent bond.
When a metal (elements on the left and lower part of the periodic table, such as sodium or iron) combines with a nonmetal, electrons are transferred from one atom to another. The atom that lost one or more electrons is left with a positive charge while the atom that has gained one or more electrons is left with a negative charge. Since opposite charges attract, these atoms now become tightly bound to each other in a highly ordered crystalline lattice that would resemble a house of mirrors. This type of interaction is called an ionic bond.
In this experiment, you will carry out a simple test to determine whether various water soluble substances around your home are ionic or covalent.
Ionic compounds that dissolve in water will conduct electricity. Covalent compounds will not conduct electricity. If your compound conducts electricity, you can be confident that it is ionic!
SUPPLIES, TOOLS, MATERIALS
- 8 oz plastic disposable cup
- 2 all metal flat-head thumb tacks
- 9 volt battery
- Various water soluble chemicals to test (salt, sugar, baking soda, artificial sweetener, etc.)
- Distilled water*
- Phenolphthalein indicator
*Tap water works fine but could confuse your results if there are dissolved ions in your water system.
**This will cause the solution to turn pink for ionic compounds. It isn't necessary but highly recommended to make the experiment super cool.
- Push two tacks into the bottom of your plastic cup. The pointed sides will be inside the cup. Make sure the tacks are not touching each other and that they are about an inch apart.
- Fill your cup about three fourths full with water (preferably distilled).
- Add a spoonful of your test chemical and stir until it is fully dissolved.
- Add 2-3 drops of phenolphthalein indicator and stir.
- Touch the 9-volt battery to the bottom of the cup. Record your observations!
- Thoroughly rinse the inside of your cup and your spoon before testing another compound. Any residual material could contaminate your solution, making it conduct electricity and giving you a false positive result for ionic bonding.
- Repeat for as many compounds as you want to test!
WHAT IS HAPPENING?
What you observed when you tested your ionic compounds is actually a process called electrolysis of water. In fact, there were two chemical reactions occurring in your cup, one at the anode and one at the cathode.
At the anode,
4 OH−(aq) → O2(g) + 2 H2O(l) + 4 e−
At the cathode,
2 H2O(l) + 2e− → H2(g) + 2 OH−(aq)
Notice the hydrogen gas formed at the cathode. This hydrogen is responsible for the bubbles you observed! Also, if you used an indicator, the OH− (called "hydroxide ion") is responsible for the color change you observed.
What about the oxygen gas formed at the anode? There are a number of reasons this gas is either not observed or only slightly observed, ranging from side reactions, to nano-bubbles, to the oxidation of the tack itself (did it turn black?).
QUESTIONS TO CONSIDER
- Look up images online of ionic crystals and describe them in detail.
- Using a periodic table, guess whether each compound below would be ionic (metal(s) and nonmetal(s)) or covalent (all nonmetals).
- Record your observations for each compound you tested and determine whether each compound is ionic or covalent.
- If possible, find the chemical formula for the compounds you tested online. Do your results agree with what you would have predicted from the formula? If not, why not?