Color Changing Powder
Science Concept: A double replacement reaction will occur between two solids, resulting in a color change.
Materials & Pre-Demo Preparations:
Potassium iodide powder
Lead nitrate powder
Before the demonstration, measure out a moderate amount of both powders. No specific amount is required as long as both amounts are about the same. The powders need only cover the bottom of the flask. If using a crystal form of the chemicals, be sure to grind the two (separately) into fine particles.
Directions: Pour about 5 g of the potassium iodide powder into the test tube. Next, pour the same amount of lead nitrate powder into the test tube. After both powders have been added, put a stopper on the test tube, and shake the test tube until the powders turn a bright yellow color.
Introduction & Commentary: I have two powders here that look almost identical. Both are white. What do you think will happen if I put these two powders together? What kind of changes do you think might take place?
In most chemical reactions, the two chemicals are already dissolved in a water solution. This is usually necessary to let the molecules and ions move so that can collide and have a chemical reaction. In this case no water is to be used. The two solids are simply mixed together to see if a reaction can occur. The reaction can take place only on the surfaces of the crystals - this is why the tube is shaken vigorously.
The two powders undergo a chemical reaction in which a double replacement occurs. The lead ions combine with the iodide ions to make the yellow compound of lead iodide. The potassium ions combine with the nitrate ions to make the compound potassium nitrate which is white and cannot be seen because the yellow compound covers up the white compound. The chemical reaction is shown below:
2 KI + Pb(NO3 )2 ----------------> PbI2 + 2 K(NO3)
potassium iodide, lead nitrate, -------------> yellow lead iodide, colorless potassium nitrate
The double replacement reaction results in a white potassium nitrate and yellow lead iodide.
This reaction is unique because it involves solids undergoing a change. In order for a reaction to take place, molecules must collide with each other. Because of this, usually only liquids and gases have chemical reactions because their molecules are free to move about and collide with each other. In the case of solids, the molecules are so tightly packed together that they often times do not have enough space to collide and react with each other. In the case of this particular demonstration, the chemical reaction between the solids is possible as a result of the vigorous shaking of the powders. By shaking them, the surface of the crystals collide with each other and allow for a chemical reaction to take place.
Safety Precautions: Because this experiment involves lead, some may wish to wear rubber gloves. Also, when pouring these powders into the flask, lay out paper towels so that the powders do not get on the counter. If they do get on the counter and react with each other, a yellow color may stain the countertops.
Waste Disposal: The contents of the flask cannot be poured down the sink or put in the garbage. Special disposal is required because of the lead within the flask. See an instructor for special directions on the disposal of the lead.