Question

Identify the following as either a chemical change or physical change, and provide evidence to support your answer:

1. Blue Cu(II) cations of the salt solution react with green Ni(II) cations of its salt solution to produce a turqouise colored solution

2. A blue Copper 2 cation is heater, and the salt color became white and drops of a clear liquid were seen on the side of the test tube. When water was added, the solution turned blue. However, once water was evaporated through heating, the blue salt appeared again.

3. A blueish-green cation is heater and gas appears and the salt of this cation turned nearly black.

Part B.

1. Describe a way you could determine if Pb 2+ had leaked into public water pipes, in which lead was used in the past to connect the pipes.

2. Indicatre what would occur in the events that Pb 2+ had indeed leaked into the water

3. Formulate the net ionic equation for a positive test

4. What would occur if there was no Pb 2+ in the water? Would this allow us to conclude that there is definitely no Pb 2+ in the water, or would there still be uncertainty? Explain

Answer 1

1. Physical change, you are mixing two solution of two different colors and the result is the mixing of the colors, the ions are still in the same chemical form, you can separate both solutions by physical means.

2. Chemical change, you are adding a removing water from the crystal promoting the change of color, the way to add and remove water is by breaking bonds with heat.

3. Chemical change, with heat you are changing the composition of the salt, into gas and a new black salt.

Part B

1.The most common way to test for Pb2+ is using Potassium Iodide, the reaction will form a yellow precipitate.

2.There are reports that the leaked Pb into the water caused a public health crisis in some cities.

3.

4. most of the iodides are soluble, if the test is negative there will not be precipitate. This does not mean that there isn't Pb, it just means the test cannot detect at the concentration lead is present. Since the reaction is based on the equilibrium of the two ions, you need to reach a certain concentration to start the precipitation, if the Pb concentration is less than the concentration required to satisfy the product of solubility the two ions will remain in solution, thus you won't see a change and the test will be negative. You will need to take a sample to the lab to determine the concentration of Pb.