Blood and poison

Solutions of soluble salts are prepared. Mixing the solutions leads to the startling formation of a "blood red" precipitate.

Ingredients: ferric sulfate, potassium thiocyanate

Procedure: A complete recipe follows.

1. Prepare separate solutions of soluble potassium thiocyanate and ferric sulfate salts.

2. Transfer a quantity of ferric sulfate solution into an Erlenmeyer flask or test tube.

3. Add a few drops of potassium thiocyanate solution and observe the reaction.

Understanding: The soluble salts form lightly tinted (ferric sulfate) or colorless (potassium thiocyanate) solutions. Mixing the two solutions leads to the dramatic formation of a deeply colored "blood red" precipitate. What must the precipitate be?

Think of the four ions initially in solution: sulfate, potassium, thiocyanate, and ferric. There are six unique pairings of the four ions. Only two of the six pairings make sense, as we need to combine a positively charged cation (ferric or potassium) with a negatively charged anion (sulfate or thiocyanate).

We have tried one of those pairings (ferric sulfate and potassium thiocyanate) and know that the result is soluble salts. The other pairing (ferric thiocyanate and potassium sulfate) must lead to the precipitate. From our experience with sodium salts (like table salt, sodium chloride) we expect that sodium sulfate is soluble. So we identify ferric thiocyanate as the precipitate.

The balanced equation for the reaction is

6 KSCN(aq) + Fe2(SO4)3(aq) → 6 K+(aq) + 3 SO42-(aq) + 2 Fe(SCN)3(s)

This reaction provides an excellent means of analyzing a sample for the presence of ferric ion, through the addition of a small quantity of soluble thiocyanate salt.

Solutions of soluble salts are prepared and mixed. Cations and anions combine to form a bright yellow precipitate.

Ingredients: lead nitrate, potassium dichromate

Procedure: A complete recipe follows.

1. Prepare separate solutions of soluble potassium dichromate and lead nitrate salts.

2. Transfer a quantity of lead nitrate solution into an Erlenmeyer flask or test tube.

3. Add a few drops of potassium dichromate solution and observe the reaction.

Understanding: The soluble salts form colorless (lead nitrate) or lightly tinted (potassium dichromate) solutions. Mixing the two solutions leads to the dramatic formation of a bright yellow precipitate. What is the precipitate?

Consider the four ions initially in solution: nitrate, potassium, dichromate, and lead. There are six unique pairings of the four ions. Only two of the six pairings make sense, as we need to combine a positively charged cation (lead or potassium) with a negatively charged anion (dichromate or nitrate).

We have tried one of those pairings (lead nitrate and potassium dichromate) and know that the result is soluble salts. The other pairing (lead dichromate and potassium nitrate) must lead to the precipitate. If you have some experience with salt peter (potassium nitrate), you know that it is soluble. It must be that lead dichromate is the bright yellow precipitate.

The balanced equation for the reaction is

Pb(NO3)2(aq) + K2Cr2O7(aq) → 2 K+(aq) + 2 NO3-(aq) + PbCr2O7(s)

This reaction provides an excellent means of analyzing a sample for the presence of lead ion, a well known poison, through the addition of a small quantity of soluble dichromate salt.