Mixing 3.0 mL of AgNO3 (silver nitrate) and K2CO3 (potassium carbonate) in a test tube results in a chemical reaction to form a precipitate of silver carbonate (Ag2CO3). The overall reaction can be represented as follows:

2AgNO3 (aq) + K2CO3 (aq) --> Ag2CO3 (s) + 2KNO3 (aq)

Here's what you would observe when you mix these two solutions:

1. Formation of Precipitate: As soon as the solutions are mixed, you will notice the formation of a white precipitate. This precipitate is silver carbonate (Ag2CO3), which is insoluble in water. The precipitate appears as a cloudy suspension in the test tube.

2. Color Change: Initially, the AgNO3 solution is colorless, and the K2CO3 solution may also be colorless or slightly basic (depending on its concentration). After mixing, the solution will turn milky white due to the suspended precipitate of Ag2CO3.

3. Effervescence: During the reaction, you may observe some effervescence or bubbling in the test tube. This is caused by the release of carbon dioxide gas (CO2). The reaction between carbonate ions (CO3^2-) from K2CO3 and silver ions (Ag+) from AgNO3 produces CO2 gas as a byproduct.

4. Settling of Precipitate: Over time, the precipitate of Ag2CO3 will settle down at the bottom of the test tube, leaving a clear supernatant liquid above it. The supernatant contains the soluble product, potassium nitrate (KNO3), which is formed during the reaction.

5. Confirmation of Precipitate: To confirm the formation of Ag2CO3 precipitate, you can perform additional tests. For example, you can add a few drops of dilute nitric acid (HNO3) to the test tube. The precipitate should dissolve in nitric acid, indicating that it is silver carbonate.