Calcium carbonate crystals can form what when mixed with acetic acid?

When calcium carbonate (CaCO3) is mixed with acetic acid (CH3COOH), a chemical reaction occurs that produces carbon dioxide gas (CO2) along with water (H2O) and calcium acetate (Ca(CH3COO)2). This reaction can be represented by the equation:

CaCO3 + 2CH3COOH → Ca(CH3COO)2 + CO2 + H2O.

The evolution of gas is a notable characteristic of this reaction, which can often be observed as bubbling or fizzing. This is a classic example of an acid-carbonate reaction, where the acid reacts with the carbonate to release carbon dioxide gas, demonstrating the effervescence associated with such reactions. This is why gas is the correct answer and is a key point when studying chemical reactions, particularly in the context of urine and body fluid analysis, where such interactions can be relevant in interpreting certain findings.