If acid is added to a piece of sandstone and it begins to bubble, the mineral likely present is calcite.

Here's why:

* Calcite (CaCO3) is a carbonate mineral that reacts with acids, releasing carbon dioxide gas (CO2), which causes the bubbling. This reaction is often used as a field test to identify calcite.

* Sandstone is a sedimentary rock primarily composed of sand grains, which are typically made of quartz (SiO2). Quartz does not react with acids.

Important Note: While calcite is a common mineral found in sandstone, it's not always present. The bubbling reaction is a strong indicator of calcite, but other minerals might also react with acid, so it's not foolproof.