Final answer:
Limestone's hardness can be determined using the Mohs scale and is approximately 3, meaning it can be scratched by steel, but not by softer materials like a copper penny. A dilute HCl acid test can also be performed to confirm the presence of calcite, though it does not reflect hardness directly. Chalk is a softer form of limestone indicative of its hardness.
Step-by-step explanation:
To find out how hard limestone is, one common method is the Mohs scale of mineral hardness, which characterizes the scratch resistance of various minerals. Typically, limestone, being composed mostly of the mineral calcite, has a Mohs hardness of about 3, which means it can be scratched by harder materials like a steel knife but not by a copper penny.
Limestone is also chemically reactive with acids. An easy way to test this is to apply a few drops of dilute hydrochloric acid (HCl) on the limestone; if it fizzes, it confirms the presence of calcite. This reaction, however, doesn't directly tell us the hardness but rather the chemical composition of the limestone.
Finally, chalk, which is a form of limestone composed of the shells of microscopic marine organisms, is typically soft and friable, so it's consistent with the general hardness level of limestone.