How can you tell if a compound is an acid or base?

What causes the appearance of lines in a emission spectrum

Achemist who studies water samples did a demonstration of how to test for lead in water. she added a clear solution of potassium iodide to a clear solution of lead nitrate. then a yellow swirling solid formed in the liquid. what is most likely true about the yellow solid?

Ajar contains a certain substance. which observation would show that the substance must be either a solid or a liquid?