Which of these is an indication that a chemical reaction has occurred? a. melting of a substance b. boiling of a substance c. formation of a gas d. freezing of a substance

Write the symbol for every chemical element that has atomic number greater than 3 and atomic mass less than 12.0 u.

Which of the following statements is true? a. elements in the last period are radioactive. b. atomic weight is the same as atomic mass. c. elements in the same group have the same number of electron shells. d. atomic number equals the number of neutrons in the nucleus of an atom.

Astudent carefully transfers 30 g of water and 30 g of alcohol in a glass tube, forming two layers and filling the tube completely. after sealing the tube, the student mixes the solutions, and notices a bubble that forms in the tube. what is the mass of the contents in the glass tube after mixing?