Zinc + Hydrochloric Acid → Zinc Chloride and Hydrogen Gas Zn + 2HCl → ZnCl2 + H2

Explain why it is necessary to place a "2" in front of HCl in the reactants in the chemical equation. Use "evidence" and "scientific" reasoning to support your answer.

This is because you need the same amount of atoms on each side and when you go to the reaction side you see that by adding the coefficient in from the H, 2, the hydrogen and chlorine react to make HCI :))

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.

a gas, having a molecular weight of 13 kg/kmol and a kinematic viscosity of 0.25 cm2/s, flows through a pipe 0.25 m internal diameter and 5 km long at the rate of 0.4 m3/s and is delivered at atmospheric pressure.