During an experiment, mr. lehman tells his students to be alert for signs that an endothermic reaction is happening. which of the following scenarios would his students most likely observe? -a test tube becomes cool to the touch -a liquid appears to glow -a new odor comes from a beaker -a solid forms from a mixture of solutions

Option (a) is the correct answer.

Explanation:

A reaction is endothermic reaction in which the reactant molecules absorb energy from the surrounding and no heat is released during the completion of reaction.

For example, melting of ice is an endothermic reaction.

Thus, we can conclude that out of the given options, a test tube becomes cool to the touch is the scenarios that his would students most likely observe.

it's c. elements are placed in rows based on how may valencies electrons have.

considering the question as per the attachment

here we can see that the radon nuclide has 222 as the atomic mass number and 86 as the atomic number.

when it undergoes nuclear fission it gives polonium which has atomic mass number as 218 and atomic number will be 84 along with a helium nucleus which has 4 atomic mass number and 2 as its atomic number.

222 = 218 + 4 (atomic mass number)

86 = ? + 2; therefore ? = 86-2 = 84.

the atomic mass number of polonium will be 84.