Which of the following is a sign a chemical reaction has occurred?

The answer is not B because a new substance is not created, they just produced sparks and gas. The answer is not C because the liquid is still THE liquid even if it starts to boil. The answer is not D because that is a physical change called dissolving. So the answer would have to be A; blue liquid is mixed into a yellow liquid, and the mixture turns green. The two substances are being mixed together and created a new substance. Also a sign of chemical reaction is color change.

explanation:

let the different atoms be x and y. each with four protons and same mass will be represented as follows.

mass number is number of protons + number of neutrons.

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since, it is given that one has a negative charge while the other is neutral. therefore, there will be increase in number of protons and the atom will be represented as follows.

according to periodic table, mass number of beryllium is 9 and it has atomic number 4 so, the atoms will be and .  

the protons and neutrons will be located inside the nucleus whereas the electrons will be revolving around the nucleus.