The only definite evidence of a chemical reaction is

a) the production of new substances

b) changes in phase

c) formation of a gas

d) a color change

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.

the answer is   c.) 1575 j.

explanation:

i do not get it   at all