In exothermic chemical reactions the combined masses of the resulting molecules is indeed slightly less than that of the inputs. But this does not make \u201cthe system\u2019s mass appear to decrease\u201d for two reasons. The superficial reason is that in chemical reactions the mass decrease due to energy loss from the system is far too small for us to detect, and so we did originally learn the very slightly wrong conclusion \u201cthat mass is conserved\u201d. But the deeper reason is that, so long as the system is isolated (eg by including walls that reflect or absorb photons so that none can escape), then the system\u2019s mass does not in fact decrease at all. This is because the system\u2019s mass includes not just the masses of the individual molecules but also all of the energy in the system (including both photons and the thermal kinetic energy of relative motion of molecules). And if we do allow energy to escape, then the system\u2019s mass is not<\/i> conserved, as what we have learned more recently (though well over a hundred years ago now) is that what is conserved is not just the total mass of constituent particles but rather the total energy (where that includes a contribution from the masses of any particles within the system).<\/p>\n
Source: (1001) Alan Cooper’s answer to In chemical reactions like CH\u2084 + 2O\u2082 \u2192 CO\u2082 + 2H\u2082O, we learn that mass is conserved.But since heat energy is released, E=mc\u00b2 implies a tiny mass decrease. Heat is emitted as massless photons, so why does the system\u2019s mass appear to decrease? – Quora<\/a><\/em><\/p>\n","protected":false},"excerpt":{"rendered":" In exothermic chemical reactions the combined masses of the resulting molecules is indeed slightly less than that of the inputs. But this does not make \u201cthe system\u2019s mass appear to decrease\u201d for two reasons. The superficial reason is that in chemical reactions the mass decrease due to energy loss from the system is far too … Continue reading Mass “Loss” in Chemical Reactions<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1,4],"tags":[],"topics":[],"class_list":["post-1825","post","type-post","status-publish","format-standard","hentry","category-all","category-quora-answers"],"_links":{"self":[{"href":"https:\/\/qpr.ca\/blogs\/physics\/wp-json\/wp\/v2\/posts\/1825","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/qpr.ca\/blogs\/physics\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/qpr.ca\/blogs\/physics\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/qpr.ca\/blogs\/physics\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/qpr.ca\/blogs\/physics\/wp-json\/wp\/v2\/comments?post=1825"}],"version-history":[{"count":1,"href":"https:\/\/qpr.ca\/blogs\/physics\/wp-json\/wp\/v2\/posts\/1825\/revisions"}],"predecessor-version":[{"id":1827,"href":"https:\/\/qpr.ca\/blogs\/physics\/wp-json\/wp\/v2\/posts\/1825\/revisions\/1827"}],"wp:attachment":[{"href":"https:\/\/qpr.ca\/blogs\/physics\/wp-json\/wp\/v2\/media?parent=1825"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/qpr.ca\/blogs\/physics\/wp-json\/wp\/v2\/categories?post=1825"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/qpr.ca\/blogs\/physics\/wp-json\/wp\/v2\/tags?post=1825"},{"taxonomy":"topics","embeddable":true,"href":"https:\/\/qpr.ca\/blogs\/physics\/wp-json\/wp\/v2\/topics?post=1825"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}