The quality or property of being additive; the characteristic of things that can be combined through addition without changing their properties.
From 'additive' plus the suffix '-ity' (forming abstract nouns). This mathematical and scientific term describes when things combine predictably through simple addition.
Additivity is crucial in science—if you mix two cups of water with three cups of water, you get five cups (additivity works!), but mix two cups of water with three cups of alcohol and you get less than five cups because molecules pack differently.
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