1、一般地形容酸气味,两者可通用.
2、acid可作名词,sour不行.
3、acid可形容土壤（主要是化学概念上的酸）,sour主要形容食物；
4、就引申义说,acid形容说话尖刻；sour形容不友好的’关系、眼神‘等
acid (LIQUID SUBSTANCE) 可作名词 noun [C or U],意为’any of various usually liquid substances which can react chemically with and sometimes dissolve other materials:
例如：Vinegar is an acid.醋是一种酸..
acid 也可作形容词 adjective,有两种意思：1.‘containing acid,or having similar qualities to an acid':例如：an acid taste/smell 酸的口味/气味；acid soil酸性土壤（这里不能用sour）
第二个意思用来形容尖刻的说话方式2.'describes a remark or way of speaking that is cruel or criticizes something in an unkind way':
例如When she spoke her tone was acid她说话时语气很尖刻.
sour 只能作形容词：adjective
有两个意思：
1.主要是形容食物的酸味酸气：'having a sharp,sometimes unpleasant,taste or smell,like a lemon,and not sweet:例如：These plums are a bit sour.这些李子有点酸.
2.不友好的恼人的,'unfriendly or easily annoyed':
例如：Overnight,it seemed,their relationship had turned sour.过了一晚上似乎他们的关系变糟了.
She gave me a sour look.她朝我不友好的看了一眼.
参考：Cambridge Learner's Advanced Dictionary