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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。