Agrio vs. ácido
The Spanish words 'agrio' and 'ácido' both refer to the taste sensation of sourness, but they are used in different contexts. 'Agrio' directly translates to 'sour', while 'ácido' translates to 'acidic'. In general, 'agrio' is used more frequently to describe foods or flavors, while 'ácido' is used more in scientific or chemical contexts. However, both can be used interchangeably to some degree when talking about tastes.
Agrio
'Agrio' generally translates into English as 'sour'. It's often used to describe the flavor of foods and beverages that have a sharp or tart taste.
El limón es una fruta muy agria.
(The lemon is a very sour fruit.)
Esta cerveza tiene un sabor agrio.
(This beer has a sour taste.)
Los pepinillos son agrios y crujientes.
(The pickles are sour and crunchy.)
Ácido
'Ácido' typically means 'acidic', but can also mean 'sour' in terms of taste. It is frequently used within more formal or scientific contexts when referring to the actual pH level of a substance.
El vinagre es ácido.
(Vinegar is acidic.)
El café tiene un sabor ácido.
(Coffee has an acidic taste.)
Las naranjas son ácidas pero algunas son dulces también.
(Oranges are acidic but some are sweet as well.)
Summary
'Agrio' and 'ácido' both relate to the concept of sourness in Spanish, although their usage can slightly differ. While both words are interchangeable for describing sour tastes in food & drinks, 'agrio' carries a simpler connotation and is mostly confined to general daily life usage. On the other hand, besides denoting sourness, 'ácido' has broader scientific implications pertaining to acidity as well.