das Salz
salt
A white, crystalline substance (sodium chloride) used mainly to season and preserve food.
Die Suppe schmeckt fad, es fehlt noch etwas Salz.
The soup tastes bland; it still needs some salt.

salt
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salt
A white, crystalline substance (sodium chloride) used mainly to season and preserve food.
Die Suppe schmeckt fad, es fehlt noch etwas Salz.
The soup tastes bland; it still needs some salt.

You can tell from context whether the kitchen seasoning or the chemical compound is meant. In everyday life and cooking, it always refers to table salt. In scientific contexts, often together with 'Säure' (acid) or 'Base' (base), it refers to the chemical meaning.
Gibst du mir bitte mal das Salz?
Could you please pass me the salt?
As a seasoning, 'Salz' (salt) is usually an uncountable noun. In chemistry, however, 'Salz' (salt) is countable, and the plural is 'die Salze' (the salts) when referring to different kinds of salts.
Für das Rezept brauchst du nur eine Prise Salz.
For the recipe, you only need a pinch of salt.
In connection with cooking, 'Salz' (salt) is often used with the verb 'würzen' (to season) or in the phrase 'eine Prise Salz' (a pinch of salt). A pinch is a very small amount that you take between two fingers.
Die Soße muss noch mit Salz und Pfeffer gewürzt werden.
The sauce still needs to be seasoned with salt and pepper.
The phrase 'das Salz in der Suppe' (literally: the salt in the soup) is a fixed idiom. It describes an ingredient or event that is what really makes something interesting or special.
Für ihn sind die Reisen mit Freunden das Salz in der Suppe des Lebens.
For him, traveling with friends is the special ingredient in life.
The meaning of 'Salz' (salt) as a chemical compound is technical language. It is mainly used in chemistry class, at university, or in scientific texts.
Bei der Neutralisation entsteht Wasser und ein Salz.
During neutralization, water and a salt are formed.
appeal / stimulus / irritation
'das Salz' and 'der Reiz' are connected here mainly in a figurative sense. In the expression 'das Salz in der Suppe' (the special ingredient), 'Salz' does not mean the actual food ingredient, but something that makes something interesting or special in the first place. 'der Reiz' names the appealing or interesting side of something more directly. 'Salz in der Suppe' is more figurative and is often used for an important extra. 'Reiz' is more generally suitable when you want to explain why something is exciting or attractive.

Die kleinen Überraschungen waren das Salz in der Suppe der Feier.
The little surprises were the special ingredient that made the party interesting.
Der Reiz dieser Reise liegt für mich in den unbekannten Orten.
For me, the appeal of this trip lies in the unfamiliar places.