konzentriert
focused
Being fully attentive and mentally engaged with something; focused.
Sie arbeitete den ganzen Nachmittag hoch konzentriert an ihrer Abschlussarbeit.
She worked in a highly focused way on her thesis all afternoon.

focused / concentrated
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focused
Being fully attentive and mentally engaged with something; focused.
Sie arbeitete den ganzen Nachmittag hoch konzentriert an ihrer Abschlussarbeit.
She worked in a highly focused way on her thesis all afternoon.

The word 'konzentriert' (focused/concentrated) is the past participle of the verb 'sich konzentrieren' (to concentrate). It therefore describes either the state of a person who is concentrating, or the result of a concentration process in substances.
Ein konzentrierter Schüler macht weniger Fehler.
A focused student makes fewer mistakes.
You can easily recognize the meaning of 'konzentriert' (focused/concentrated) from the context. If it refers to a person or an activity, it means 'focused'. If it refers to a liquid or substance, it means 'not diluted'.
Der Sportler war vor dem Start voll konzentriert.
The athlete was fully focused before the start.
If you want to express what the concentration is directed at, use the preposition 'auf' followed by the accusative. This structure comes from the verb 'sich konzentrieren auf' (to concentrate on).
Der Detektiv war ganz auf die Lösung des Falles konzentriert.
The detective was completely focused on solving the case.
In the chemical or culinary meaning, 'konzentriert' (concentrated) is often combined with nouns such as 'Saft' (juice), 'Lösung' (solution), 'Säure' (acid), or 'Brühe' (broth/stock).
Vorsicht, das ist hoch konzentrierte Säure!
Careful, that is highly concentrated acid!
strong
'stark' and 'konzentriert' can both say that a liquid contains a lot of a substance. 'stark' is often used in everyday language for taste or effect, for example with coffee, tea, or alcohol. 'konzentriert' is more precise when something is not diluted, or only barely diluted, especially with juices, cleaners, or chemical solutions. Learners may confuse the words because both express 'a lot of something'. As a simple guide: 'stark' describes more how strong something tastes or acts; 'konzentriert' describes more the high amount of a substance in a mixture.

Der Saft ist konzentriert und muss vor dem Trinken mit Wasser gemischt werden.
The juice is concentrated and must be mixed with water before drinking.
Der Kaffee ist mir zu stark, deshalb nehme ich etwas Milch dazu.
The coffee is too strong for me, so I add a little milk.