
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.

concentrated
Having a high density or a high proportion of a particular substance; not diluted. Often used for liquids or chemical solutions.
Für das Rezept benötigst du konzentrierte Tomatensuppe aus der Dose.
For the recipe, you need concentrated tomato soup from a can.
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
When describing a liquid, stark can refer to a high concentration of a substance, but it can also describe an intense effect. Konzentriert more precisely indicates that a large amount of a substance is present and that the liquid is not diluted. When the ratio of substances is explicitly meant, konzentriert is the better choice.

Die konzentrierte Reinigungslösung muss vor dem Entfernen alter Flecken mit Wasser verdünnt werden.
The concentrated cleaning solution must be diluted with water before removing old stains.
Die starke Reinigungslösung entfernt selbst alte Flecken.
The strong cleaning solution removes even old stains.