Learn Italian: La sintesi chimica in laboratorio è molto pericolosa. - Chemical synthesis in the laboratory is very dangerous.

Normally, "-i" marks a plural noun in Italian. However, Italian has several nouns of Greek origin that end in "-i", such as sintesi, crisi, and analisi. These are almost always feminine. You can tell sintesi is singular here because it uses the feminine singular article la.

If I wanted to talk about multiple chemical syntheses, how would I make sintesi plural?

Feminine nouns of Greek origin ending in "-i" are invariable, meaning the noun itself doesn't change spelling in the plural. To make it plural, you only change the words around it. "The chemical syntheses" would become le sintesi chimiche.

Why do the adjectives chimica and pericolosa end in "-a" when the noun they describe ends in "-i"?

Adjectives always match the actual gender and number of the noun, regardless of how the noun is spelled. Because la sintesi is a feminine singular noun, its adjectives take the standard feminine singular "-a" ending.

Why does the sentence use in laboratorio instead of nel laboratorio?