Water is and is not H2O, depending on who you ask. Conceptualisations of water vary across chemists and laypeople

Conceptual representations can be shaped by multiple factors, including expertise. In this preregistered study, we tested whether the concept of “water” is represented differently across laypeople and chemists focusing on psychological essentialism. Essentialised categories are thought to be determined by internal factors (e.g., chemical composition). Previous research suggests laypeople do not essentialise “water”. Here, we sought to verify whether extensive experience with chemicals might lead to more essentialist conceptions. In the first two experiments participants provided H2O estimates, typicality, centrality, and frequency ratings for water examples, which showed that chemists partially incorporate H2O in their conceptual representation of “water”. Experiment 3 underlined qualitative differences in the semantic organisation of “water” across the two groups using similarity ratings. Experiment 4 consolidated these results with a sentence acceptability task, underlying the importance of chemical composition in determining what counts as “water” for chemists. Finally, Experiment 5 showed that laypeople consider both “H2O” and “water” as more abstract compared to chemists. Our results provide evidence on the variability of both psychological essentialism and conceptual representation overall, that can vary as a function of expertise.