The public image of chemistry is a relevant issue for chemical stakeholders. It has been studied throughout history by means of document analysis and more recently through surveys. Twitter, a worldwide online social network, is based on spontaneous opinions. We tried to identify the public perception of chemistry on Twitter, what it explains, and which sentiments are perceived. We gathered 256 833 tweets between 1st January 2015 and 30th June 2015 containing the words ''chemistry'', ''chemical'' or ''chem''. We cleaned and filtered them down to 50 725 tweets with textual information in English and clustered them using spherical k-means. The resulting clusters were categorised according to six topics by 18 chemistry experts. The prevailing topics were the learning environment topic, related to activities and tasks in chemistry courses, and the human activity topic, referring to facts and news about the chemical industry. The scientific knowledge topic, concerning communication of chemistry knowledge, only accounted for a small percentage of the tweets. We classified the tweets of most relevant topics based on their sentiment values and obtained more positive than negative perceptions. Nevertheless, the analysis of the unigrams and bigrams word clouds revealed a significant presence of chemophobia-related terms in the human activity topic, both in positive and negative classified tweets. It also revealed specific elements of chemistry courses negatively perceived in the learning environment topic.