This study, based on the theory of Technological Pedagogical Content Knowledge investigated the effectiveness of introducing open source YouTube videos in the teaching and learning of the Chemistry topic 'Group Properties' at a high school in Lesotho. A quasi-experimental design was used for two conveniently selected Form D classes; one class used as the experimental group (n = 49) and the other as a control group (n = 60). The questions in the pre-test and the similar post-test were formulated to establish the learners' cognitive abilities in line with various levels of Bloom's Cognitive Domain. The mean performance of the two groups before the intervention was not statistically different. After introducing the experimental group to YouTube videos the performance of the group was significantly better than that of the control group during the post-test. The performance of the experimental group was also significantly better than that of the control group at the higher cognitive levels of Bloom's Cognitive Domain, namely, at application to evaluation. The incorporation of videos in teaching Chemistry provided a better option than the traditional method of science teaching. The use of the YouTube videos resulted in a doubling of the percentage of the experimental group learners passing the post-test (from 12 to 27%), while the percentage of control group learners passing the tests remained constant at 5% for both the pre-test and the post-test. In light of the significant benefits of using YouTube videos in this context of poor laboratory resources, it is recommended that teachers should be trained to integrate technology in their teaching in order to supplement practical work.