We have developed assessment tasks aligned to NGSS that require students to use practices along with disciplinary core ideas to make sense of energy-related phenomena. In this paper, we present an analysis of field test data and feedback from expert reviewers on the validity and reliability of a set of elementary school tasks. These tasks focused on assessing students' ability to write explanations or arguments about energy-related phenomena. Field test data were scored using rubrics based on the claim, evidence, reasoning (CER) framework. Using Rasch modeling, we evaluated the reliability of the task's rubric categories. We found that rubric categories fit well to the Rasch model. Categories were found to cluster in a hierarchy of difficulty in which reasoning and applying science idea categories were more difficult than evidence, which were more difficult than claim. The observed hierarchy in difficulty of CER categories is consistent with other studies and validates the tasks as measures of the CER framework. In addition, a panel of experts agreed that the tasks were aligned to the targeted NGSS practices and ideas. Overall, our results show that our procedure for task development resulted in valid and reliable NGSS-aligned assessment tasks.