Jessi and Squeaks grab a snack and learn all about why boiling eggs makes them hard! Second Grade Next Generation Science Standards Disciplinary Core Ideas: PS1.A: Structure and Properties of Matter - Different kinds of matter exist and many of them can be either solid or liquid, depending on temperature (2-PS1-1). PS1.B: Chemical Reactions - Heating or cooling a substance may cause changes that can be observed. Sometimes these changes are reversible, and sometimes they are not (2-PS1-4). Performance Expectations: 2-PS1-1. Plan and conduct an investigation to describe and classify different kinds of materials by their observable properties. 2-PS1-4. Construct an argument with evidence that some changes caused by heating or cooling can be reversed and some cannot.