Model NGSS Lessons: Kindergarten - 3rd Grade

Document Type

Teacher Resource

Grade Level

Grade Level: 2nd Grade

Publication Date


Background Information

Heating or cooling an object will always change its temperature and may alter other properties as well. The temperature change is reversible, but changes to other properties might be irreversible. Heating, in particular, often causes chemical changes in which atoms alter their bonding to form new substances. Cooking an egg changes the egg in ways that cannot be undone. It cannot be “uncooked” into a raw egg.

Phase changes are generally reversible. Water can be frozen, melted back into liquid, boiled into vapor, and then condensed back into its original liquid form. When considering a manufactured object, however, phase changes often alter the shape of the object in irreversible ways that leave it unsuitable for its intended purpose. Many materials may be cooled without permanent impact. If they contain water, however, freezing will cause the water to expand, possibly rupturing solid structures in irreversible ways.

Performance Expectations

2-PS1-4 Physical Science

Construct an argument from evidence that some changes caused by heating and cooling can be reversed and some cannot.



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