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In the Next Generation Science Standards (NGSS), "states of matter" are not taught as a single isolated unit memorized in one grade. Instead, they are introduced progressively through a banded grade-level progression under the Disciplinary Core Idea PS1.A: Structure and Properties of Matter.

The concept scales up from macro-level observable properties in elementary school to microscopic, particle-level kinetic energy models in middle school.

1. 2nd Grade: Macro-Level Observations

• Performance Expectation: 2-PS1-1 — Plan and conduct an investigation to describe and classify different kinds of materials by their observable properties.

• How States Appear: Students focus on the tangible, everyday characteristics of matter. The core idea introduces that “different kinds of matter exist and many of them can be either solid or liquid, depending on temperature.”

• Focus: Sorting, classifying, and recognizing basic changes (like melting or freezing) using descriptive words like color, texture, hardness, and flexibility, without diving into atomic structures.

2. 5th Grade: The Particulate Nature of Matter (Gases & Conservation)

• Performance Expectations:

  • 5-PS1-1 — Develop a model to describe that matter is made of particles too small to be seen.

  • 5-PS1-2 — Measure and graph quantities to provide evidence that regardless of the type of change that occurs when heating, cooling, or mixing substances, the total weight of matter is conserved.

• How States Appear: Students transition from just seeing solids and liquids to realizing that gases are also matter made of particles moving freely in space.

• Focus: Building un-seen particle models to explain phenomena (like a balloon inflating or air exerting pressure) and tracking the conservation of mass during phase changes. (Note: NGSS boundaries at this level explicitly state that teachers should not yet try to explain the atomic-scale mechanisms of evaporation and condensation).

3. Middle School (MS): Kinetic Energy and Particle Motion

• Performance Expectation: MS-PS1-4 — Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.

• How States Appear: This is where formal states of matter (solid, liquid, gas) and phase transitions are deeply anchored. Students move from describing states to explaining them via molecular motion:

  • Solids: Atoms/molecules are closely spaced and vibrate in position, maintaining fixed relative locations.

  • Liquids: Molecules are constantly in contact with one another but move about relative to each other.

  • Gases: Molecules are widely spaced, moving freely except when colliding.

• Focus: Thermal energy transfer. Adding or removing heat changes the kinetic energy of these particles, directly driving phase changes and altering pressure/volume relationships.

4. High School (HS): Intermolecular Forces and Bulk Interactions

• Performance Expectations: HS-PS1-3 and secondary connections to HS-PS2-6.

• How States Appear: While high school chemistry shifts heavily toward atomic structure, electrons, and the periodic table, states of matter are governed by electrical forces within and between atoms (intermolecular forces, polarity, and Coulomb’s law).

• Focus: Explaining why substances exist as solids, liquids, or gases at room temperature based on the strength of electrostatic attractions holding the particles together.