Students who demonstrate understanding can: 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.
Clarification statement: Emphasis is on qualitative molecular-level models of solids, liquids, and gases to show that adding or removing thermal energy increases or decreases kinetic energy of the particles until a change of state occurs. Examples of models could include drawing and diagrams. Examples of particles could include molecules or inert atoms. Examples of pure substances could include water, carbon dioxide, and helium.
Official wording from the Next Generation Science Standards (NGSS Lead States, 2013). NGSS is a registered trademark of WestEd. Neither WestEd nor the lead states and partners that developed the NGSS were involved in the production of this page, and they do not endorse it. View on nextgenscience.org
Particles of matter are always moving. In a solid they vibrate in place, in a liquid they slide past one another, and in a gas they fly around freely and spread out to fill their container. Adding thermal energy makes the particles move faster on average, which we measure as a rise in temperature. Removing thermal energy slows them down.
Keep heating a solid and eventually the particles move enough to break out of their fixed positions: the solid melts. Keep heating the liquid and particles escape into the gas. While a pure substance is changing state, the energy goes into rearranging the particles, so the temperature holds steady until the change is complete. Water stays at 0 degrees Celsius while it melts, even though heat is still flowing in.
Students should build drawings or diagrams that show particle spacing and motion in each state and use them to predict what will happen to temperature and state as energy goes in or out. Any pure substance works, from water to carbon dioxide to helium.
Students often say particles get bigger when a substance warms up. The particles stay the same size; they move faster and the spaces between them grow.
Particles in a solid are fixed in place but still vibrate. Cooling makes the vibration smaller, not zero.
Students expect a steady rise. Show a heating graph: the flat section at 0 degrees Celsius is where the energy goes into breaking the solid arrangement.
The bubbles are water vapor, water particles that have gained enough energy to become a gas inside the liquid.
Ice at minus 10 degrees Celsius is heated steadily until it becomes steam above 100 degrees Celsius. Describe what the particles are doing in each part of the temperature graph.
Answer: Sloped parts of the graph are changes in particle speed (temperature); flat parts at 0 and 100 degrees Celsius are changes of state, where added energy changes the arrangement of particles instead of their average speed.
Let students act as particles: standing close and jiggling for a solid, shuffling around each other for a liquid, and spreading across the room for a gas. Then have them draw the same three states as boxes of dots with motion lines, and annotate a heating curve with those drawings.
Questions often show three particle boxes and ask which shows a gas after energy is removed, or show a flat heating curve and ask what is happening. Emphasize that the model must show both arrangement and motion.
Original questions written for this standard. Choose an option or type your answer, then press Check. Every question has a worked explanation.
Answer: C) They move faster on average
Adding thermal energy increases the average motion of the particles, which shows up as a rise in temperature.
Answer: B) Far apart and moving freely
Gas particles are widely spaced and move quickly in all directions, so a gas fills its container.
Answer: A) The energy is changing the arrangement of particles, not their average speed
During a change of state, energy goes into breaking particles out of the solid arrangement, so the average motion, and the temperature, holds steady.
Answer: condensation (also accepted: condensing)
As gas particles lose energy, they slow and move closer together, forming a liquid. This is condensation.
Answer: C) Particles in ice vibrate in fixed positions
Even in a solid the particles vibrate. They just cannot leave their positions.
Answer: boiling (also accepted: evaporation, vaporization, evaporating)
Liquid becoming gas is vaporization, which happens as boiling throughout the liquid or as evaporation at the surface.
Answer: A) They slow down on average
Removing thermal energy lowers the average motion of the particles, so they slow down and the temperature falls.
Develop a model to describe that matter is made of particles too small to be seen.
A full lesson with slides, activities and an exit ticket on thermal energy and changes of state, pitched to grades 6-8 and editable in PowerPoint or Google Slides.
Make a lesson βA printable, differentiated worksheet on MS-PS1-4 with an answer key, ready in about a minute.
Make a worksheet βTurn thermal energy and changes of state into a quiz students answer online that marks itself, with a class summary for you.
Build a test βNo. The emphasis is on qualitative particle models of solids, liquids and gases and on explaining temperature and state changes, not on calculating energy.
Any pure substance. Water is the most common, but carbon dioxide (dry ice) and helium are good examples of substances with very different melting and boiling points.