Physical and Chemical Changes: Is It Still the Same Stuff?

Warm-Up

6 minutes. Hold up the sheet of foil. Fold it, crumple it, tear it in half. Ask: is this still aluminum?

Everyone will say yes, and they will be right, and it will feel too easy. Good — that is the baseline you need.

Now pour the vinegar onto the baking soda. Let it fizz. Ask the same question: is this still baking soda and vinegar?

Take answers and push for reasons rather than labels. Students will point at the bubbles. Ask what the bubbles are made of, and let the uncertainty sit — the gas is a substance that was not in either cup a moment ago, and noticing that something NEW is present is the whole idea, arrived at before it has a name.

Direct Instruction

13 minutes. Give the two names and the one question that separates them. Write on the board:

• Physical change — a new look. Same substance.
• Chemical change — a new substance.

Then the test, underneath: is it still the same stuff? Run the foil and the fizz back through it out loud so the class sees the question do the work.

Teach the signs of a chemical change as evidence rather than as a checklist: a new color, a new smell, bubbles of a gas, or heat and light given off. Say plainly that these are clues that usually mean a new substance, not a definition of one — the definition is the new substance itself.

Now the demonstration the lesson is built on. Show the cup of salt water made at the start. Ask where the salt went. Take a vote: is the salt gone, or is it still there?

Classes split on this, and the split is the point. Salt dissolving looks like a disappearance, and a disappearance looks like the most dramatic change available.

Bring out the dish you set up earlier and show the salt sitting in it. The water left; the salt did not go anywhere. It was salt the whole time, so dissolving is a physical change — and the class just watched the evidence rather than being told.

Then close the loop on drama. Put two changes side by side: a dropped plate shattering, which is loud and fast and physical, and a silver spoon slowly going dark in a drawer, which is silent and slow and chemical. How exciting a change looks tells you nothing about which kind it is. That sentence is worth writing on the board.

Be honest about the limits of the reversibility test. Getting the original back is a strong clue that nothing new was made, and it is the most useful question a student in this grade can carry. It is not a law: plenty of physical changes cannot practically be undone. When the two questions disagree, the substance question is the one that decides.

Guided Practice

12 minutes. Read the worksheet’s reference box together, then work Section A’s first two items as a class before students continue in pairs.

• Require a spoken reason for every item, in one fixed form: physical, because it is still the same ___, or chemical, because now there is ___ that was not there before. The sentence frame is doing real work — it makes a student name the substance, which is exactly the step that guessing skips.
• Circulate and listen for reasons that are really about drama: chemical, because it changed a lot. That is the misconception, still intact, wearing the right answer. Ask those students the salt question again.
• Two items are worth pausing on with the whole class if the pairs disagree, because they are the ones where looks and substance pull in opposite directions rather than pointing the same way.

Independent Practice

9 minutes. Students finish Section A independently and complete Section B, which asks them to explain the difference in their own words, name a sign of a chemical change, and give an example they have seen themselves.

The example item is the one to read first when you collect these. A student who supplies something from their own kitchen or yard has generalized; a student who returns one of your demonstrations has not yet, and is worth a follow-up question rather than a mark.

Assessment

3 minutes. Exit ticket, three items. (1) A candle is left in a hot car and slumps out of shape. Physical or chemical, and why? (2) A shiny copper roof turns green over many years. Physical or chemical, and why? (3) Which of those two looked like a bigger change, and did looking bigger make it chemical?

Items 1 and 2 test the classification. Item 3 tests whether the lesson landed, because a student can get both classifications right by memory and still believe that drama is what decides.

Closure

2 minutes. Hold up the dish of dried salt one more time and ask what it proves. Listen for an answer about the salt never having stopped being salt — not for the words physical change, which a student can say without having believed any of it.