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Matter Lab 6-8 / Week 07 / Wednesday
3/6
Week 07 · Physical Change or Chemical Change?

Wednesday

Bench Lab: Four changes
// Same stuff in a new form, or something new?
⏱ about 20 min

Wednesday: Bench Lab: Four Changes

Rocket lines up four clear cups and labels them A, B, C and D with tape. "Four changes," he says. "Predictions first."

Raven holds the pencil over the Bench Log. "What do you notice before we start? Every cup is clear, and every powder is white."

"So color will not help much," Rocket says. "We watch for bubbles, new solids, and warm or cool."

Nova lowers herself to cup level. "And remember the catch," she says. "After each change, ask what is left. Is it the stuff you started with?"

Rocket grins and stirs the first cup. "Salt, water, go."

Raven starts a timer. "Thirty seconds of watching for every cup. No skipping."

Your mission

You will run four changes at the Bench and decide which are physical and which are chemical.

For each cup, record what you see and which clue appears, if any. Then decide whether the substance at the end is the one you started with.

  • Four clear cups, labeled A, B, C and D
  • Water (cool from the tap) and warm water from the tap
  • Salt, sugar, baking soda, vinegar and lemon juice
  • Teaspoon and tablespoon measuring spoons
  • One ice cube
  • A tray or baking sheet to catch spills, and a towel
  • Your Bench Log
Safety first
Wear safety glasses or sunglasses. Cup C and cup D can foam and splash.
Nothing is tasted, and nothing is smelled up close. Nothing goes in the mouth.
Warm water from the tap is as warm as it gets. A grown-up handles any hotter water.
Only kitchen materials are used. Never add any cleaner.
Pour leftovers down the sink with running water. Wash your hands when you finish.
  1. Cup A: add half a cup of cool water and one teaspoon of salt. Stir for thirty seconds and watch.
  2. Cup B: add half a cup of warm tap water and drop in the ice cube. Watch for two minutes.
  3. Cup C: add one teaspoon of baking soda. Pour in two tablespoons of vinegar and watch.
  4. Cup D: add one teaspoon of baking soda. Pour in two tablespoons of lemon juice and watch.
  5. Touch the outside of each cup. Note whether it feels warmer, cooler or the same as before.
  6. For each cup, write what you saw, which clue appeared, and your verdict in the data table.
  7. Cup A only: with a grown-up, set a spoonful of the salt water in a sunny spot. Check it tomorrow.
CupWhat I mixedWhat I sawClue (if any)Physical or chemical?
Asalt and water
Bice and warm water
Cbaking soda and vinegar
Dbaking soda and lemon juice
PREDICT BEFORE YOU POUR
  • Read the question.
  • Tap your answer.
In cup A, the salt disappears into the water. What do you predict happens if the water is left to dry up?
In cup C, baking soda and vinegar foam up. Which clue is that?
Cup B: the ice melts into the water. Is the water at the end the same substance as the ice?

Reading your results

Cups A and B show physical changes. The salt and the ice changed form, but nothing new was made.

Cups C and D fizz. A gas was produced, which is a strong clue that a chemical reaction happened.

A chemistry demonstration described by the University of California, Davis, confirms it. Baking soda and vinegar react and produce carbon dioxide gas.

Carbon dioxide was not in the cup before. A new substance formed, so cup C was a chemical change.

What the lab showsTrue or false?
Cup A, salt in water, was a physical change.?
Cup C made a gas that was not there before.?
Cup B, melting ice, made a new substance.?
Fizzing is a clue that a chemical reaction happened.?
WHY THIS EXERCISEYour four verdicts are the evidence this week's standard asks you to collect.
Baking soda and vinegar produce ____ dioxide gas.
How many of the four cups showed a chemical change? Type a number.
Cup A can be undone by letting the water ____ away. (dry or burn)
Draw cup C before and after. Label the baking soda, the vinegar, the foam and the gas leaving the cup.

Great lab work. Tomorrow you will read a real reference table and see how properties identify a substance.

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