The Four-Station Change Lab
Fifth graders at Pine Ridge Elementary rotate through four stations during a 40-minute lab. At Station A, a 25-gram ice cube melts in an open cup; the water left behind still has a mass of 25 grams. At Station B, a sugar cube is stirred into 100 mL of water until it disappears; the water tastes sweet and the total mass is unchanged, and the teacher notes the sugar could be recovered by leaving the cup in the sun. At Station C, a slice of bread is toasted until it browns and gives off a burnt smell; no one can turn it back into bread. At Station D, 10 mL of vinegar and 5 grams of baking soda are sealed in a zipper bag; the bag puffs up, fizzes, and feels slightly cool. Each group must write a claim for every station. During share-out, Marcus says, 'Melting and dissolving are the same thing, because in both the solid disappears.'
1. For each station, write a claim (physical change or chemical change) and give two pieces of evidence that support the claim.
Read the worked answer
Station A - physical change. The ice changed from solid to liquid but is still water. Evidence: the mass stayed at 25 grams, and the water could be frozen back into ice.
Station B - physical change. The sugar spread through the water but did not become a new substance. Evidence: the water tastes sweet, and the sugar can be recovered by evaporating the water. Nothing new was produced.
Station C - chemical change. Toasting makes a new substance. Evidence: the bread browns, releases a burnt smell, and cannot be turned back into bread. A change you cannot reverse is a clue that new matter formed.
Station D - chemical change. Evidence: a gas is produced (the bag puffs), the mixture cools, and a new smell appears. A gas that was not there before is strong evidence that a new substance formed.
The useful question is not 'did it look different?' but 'is it still the same substance, and can I get it back?'
2. Respond to Marcus's statement. Explain the difference between melting and dissolving, and describe one observation that would settle the question for a classmate.
Read the worked answer
Marcus is noticing a real pattern - the solid is no longer visible in both cases - but 'disappearing' is not the same as 'becoming something new.'
In melting, a solid becomes a liquid, but it is the same substance: ice becomes water, and cooling it returns ice. No second material is involved.
In dissolving, a solid breaks into pieces too small to see and spreads through a liquid. The sugar is still sugar; it is just mixed evenly with water. Evaporating the water leaves sugar crystals behind.
To settle it, the class could set both cups in a warm windowsill for two days. The melted ice will evaporate and leave nothing (or refreeze into ice if placed in a freezer), while the sugar water will leave a sticky film of sugar crystals. Recovering the original substance proves no new substance formed, so both are physical changes.
3. At Station E, a nail sits in salt water for a week. It comes out coated in orange-brown flakes, the water is cloudy, and the nail's mass has increased by 0.4 grams. Write a claim and explain why the mass increase does not show that a physical change happened.
Read the worked answer
Claim: this is a chemical change. The orange-brown coating is rust, a new substance that is not iron and not salt water. Evidence: the color and texture are new, the water turned cloudy, and the change cannot be undone by drying or cooling the nail.
The added 0.4 grams can confuse students because mass is conserved in physical changes like melting. But mass is conserved in chemical changes too - it just comes from the environment. Atoms of oxygen from the air and water joined with iron atoms to form rust, so the nail-plus-rust system now includes atoms it did not have before.
A true physical change would leave the same substance, only reshaped or in a new state. Rust is a different substance with different properties (it flakes, it is brittle, it does not conduct or hold together like iron), so the mass gain points to a reaction, not a change of state.