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Deciding by evidence whether a new substance was made, which is the claim every chemical equation rests on.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
By the end of this lesson you will be able to say whether a described change was physical or chemical, and — which matters more — to name the observation that settles it. You will be able to handle the cases that look like the opposite of what they are: dissolving, subliming, and melting against burning.
You can place a sample as an element, a compound or a mixture, and you know that the word doing the work is joined. This lesson asks the next question: when something happens to a sample, did the substances in it change into different substances, or are the same substances still there in a different arrangement?
| Term | What it means |
|---|---|
| Physical change | A change of state, shape or arrangement that leaves the substance itself the same. |
| Chemical change | A reaction: one or more substances turn into different substances. |
| Reactant | A substance present at the start of a reaction. |
| Product | A substance present at the end of a reaction that was not there before. |
| Sublimation | A solid turning straight into a gas; a physical change. |
| Physical means | Heating, cooling, filtering or evaporating, with no reaction involved. |
Six benches, and all you are given is what an observer could see. No list of rules yet.
| Bench | What was seen |
|---|---|
| 1 | white crystals stirred into water until the water looked empty |
| 2 | a silver-gray ribbon burned with a white flame, leaving a white powder |
| 3 | wax held over a flame until it ran, then left to set |
| 4 | two clear solutions poured together; the mixture turned cloudy yellow |
| 5 | a purple solid on a warm plate gave off purple vapor, which settled as purple crystals on a cold lid |
| 6 | a nail left three weeks on a damp sill, now orange and flaky |
Sort them into a different substance is there now and the same substance is still there, naming the observation you used each time.
Three are built to catch you. Bench 1 disappeared — is that the same as being destroyed? Bench 5 is the most dramatic thing here, and what settles it is on the cold lid. Bench 6 had nothing to watch at all.
Then: what single test would settle all six, without your having to remember which observations count?
The question is: is there a substance there afterwards that was not there before?
If yes, the change was chemical. If no — if the same substances are there, in a different state or a different shape or spread through a liquid — the change was physical.
That sounds easy and is not, because you usually cannot see substances. So here is the list of observations that answer the question for you. Any one of them is good evidence of a chemical change:
And one question beats all of them for reliability:
None of the five signs is perfect on its own. A gas comes off when water boils. A color changes when a wet shirt dries. The temperature drops when ice melts in your hand. That is why the list is a list, and why the last question is the one to reach for when the list disagrees with itself.
Another way: picture
Think of a box of lettered tiles. Rearranging the tiles from a heap into a line is physical: the same tiles, moved. Melting them so they run together and set as one lump is still physical, because breaking the lump apart gives the tiles back. Now imagine the tiles are letters and you can glue them into words: CAT becomes ACT becomes a different word made of exactly the same letters. That is a chemical change — every atom is still there, nothing was created or destroyed, and what you have is not what you had.
Another way: steps
To judge any change:
| What happens | Verdict | What settles it |
|---|---|---|
| salt dissolves in water | physical | evaporate the water and the same salt, the same mass of it, is in the dish |
| candle wax melts | physical | it sets again lower down the candle and is the same wax |
| candle wax burns | chemical | the wax is gone; carbon dioxide and water vapor leave the flame and the candle is shorter for good |
| dry ice disappears from a bench | physical | what leaves is carbon dioxide gas — the same substance the solid was |
| iodine turns straight to purple vapor | physical | the vapor settles as iodine crystals again on anything cold |
| blue copper sulfate crystals turn white on heating | chemical | water is driven out of the crystal, so the white solid is a different substance from the blue one |
The last one is the interesting one, and it is worth a paragraph. Blue copper sulfate crystals have water molecules built into the crystal — the dot in $\mathrm{CuSO_4 \cdot 5H_2O}$. Heating drives that water out, and what is left, $\mathrm{CuSO_4}$, is a different substance: different color, different formula, different mass. Drip water back on and it goes blue again and gets warm, which is itself a chemical change in the other direction.
So reversible and physical are not the same word. Plenty of chemical changes are reversible. What makes a change physical is that it can be reversed by physical means — by heating, cooling, or taking a solvent away — and not by another reaction.
Name the starting substances. Write down what was there before anything happened, including anything the sample was in contact with, such as the air around a burning ribbon.
Name what is there afterwards. As far as the description allows, list the substances present at the end. A white powder, a gas, a new solid in a clear liquid, a new smell all count as candidates.
Compare the two lists. If something on the second list is not on the first, a new substance was made and the change is chemical.
If the lists look the same, try the reversal test. Ask whether cooling, warming, filtering or evaporating would bring back exactly what you started with. If it would, the change is physical.
Name the observation. Write the one observation that settled the verdict beside it.
Check the work. Does the verdict match the answer to was a new substance made? A row that says chemical and no, or physical and yes, has gone wrong. Did the observation you named concern the substance, or only the event? And would the same verdict follow if the change had happened quietly and slowly?
Naming the starting substances is needed because a new substance can only be recognized against a list of old ones. The white powder on the tongs is only evidence because it was not on the list before the flame.
The reversal test is allowed because physical changes leave the substances themselves untouched. If nothing was changed into anything else, then reversing the conditions — cooling what was heated, evaporating what was dissolved into — returns the same substances, by the same mass.
Insisting on physical means is needed because many chemical changes are reversible too, but only by another reaction. Without the word physical, the copper sulfate pair would be wrongly called a physical change.
Naming the observation is needed because the verdict is a claim, and a claim with no evidence behind it cannot be checked by anyone else. It also catches the commonest error, which is to reach the verdict from how dramatic the change looked.
When magnesium ribbon burns, the white powder left behind weighs more than the ribbon did. A strip of 48 mg leaves 80 mg of powder: the extra 32 mg is oxygen from the air, now joined to the magnesium as magnesium oxide. A gain in the solid's mass, with nothing added by hand, is a strong hint that something from the air has reacted.
But mass alone cannot decide the verdict, because mass is conserved in physical and chemical changes alike. Melt 100 g of wax and you have 100 g of wax. Burn 100 g of wax in a sealed flask and the flask still holds 100 g of material — carbon dioxide and water in place of wax. The total never changes; what changes is the identity of what you have. So a change in the mass of one part of a system tells you material moved, and the question is still whether it moved as the same substance or became a new one.
A chemical equation says that these reactants became those products. Writing one for a physical change is not a small error: it is a claim that a substance turned into a different substance when it did not.
The mass bookkeeping is the same in both cases, which is why it is no help in telling them apart. Nothing is created and nothing is destroyed either way — that is the next unit's subject, and it is true of physical and chemical changes alike.
What is different is the identity of what you have. That is the only thing this lesson asks you to decide, and every question in it comes back to it.
A bakery in Pittsburgh runs both kinds of change every morning, and the head baker has to know which is which because they are handled differently. Butter softening on the counter, chocolate melting over a water bath and sugar dissolving in warm milk are all physical: chill the butter and it sets, cool the chocolate and it hardens, boil off the milk and the sugar is there. None of them has made anything new, so a mistake in timing can usually be undone.
Baking is different. In the oven, baking soda breaks down to give carbon dioxide gas that lifts the dough, sugars on the crust react with proteins in the browning reactions that give bread its color and flavor, and starch sets into a new structure. A loaf that has been baked cannot be unbaked by cooling it.
The bakery's figures show the difference too. A 900 g batch of dough comes out of the oven as about 800 g of bread: the missing 100 g is mostly water that left as steam, a physical change happening alongside the chemical ones. The baker's rule of thumb — if cooling it would put it back, you can fix it; if not, you start again — is the reversal test this lesson uses.
Highway engineers on the Gulf Coast treat rust as a chemical change, because it is one: iron reacts with oxygen and water to make iron oxide, a new substance that flakes away and takes the metal's strength with it. Repainting a bridge is physical, but removing rust needs blasting or chemical treatment, not just drying the steel.
Drama is taken as evidence. A bright flame, a loud bang or a strong smell feels like a reaction and often is one — but a kettle boiling is dramatic too, and a rusting gate is not dramatic at all and is chemical. How exciting a change looks is not on the list.
Dissolving is taken as reacting. The salt disappears, so something must have happened to it. Nothing did: it is dispersed through the water and it comes back unchanged when the water goes. Salt water is a mixture.
Reversible is taken to mean physical. The blue-and-white copper sulfate pair is reversible and both directions are chemical. The test is whether physical means will reverse it.
A missing sign is taken as proof. "No gas came off, so nothing happened." Iron rusting gives off no gas at all and takes months. Absence of a sign is not evidence.
Change of state is taken as chemical because the substance looks so different. Steam looks nothing like ice. Both are water, and both give the other back on cooling or warming. A change of state is physical every single time.
Name what was there before.
$\text{A block of solid carbon dioxide.}$
You cannot spot a new substance without naming the old ones.
Name what is there afterwards.
$\text{Nothing visible on the bench; a cold mist nearby.}$
Describe the end of the change before judging it.
Identify what left the block.
$\text{Carbon dioxide gas.}$
The same substance the block was, in a different state.
Apply the reversal test.
$\text{Cooling the gas enough gives the solid back.}$
No reaction is needed to undo it.
Give the verdict with its observation.
$\text{Physical: sublimation of carbon dioxide.}$
Going straight from solid to gas is still a change of state.
Name what was there before.
$\text{Magnesium ribbon and the air around it.}$
The air counts, because oxygen can join the solid.
Name what is there afterwards.
$\text{A white powder on the tongs; the ribbon is gone.}$
White powder was not on the first list.
Weigh the evidence of mass.
$80 - 48 = 32$ mg gained by a 48 mg strip.
Something from the air is now part of the solid.
Apply the reversal test.
$\text{Cooling the powder gives no ribbon back.}$
Physical means cannot undo the change.
Name the new substance.
$\text{Magnesium oxide.}$
A substance you can name is the strongest sign.
Give the verdict with its observation.
$\text{Chemical: a white oxide where silver metal was.}$
The two tests agree.
Name what was there before.
Blue crystals, $\mathrm{CuSO_4 \cdot 5H_2O}$, 250 mg.
The formula shows water built into the crystal.
Name what is there afterwards.
$\text{A white powder, 160 mg, and steam leaving the dish.}$
Both the color and the mass changed.
Find the mass that left.
$250 - 160 = 90$ mg of water.
Mass before less mass after is what left.
Identify the white solid.
$\mathrm{CuSO_4}$, a different substance.
Different color, formula and mass from the blue crystal.
Apply the reversal test.
$\text{Cooling alone does not bring the blue back.}$
Only adding water, a reaction, does.
Check the reverse direction.
$\text{Water dripped on turns it blue and warm.}$
A temperature change of its own is a sign of reaction.
Give the verdict with its observation.
$\text{Chemical, in both directions.}$
Reversible is not the same as physical.
Name both ends of the change.
$\text{Milk before; sharp-tasting, curdled milk after.}$
Name both ends before judging.
Look for a new substance.
$\text{An acid that was not there before.}$
A substance you can name is evidence of reaction.
Apply the reversal test.
A class watched a clean iron nail left standing in copper sulfate solution and wrote down four things. Mark the two that are evidence that a new substance was made.
This task has no paper form; do it on a device.
Complete the worked solution: a strip of magnesium ribbon of mass $48$ mg is held in a flame and leaves $80$ mg of white powder. Find the mass the solid gained, and how much magnesium ribbon cooling the powder gives back.
Find the mass gained.
$(\text{powder}) - (\text{ribbon}) =$ o mg
Oxygen from the air is now part of the solid.
Find what cooling gives back.
$\text{ribbon recovered on cooling} =$ r mg
The white powder stays white powder however cold it gets.
Give the verdict.
$\text{A new substance, magnesium oxide: chemical.}$
Physical means will not undo it, and the solid's identity changed.
Match each change to the observation that settles what kind of change it is.
| the orange solid flakes off and is not iron: it outweighs the iron it came from | a brown crust forms and a smell of baking appears, neither of which dough has | the wax is gone, and what leaves the flame is carbon dioxide and water vapor | |
|---|---|---|---|
| an iron gate going orange in the rain | |||
| a tray of bread dough baking in an oven | |||
| candle wax burning at the wick |
A spoonful of salt is stirred into a beaker of water and disappears completely. Which statement gives the right verdict for the right reason?
Three changes are described. For each one, say whether it is a physical or a chemical change, and whether a new substance was made.
| physical or chemical? | was a new substance made? | |
|---|---|---|
| purple iodine crystals turning straight to vapor on a warm plate | ||
| milk souring in a warm kitchen | ||
| candle wax running down the side of a candle |
A high school lab in Denver heats $1500$ mg of blue copper sulfate crystals in a dish until they turn white. The white solid left weighs $960$ mg. How many milligrams of water were driven out of the crystals?
The answer: a.
Three things happen in a working day at a forge and a bakery. Nothing here is a laboratory, and the question is the same one it always was.
| physical or chemical? | was a new substance made? | |
|---|---|---|
| a bar of steel glowing red as the smith heats it | ||
| the coke in the forge burning down to ash | ||
| the blue-black scale flaking off the bar as it is hammered |
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
Three changes are described. For each one, say whether it is a physical or a chemical change, and whether a new substance was made.
| physical or chemical? | was a new substance made? | |
|---|---|---|
| purple iodine crystals turning straight to vapor on a warm plate | ||
| a strip of magnesium ribbon burning with a white flame | ||
| salt disappearing as it is stirred into water |
You can judge a change by asking whether a new substance is there afterwards, and back the verdict with an observation rather than an impression. Say out loud why melting wax and burning wax are different kinds of change, when both happen at the same candle. Next: how a mixture is taken apart again, and why each technique works.
17. Your turn: milk left out in a warm kitchen goes sour. Physical or chemical, and what settles it?, step 3
$\text{Chilling does not unsour it: chemical.}$
Physical means will not reverse it.