Monday
Monday: Two Kinds of Change
Rocket sets three clear cups on the Bench before anyone else sits down. Ice in the first, sugar water in the second, and a spoon of baking soda in the third.
"Three changes," he says. "Ice melts, sugar vanishes, and this one is going to fizz."
Raven leans in with the Bench Log open. "What do you notice? The melted ice is still water. The sugar is still sugar, just hidden."
Rocket pours vinegar on the baking soda. It foams up and over the rim. "And this one?" he says. "Is this still baking soda?"
Nova hovers above the foam, her light flickering. "Some changes keep the same stuff," she says. "Some make stuff that was not there before. Which is this?"
Raven writes a new heading in the log: physical or chemical?
Physical properties
A physical property is a characteristic of a substance that can be observed or measured without changing the identity of the substance.
Color, hardness, solubility, density, melting point and boiling point are all physical properties. Silver is shiny and conducts electricity. Salt is dull and brittle.
Melting point and boiling point are especially useful identifiers. Two samples with the same melting point and boiling point are very likely the same substance.
This week you will use properties as evidence. If the properties change, something new may have formed.
Physical change
A physical change is a change to a sample of matter in which some properties change. The identity of the matter does not change.
As an ice cube melts, its shape changes and it gains the ability to flow. Its composition does not change. It is still water.
Changes of state, such as melting, freezing, boiling and condensing, are all physical changes. So is dissolving. Salt dissolved in water can be regained by boiling the water away.
Some physical changes cannot be undone. Grinding wood into sawdust is a physical change, but you cannot put the sawdust back together.
Chemical change
A chemical change produces different materials than the ones you started with. Another name for a chemical change is a chemical reaction.
A chemical reaction is a process in which one or more substances are changed into one or more new substances.
The substances you start with are the reactants. The new substances are the products. Chemists write it as reactants, an arrow, then products.
Baking bread is a chemical change. Flour, sugar, yeast and water become bread, a substance with different characteristics from the ingredients.
- Read the question.
- Tap your answer.
| Statement | True or false? |
|---|---|
| A physical property can be measured without changing what the substance is. | ? |
| Melting ice changes the identity of the water. | ? |
| A chemical change produces different materials than the ones you started with. | ? |
| Every physical change can be undone. | ? |
| Melting point and boiling point help identify a substance. | ? |
Check it at the end of the lesson. Describe what changed and what stayed the same.
Write "physical change" or "chemical change" beside your note, with one reason.
Only kitchen materials are used: water, salt, sugar, baking soda, vinegar and lemon juice. No cleaners of any kind.
Definitions this week come from the CK-12 chemistry text hosted by Chemistry LibreTexts, at chem.libretexts.org.
Good start. Tomorrow you will learn the four clues chemists watch for when something new may be forming.