An element contains only one kind of atom and cannot be broken down chemically. There are about ninety natural ones.
Element A substance made of only one kind of atom.
Step 1: Something to Notice
Watch first. The explanation comes later.
Two of the most dangerous substances in a school laboratory combine to make the substance you put on chips.
Where did the properties of sodium and chlorine go?
Step 2: Find Out
Isolate the method that separates a sample. What works tells you what kind of matter you have.
Step 1 — Predict
Sodium chloride resembles neither sodium nor chlorine. Why?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 3: So Here Is Why
Now the explanation, after you have seen it happen.
A compound is a new substance
Elements chemically bonded in a fixed ratio, with properties of its own that are not inherited from the parts.
Compound Elements chemically bonded in a fixed ratio.
A mixture keeps its components
No chemical bonding, no fixed ratio, and each component still behaves as it did.
Mixture Substances combined without chemical bonding, in any proportion.
Which is why mixtures separate physically
Filtering, evaporating or a magnet each exploits a property one component still has.
Physical and chemical change
A physical change alters form; a chemical change makes a new substance. A new substance is the test.
One question sorts almost everything
Can it be separated by physical means? If yes it is a mixture; if no it is a pure substance. Then ask whether it can be broken down chemically — if no, it is an element; if yes, a compound.
A mixture is not a compound
Iron filings mixed with sulfur can be separated with a magnet and retain their own properties. Heated together they form iron sulfide, which has new properties and cannot be separated magnetically.
Step 4: A Common Mistake
Lots of people think
“A compound is a mixture of its elements, so a compound should behave a bit like each of them.”
Step 5: Why Alloys Are Mixtures
The same idea somewhere new.
Steel is iron with carbon dissolved in it, and brass is copper with zinc — both are mixtures rather than compounds, because the proportions vary continuously and are chosen for the job. That variability is exactly what makes alloys useful: a steel can be tuned to be hard or springy by adjusting a ratio, which a compound with fixed composition can never be.
Step 6: Think It Through
Practice makes it stick.
Fixed or Not
Problem 1 of 2
Salt water can be 1% salt or 10%. Sodium chloride is always 39% sodium. What does that show?
Steel
Problem 2 of 2
Steel is iron with carbon dissolved in it, in proportions chosen for the job. Compound or mixture?
Sorting Matter
1 of 5
Match each term to its definition.
Tap a card on the left to start.
2 of 5
Why can a mixture be separated physically?
3 of 5
What is the test for a chemical change?
4 of 5
Which of these are chemical changes?
5 of 5
Does sodium chloride share properties with sodium?
Step 7: Quick Check
Show what you know.
Question 1 of 1
Why does table salt resemble neither sodium nor chlorine?
Step 8: Explain It in Writing
Claim, evidence, then reasoning.
The question
Explain why sodium chloride resembles neither sodium nor chlorine, and how you could tell a compound from a mixture experimentally.
Fill in all three boxes. The reasoning box is the one that matters most.
What You Found Out
- An element has one kind of atom; a compound is elements bonded in a fixed ratio.
- A compound has its own properties, unrelated to those of its elements.
- A mixture keeps its components’ properties, which is why physical separation works.
- A chemical change makes a new substance; a physical change only alters form.