Element vs Compounds
Which of the following is an element?
A) H₂O
B) NaCl
C) O₂
D) CO₂
C) O₂
Molecule
Two or more atoms bonded together (can be the same element like O₂, or different elements like H₂O).
2NaCl
Elements: Sodium (Na), Chlorine (Cl)
Atoms in 1 NaCl = Na = 1, Cl = 1 → total = 2
Multiply by 2 → Na = 2, Cl = 2
Total atoms = 4
A student notices broken glassware on the floor. They decide to pick it up with their bare hands.
Question: Is this safe or unsafe? What should the student do instead?
Unsafe ❌. The student could get cut. They should tell the teacher and use a broom and dustpan to clean it up.
Which of the following is a solution?
A) Sand and water
B) Salt and water
C) Oil and water
D) Trail mix
B) Salt and water ✅
Salt and water, because the salt dissolves evenly.
Explain the difference between an element and a compound in your own words.
An element is a pure substance made of only one type of atom (like O₂, Fe, or Au). A compound is made of two or more different elements chemically combined (like H₂O or NaCl).
Compound
A substance made of two or more different elements chemically combined (like NaCl, H₂O).
C₆H₁₂O₆ (Glucose)
Elements: C, H, O
Atoms: C = 6, H = 12, O = 6
Total atoms = 24
While heating a test tube, a student points the open end toward a friend.
Why is this dangerous?
It’s unsafe ❌ because hot liquids or gases could shoot out and burn someone. Always point test tubes away from yourself and others.
What is the difference between a mixture and a solution?
A mixture is when substances are combined but not chemically bonded (like trail mix).
A solution is a special kind of mixture where one substance dissolves in another (like salt water).
A student mixes iron (Fe) and sulfur (S). After heating, the two elements react to form iron sulfide (FeS).
Before heating, are iron and sulfur elements or a compound?
After heating, what do they form?
They are elements (Fe and S).
They form the compound FeS (iron sulfide).
Chemical Formula
A shorthand way to represent a substance using element symbols and numbers (like H₂O for water, CO₂ for carbon dioxide).
H₂O
Elements: Hydrogen (H), Oxygen (O)
Atoms: H = 2, O = 1
Total atoms = 3
A student finishes an experiment and pours leftover chemicals down the sink without asking the teacher.
What’s wrong here?
Unsafe ❌. Some chemicals must be disposed of in special waste containers. Always follow teacher directions for cleanup.
A student mixes sugar into tea. The sugar disappears and the tea tastes sweet.
This is a solution because the sugar dissolved in the tea.
Which of the following is a compound?
A) Oxygen gas (O₂)
B) Gold (Au)
C) Water (H₂O)
D) Silver (Ag)
C) Water (H₂O)
Chemical Property
A characteristic that describes a substance’s ability to undergo a chemical change (flammability, reactivity).
3Mg(OH)₂
Inside (OH)₂ → O = 2, H = 2
Add Mg = 1 → Mg = 1, O = 2, H = 2
Multiply by 3 → Mg = 3, O = 6, H = 6
Total atoms = 15
During an experiment, a student notices a chemical splashes onto their skin. They quickly rinse with water and tell the teacher.
Is this the correct response?
yes ✅. Rinsing immediately and notifying the teacher is the safest action.
Which of the following is NOT a mixture?
A) Air
B) Lemonade
C) Pure gold (Au)
D) Soil
C) Pure gold (Au) ✅
Carbon dioxide (CO₂) is made of carbon and oxygen atoms.
How many elements make up carbon dioxide?
Is carbon dioxide an element or a compound?
2 elements (carbon and oxygen).
A compound.
Physical Property
A characteristic of a substance that can be observed without changing its identity (color, shape, density, melting point).
CO₂
C = 1
O = 2
Total atoms = 3
A student enters the lab, leaves their backpack in the walkway, and starts working without wearing goggles.
What are two safety problems here?
Backpack in the walkway = tripping hazard.
No goggles = risk of eye injury.
They should move belongings out of the way and always wear goggles.
What does it mean if a substance is “soluble”? Give an example.
Soluble means it can dissolve in another substance. Example: sugar dissolves in water.