Elements, Compounds and Mixtures worksheet for KS3 Science.

Here is the complete solution to the “Elements, Compounds and Mixtures” worksheet with detailed explanations.

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1. Which of the boxes contains:

*(Note: The diagrams show particles — black circles and white circles represent different types of atoms.)*

a) Molecules of an element?
→ Box 2
*Explanation:* In Box 2, all particles are identical molecules made of two white atoms bonded together (e.g., O₂ or N₂). Since both atoms in each molecule are the same type, this represents a diatomic element.

b) Molecules of a compound?
→ Box 3
*Explanation:* In Box 3, each molecule consists of one black atom and two white atoms bonded together (like CO₂ or H₂O). Since the molecules contain more than one type of atom chemically bonded, they are compounds.

c) Atoms of an element?
→ Box 1
*Explanation:* In Box 1, there are individual black atoms floating around, not bonded to anything. These are single atoms of an element (like noble gases such as He or Ne).

d) Do any of the boxes contain a mixture? Explain your answer.
→ No, none of the boxes contain a mixture.
*Explanation:* A mixture would contain two or more different substances (elements or compounds) physically mixed but not chemically bonded. Each box contains only one type of particle:
- Box 1: Only atoms of one element.
- Box 2: Only molecules of one element.
- Box 3: Only molecules of one compound.
Since each box has only one substance, none are mixtures.

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2. Look at the following substances. Circle the elements and underline the compounds.

Remember:
- Element: Made of only one type of atom (e.g., Li, N₂, Cl₂, S₈).
- Compound: Made of two or more different types of atoms chemically bonded (e.g., CO, NaCl).

a) Li → Circle (element — lithium metal)
b) N₂ → Circle (element — diatomic nitrogen gas)
c) CO → Underline (compound — carbon monoxide)
d) Cl₂ → Circle (element — diatomic chlorine gas)
e) S₈ → Circle (element — sulfur molecule made of 8 sulfur atoms)
f) NaCl → Underline (compound — sodium chloride)

✔ Final Answer:
- Circled (Elements): Li, N₂, Cl₂, S₈
- Underlined (Compounds): CO, NaCl

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3. How many elements make up each compound?

We count the *different types of elements* (symbols) in each formula.

a) HCl → Hydrogen + Chlorine → 2 elements
b) CaCO₃ → Calcium + Carbon + Oxygen → 3 elements
c) HNO₃ → Hydrogen + Nitrogen + Oxygen → 3 elements
d) H₂SO₄ → Hydrogen + Sulfur + Oxygen → 3 elements
e) CH₃OH → Carbon + Hydrogen + Oxygen → 3 elements

✔ Answers:
a) 2
b) 3
c) 3
d) 3
e) 3

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4. How many atoms are there in each of these molecules?

Count total atoms in the chemical formula.

a) N₂ → 2 nitrogen atoms → 2 atoms
b) CO₂ → 1 carbon + 2 oxygen = 3 atoms
c) O₃ → 3 oxygen atoms → 3 atoms
d) CH₄ → 1 carbon + 4 hydrogen = 5 atoms
e) SO₂ → 1 sulfur + 2 oxygen = 3 atoms
f) C₂H₄ → 2 carbon + 4 hydrogen = 6 atoms

✔ Answers:
a) 2
b) 3
c) 3
d) 5
e) 3
f) 6

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5. Circle the molecule with the most atoms, in each pair:

Compare total number of atoms in each formula.

a) HCl or POCl
- HCl: 1 H + 1 Cl = 2 atoms
- POCl: 1 P + 1 O + 1 Cl = 3 atoms → ✔ Circle POCl

b) NH₃ or HNO₃
- NH₃: 1 N + 3 H = 4 atoms
- HNO₃: 1 H + 1 N + 3 O = 5 atoms → ✔ Circle HNO₃

c) C₃H₈ or C₂H₅OH
- C₃H₈: 3 C + 8 H = 11 atoms
- C₂H₅OH (same as C₂H₆O): 2 C + 6 H + 1 O = 9 atoms → ✔ Circle C₃H₈

d) CH₃COCH₃ or C₆H₆
- CH₃COCH₃ (acetone, C₃H₆O): 3 C + 6 H + 1 O = 10 atoms
- C₆H₆ (benzene): 6 C + 6 H = 12 atoms → ✔ Circle C₆H₆

✔ Answers:
a) POCl
b) HNO₃
c) C₃H₈
d) C₆H₆

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6. What is the formula for each molecule shown below?

Look at the diagrams:

First molecule: Two oxygen atoms bonded → O₂

Second molecule: One carbon atom bonded to two oxygen atoms → CO₂

Third molecule: One oxygen atom bonded to two hydrogen atoms → H₂O

Fourth molecule: One nitrogen atom bonded to three hydrogen atoms → NH₃

✔ Answers:
- O₂
- CO₂
- H₂O
- NH₃

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✔ FINAL ANSWER SUMMARY:

1.
a) Box 2
b) Box 3
c) Box 1
d) No — each box contains only one pure substance.

2.
Elements (circle): Li, N₂, Cl₂, S₈
Compounds (underline): CO, NaCl

3.
a) 2
b) 3
c) 3
d) 3
e) 3

4.
a) 2
b) 3
c) 3
d) 5
e) 3
f) 6

5.
a) POCl
b) HNO₃
c) C₃H₈
d) C₆H₆

6.
O₂, CO₂, H₂O, NH₃

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