Worksheet elements compounds mixtures ks3 - Elements, Compounds ... - Free Printable

Let's go through each question on the "Elements, Compounds and Mixtures" worksheet step by step and provide clear explanations for the answers.

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Question 1: Analyzing the Boxes

We are given three boxes with different arrangements of particles:

- Box 1: Five black dots (●) scattered.
- Box 2: Four white double circles (○○).
- Box 3: Three molecules, each made of one black and one white circle (●○).

#### a) Molecules of an element?
- A molecule of an element consists of two or more atoms of the same type bonded together.
- Box 2 shows molecules made of two identical white atoms (○○), so this is a molecule of an element (e.g., O₂ or N₂).
✔ Answer: Box 2

#### b) Molecules of a compound?
- A compound is made of atoms of different elements chemically bonded.
- Box 3 has molecules with one black and one white atom (●○), which means two different types of atoms → compound.
✔ Answer: Box 3

#### c) Atoms of an element?
- Atoms of an element are individual atoms that are not bonded to other atoms.
- Box 1 contains single black dots — these represent individual atoms of an element.
✔ Answer: Box 1

#### d) Do any of the boxes contain a mixture? Explain.
- A mixture contains two or more substances that are not chemically bonded and can be separated physically.
- None of the boxes show different types of molecules or atoms coexisting without bonding.
- Box 1: Only one type of atom → pure element.
- Box 2: Only one type of molecule → pure element (diatomic).
- Box 3: Only one type of molecule → pure compound.
- So, none of the boxes contain a mixture.
✘ Answer: No, none of the boxes contain a mixture because each box contains only one type of particle (atoms or molecules).

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Question 2: Circle the elements and underline the compounds

Substances:
a) Li
b) N₂
c) CO
d) Cl₂
e) S₈
f) NaCl

- Elements: Made of only one type of atom.
- a) Li – lithium (element)
- b) N₂ – nitrogen gas (diatomic element)
- d) Cl₂ – chlorine gas (diatomic element)
- e) S₈ – sulfur (elemental form)
- Compounds: Made of two or more different elements chemically bonded.
- c) CO – carbon monoxide
- f) NaCl – sodium chloride

✔ Answer:
- Circle (elements): a) Li, b) N₂, d) Cl₂, e) S₈
- Underline (compounds): c) CO, f) NaCl

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Question 3: How many elements make up each compound?

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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Question 4: How many atoms in each molecule?

a) N₂ → 2 nitrogen atoms → 2 atoms
b) CO₂ → 1 C + 2 O → 3 atoms
c) O₃ → 3 oxygen atoms → 3 atoms
d) CH₄ → 1 C + 4 H → 5 atoms
e) SO₂ → 1 S + 2 O → 3 atoms
f) C₂H₄ → 2 C + 4 H → 6 atoms

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

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Question 5: Circle the molecule with the most atoms in each pair

a) HCl vs POCI
- HCl: 1 H + 1 Cl = 2 atoms
- POCI: P + O + Cl = 3 atoms
→ Circle POCI

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

c) C₃H₈ vs C₂H₅OH
- C₃H₈: 3 C + 8 H = 11 atoms
- C₂H₅OH: 2 C + 6 H + 1 O = 9 atoms
→ Circle C₃H₈

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

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

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Question 6: What is the formula for each molecule shown below?

Let’s analyze each diagram:

1. Two white circles labeled "O" → O₂
✔ Formula: O₂

2. One carbon (C) between two oxygens (O) → O=C=O → CO₂
✔ Formula: CO₂

3. One oxygen (O) bonded to two hydrogens (H) → H₂O
✔ Formula: H₂O

4. Nitrogen (N) bonded to three hydrogens (H) → NH₃
✔ Formula: NH₃

✔ Final Answers:
- O₂
- CO₂
- H₂O
- NH₃

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✔ Summary of All Answers:

#### 1.
a) Molecules of an element → Box 2
b) Molecules of a compound → Box 3
c) Atoms of an element → Box 1
d) Mixtures? → No, all boxes contain pure substances.

#### 2.
- Circle: Li, N₂, Cl₂, S₈
- 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) POCI
b) HNO₃
c) C₃H₈
d) C₆H₆

#### 6.
- O₂
- CO₂
- H₂O
- NH₃

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This worksheet helps reinforce understanding of:
- Elements vs. compounds vs. mixtures
- Molecular structure
- Counting atoms and identifying chemical formulas

Let me know if you'd like this formatted as a printable answer key!