Year 8 Science · Unit 2 · Lesson 19

Compounds vs Elements, Properties Change

Foundation Worksheet

Name

Date

Class

Learning Goals

Recall that a compound is a new substance with its own properties.
Sort the properties of sodium, chlorine and sodium chloride.
Use data to show that a compound’s properties are not an average of its elements’.

Warm Up

True or false? Fix the false ones

Circle T or F. If a statement is false, correct it on the line.

1. Table salt (NaCl) is safe to eat even though sodium and chlorine are both dangerous on their own.

Correct it:

2. Calcium carbonate (CaCO₃) is found in chalk, limestone and seashells.

Correct it:

3. Chlorine gas must be safe to breathe, because chlorine is found in table salt.

Correct it:

4. The formula H₂O shows that water is made of the elements hydrogen and oxygen.

Correct it:

5. Rust (iron oxide) is as strong as iron metal, because it contains iron.

Correct it:

Your Turn

Sort the properties

Write each property in the box of the substance it describes.

white crystals yellow-green gas soft enough to cut with a knife safe to sprinkle on food reacts violently with water toxic to breathe shiny when freshly cut dissolves in water to make a salty solution

Sodium (Na)

Chlorine (Cl)

Sodium chloride (NaCl)

Go Deeper

Table 1. Water and the two elements it is made from

Substance	Melting point (°C)	Boiling point (°C)	Burns?
Hydrogen (H₂)	−259	−253	Yes, fiercely
Oxygen (O₂)	−218	−183	No, but it helps other things burn
Water (H₂O)	0	100	No

Data: water, hydrogen and oxygen at normal air pressure.

1. Room temperature is about 25 °C. Is each substance in Table 1 a solid, liquid or gas at room temperature?

Recall 2 marks

2. Average the boiling points of hydrogen and oxygen. How far is water’s boiling point from it?

Apply 2 marks

Difference °C

3. A student says, “You can predict a compound’s properties by averaging its elements’ properties.” Does your answer to question 2 support this? Explain.

Analyse 2 marks

Show What You Know

Scenario: a campfire question

Tom puts out a campfire with a bucket of water. His friend says, “Water contains hydrogen, which burns, so water should make the fire bigger.”

4. Explain why the friend is wrong. Use the idea of a compound and evidence from Table 1.

Analyse 4 marks

Answer Key (for teachers)

True or false

1. True. 2. True. 3. False: chlorine gas is toxic; in table salt the chlorine is bonded to sodium, making a new, safe substance. 4. True. 5. False: rust is brittle and weak; as a compound it has different properties from iron metal.

Sort the properties

Sodium: soft enough to cut with a knife, reacts violently with water, shiny when freshly cut. Chlorine: yellow-green gas, toxic to breathe. Sodium chloride: white crystals, safe to sprinkle on food, dissolves in water to make a salty solution.

Go Deeper

1. Hydrogen: gas. Oxygen: gas. Water: liquid (it melts at 0 °C and boils at 100 °C). [2 marks: 1 for both gases, 1 water liquid]

2. (−253 + −183) ÷ 2 = −436 ÷ 2 = −218 °C. Water boils at 100 °C, so the difference is 100 − (−218) = 318 °C. [2 marks: 1 average, 1 difference]

3. No. The average predicts water would be a gas boiling at −218 °C, but water really boils at 100 °C, 318 °C higher. A compound is a new substance, so its properties cannot be found by averaging. [2 marks]

Show What You Know

4. Marking guide (points expected): 1 in water the hydrogen is chemically bonded to oxygen, so it is not free hydrogen gas; 1 water is a compound, a new substance with its own properties, not the properties of hydrogen; 1 evidence: Table 1 shows hydrogen burns fiercely but water does not burn; 1 extra: water is a liquid at room temperature while both its elements are gases, another sign its properties are new (or water cools the fire). [4 marks]

Common error to watch for: in the sort, putting “dissolves in water” under sodium. Sodium reacts violently with water; only the compound dissolves quietly to make salt water.