Materials Around Us — Tests and Measurements (2 of 3) · Answer guide

Lesson 1 · Test what can be seen through a sample

Classify actual samples as transparent, translucent or opaque using a fair viewing comparison.

1. A printed word is clearly readable through a sample. The sample is…

   Answer: Transparent. A clear detailed view is the key observation for transparency.

2. Light comes through a sheet, but a letter behind it looks blurred. It is…

   Answer: Translucent. Translucent materials admit light but blur the view of objects behind them.

3. To compare two sheets fairly, which should stay the same?

   Answer: The letter, lighting and viewing arrangement. Changing one sample while holding the viewing conditions steady makes the comparison interpretable.

Discuss: Use supplied viewing observations or compare safe clean plastic, tracing paper and card. Keep the printed symbol and lighting unchanged. Do not look at the Sun or a strong light source.

Teaching note: All glass is not necessarily transparent: frosted glass can be translucent. Thin and thick samples, or single and stacked layers, can behave differently. Open responses need discussion; they are not automatically marked.

Explain or demonstrate

Classify three supplied or safe actual samples. Record whether light passes through each sample and whether the word can be seen clearly; do not classify from word detail alone.

Evidence to include:

• Same viewing setup
• Light transmission and word detail both recorded
• One stated limitation

Review guide:

• 1 point: consistent setup
• 1 point: transparent clear view, translucent transmitted light without clear view, and opaque blocking of light distinguished
• 1 point: conclusion limited to actual samples

Example response: The word is sharp through A. B transmits light but the word is blurred. C blocks light passing through it and gives no view through it. I classify A as transparent, B as translucent and C as opaque. I tested only one thickness of each.

Lesson 2 · Compare how materials mix with water

Use observations to distinguish dissolving, suspended material and a separate liquid layer.

1. Small salt grains disappear from view after stirring into enough water. The best conclusion is…

   Answer: The salt dissolved and remains in the mixture. Dissolving disperses the material in water; it does not make the substance cease to exist.

2. Sand is stirred into water and later settles. This supports…

   Answer: Sand remained separate rather than dissolved. Settled grains are evidence of a separate solid phase in this test.

3. Why wait after stirring oil and water?

   Answer: Separate layers may re-form after temporary droplets. Stirring can disperse oil drops temporarily without producing lasting mixing.

Discuss: Write a prediction-observation-conclusion table from the supplied records. Add a control condition and one reason not to infer that an invisible dissolved substance has gone away.

Teaching note: Floating and sinking are not tests for solubility. A material may float, sink or be suspended while still remaining separate. A clear solution still contains its dissolved material. Open responses need discussion; they are not automatically marked.

Explain or demonstrate

Use three mixing records to classify outcomes and separate the observations from conclusions.

Evidence to include:

• A prediction for each known material
• Observed grains or layers described
• Conclusion limited to the recorded conditions

Review guide:

• 1 point: controls named
• 1 point: dissolving distinguished from settling or layering
• 1 point: no tasting or disappearance-of-matter claim

Example response: After the same stirring time, salt grains are not visible, sand settles and oil forms a layer. I conclude the small salt sample dissolved, while the other samples remained separate in this test.

Lesson 3 · Measure and compare mass

Read mass values, subtract a container mass and avoid judging mass from size alone.

1. A cup is 15 g empty and 85 g with a sample. What is the sample mass?

   Answer: 70 g. Subtract the holder: 85 − 15 = 70 g.

2. Which unit is the SI unit of mass?

   Answer: kg. The kilogram is the SI mass unit. Millilitres measure volume and centimetres measure length.

3. Two samples occupy equal volumes. Their masses…

   Answer: Can differ because the materials can differ. Equal volume is not sufficient evidence of equal mass for different materials.

Discuss: Make a two-row table with empty-container and filled-container readings. Calculate sample masses, show an addition check and explain why comparing the totals alone can be unfair if the containers differ.

Teaching note: A larger-looking object need not have more mass. Common speech often calls a balance reading “weight”, but mass and force are different quantities; formal weight is studied later. Open responses need discussion; they are not automatically marked.

Explain or demonstrate

For supplied pairs of empty and full readings, calculate two sample masses and justify the comparison.

Evidence to include:

• Number and unit on each reading
• Correct subtraction per holder
• Comparison based on sample masses

Review guide:

• 1 point: container accounted for
• 1 point: arithmetic and units correct
• 1 point: comparison justified

Example response: Holder A is 12 g and total A is 72 g, so sample A is 60 g. Holder B is 20 g and total B is 75 g, so B is 55 g. A contains more mass even though its filled total is smaller.

Final check · Set 1

Try one set after the lessons. Use the other set after reviewing any errors.

1. A display cover must let people read the notice behind it. Which tested property is needed?

   Transparency. Reading detail through a cover requires transparency.

2. Why is “every glass sample is transparent” too strong?

   Surface treatment such as frosting can blur the view. Different treatments and thicknesses affect what is seen through a sample.

3. An undissolved grain sinks. What does sinking alone tell you?

   It moved downward; solubility needs separate evidence. Motion in water does not by itself establish dissolving.

4. Which statement about gases in water is correct?

   Some gases, such as oxygen, can dissolve. Dissolved oxygen is one example; this does not imply all gases behave alike.

5. Convert 2 kg to grams.

   2000 g. Each kilogram contains 1000 grams, so 2 kg = 2000 g.

6. A 30 g container with a sample reads 120 g. Which check verifies the sample calculation?

   90 g + 30 g = 120 g. The calculated sample mass plus the holder should recover the total reading.

Final check · Set 2

Try one set after the lessons. Use the other set after reviewing any errors.

1. A learner sees a letter around a card’s edge. Does this prove the card is transparent?

   No; the view must pass through the card. A view around an edge does not test transmission through the card.

2. A single sheet is translucent; a thick stack blocks the view. What is a sound record?

   Classify each tested arrangement separately. Number of layers is part of the sample arrangement and can change the result.

3. To compare two solids, which change would make interpretation harder?

   Using very different water amounts and stirring times. Several simultaneous changes make it unclear why the results differ.

4. A cup of water has dissolved some sugar. Must any extra amount dissolve?

   No; a fixed amount of water need not dissolve unlimited sugar. The fact that some dissolves does not establish an unlimited capacity under the same conditions.

5. Two filled containers have different empty masses. What is a fair way to compare their contents?

   Subtract each container’s own empty mass. Accounting for each holder isolates the mass of each sample.

6. A big foam block and small metal piece are compared by sight. Which claim is justified without mass readings?

   Their masses still need evidence. Size alone cannot determine a mass ranking across different materials.