Lesson focus: How can we compare a material that both flows and stretches?
Use controlled measurements to compare polymerA very long molecule made by joining lots of small molecules together in a chain. materials.
Build on earlier learning
Scientific investigations: independent, dependent and control variablesUse control variables to make a materials comparison fair.
Practical
Investigating slime
- Mix PVA glue solution with borax solution and stir.
- Change one variable, such as the volume of borax solution.
- Measure how far the slime flows in 30 seconds on a tilted board.
Record: A table and a line graph of borax volume against flow distance.
Safety: Wear gloves and eye protection; borax solution is an irritant.
You will be successful when you can…
- Define a measurable flow outcome.
- Identify control variables.
- Calculate and compare means.
- Explain why rankings can depend on time.
Words that carry the science
- viscoelastic
- Showing both flow-like and elastic behaviour over the timescale observed.
- spread diameter
- Width of a sample after spreading for a stated time.
- control variable
- A factor kept the same to make a comparison fair.
- mean
- Sum of values divided by the number of values.
- repeatability
- Closeness of repeated results obtained using the same method.
Measure flow under controlled conditions
01Turn a description into a measurement
“Runny” is vague. spread diameterHow wide a sample spreads out after a set time. after 60 seconds under a fixed load gives a quantity with units that can be repeated and compared.
02Control the conditions
Use equal mass or volume, starting shape, plate surface, load, temperature and time. Temperature and time matter because polymer-chain movement can change with both.
03Measure change across time when needed
One time point compares samples only at that moment. Repeated time points reveal rates and whether the samples change ranking.
Slime: cross-linking polymer chains
PVA glue is made of long polymer chains that slide past each other, so it flows. Borax makes cross-links that tie the chains together. More cross-links: the chains cannot slide so easily and the slime is stiffer.
Blue: PVA polymer chains. Red: borax cross-links.
Controls
Numbers are illustrative. A fair test keeps the volume of PVA and the temperature the same and changes only the borax.
Calculate two means
A: 4.0, 4.2, 4.1 cm. B: 6.0, 6.2, 6.1 cm.
- A total = 4.0 + 4.2 + 4.1 = 12.3 cm.
- A meanAn average found by adding up all the values and dividing by how many values there are. = 12.3 ÷ 3 = 4.1 cm.
- B total = 6.0 + 6.2 + 6.1 = 18.3 cm.
- B mean = 18.3 ÷ 3 = 6.1 cm.
How to make the reasoning visible: B spreads 2.0 cm more on average after 60 seconds under these conditions.
Spread-test design
Use the illustrative repeats and inspect the method.
| Factor | How controlled/measured | Why | Unit |
|---|---|---|---|
| Sample amount | 5.0 g each | Equal material quantity | g |
| Load | 200 g each | Equal applied force | g |
| Temperature | 22 °C | Affects flow | °C |
| Time | 60 s | Defines comparison point | s |
| Spread | Two perpendicular diameters, averaged | Reduces shape bias | cm |
Which sample is more repeatable?
Why measure two perpendicular diameters?
Use the table. Name two control variables in the slime spread test and why each is kept the same.
Check your understanding
Answer from memory, then check against the suggested answer.
What is spread diameter?
What is a control variable?
How is a mean calculated?
What is an independent variable?
Why do we repeat measurements?
Why could warming both samples change the measured spreads?
Why might A spread less at 60 s but more than B after 10 minutes?
Why is the spread measured after exactly 60 s each time?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
| Sample | Trial 1 / cm | Trial 2 / cm | Trial 3 / cm |
|---|---|---|---|
| A | 4.0 | 4.2 | 4.1 |
| B | 6.0 | 6.2 | 6.1 |
Calculate the mean spread for sample A.
(1)Calculate the mean spread for sample B.
(1)Identify which sample spreads more under these conditions.
(1)State two variables that should be controlled.
(2)Total for this question: 5 marks
Command word: Show the calculated means with units, then make a conclusion limited to the stated conditions.