Lesson focus: Can a material made from milk help us understand polymerA very long molecule made by joining lots of small molecules together in a chain.?
Explain how processingThe steps used to turn a raw material into something useful. a natural polymerA polymer made by living things, such as starch or wool. changes its propertyA feature of a material you can observe or measure, such as how hard or bendy it is..
Build on earlier learning
FiltrationRevisit separating an insoluble solid from a liquid.
Practical
Making a plastic from milk
- Warm 100 cm³ of milk until it is hot but not boiling.
- Stir in vinegar a little at a time: the milk protein (casein) forms solid lumps.
- Filter, press the solid into a shape and let it dry.
Record: Observations at each step and how the result compares with a plastic made from crude oil.
Safety: Do not taste anything made in the laboratory.
You will be successful when you can…
- Identify casein as a protein polymer.
- Sequence the separation stages.
- Explain the effect of water removal.
- Evaluate a material using property evidence.
Words that carry the science
- casein
- A group of proteins naturally present in milk.
- curd
- The solid-rich material that separates when casein gathers together.
- filtration
- Separating an insoluble solid from a liquid using a filter.
- processing
- Steps used to change a material into a useful form.
- water absorption
- The amount of water taken into a material.
Separate an existing natural polymer, then test the new material
01Casein already exists in milk
Acid changes interactions around caseinA protein found in milk. so it gathers into curdThe soft white lumps that form when milk proteins clump together.. This visible separation does not show that new protein chains have been synthesised. In simple words: milk already contains a natural polymer called caseinA protein found in milk.. Adding acid (such as vinegar) makes the casein clump together into lumps called curdThe soft white lumps that form when milk proteins clump together.. No new polymer is made: the casein was there all along.
02Separation and drying change the material
Filtering removes much of the liquid. Pressing and drying remove more water, so the shaped material can become harder and less flexible.
03Properties decide usefulness
A casein material may be shapeable and stiff when dry but can absorb water. Suitability must be tested for the intended conditions.
Polymerisation: monomers join into a chain
Small molecules called monomers join end to end to make a very long molecule, a polymer. In ethene the double bond between the carbon atoms opens up, so each monomer can bond to the next one.
Black: carbon. White: hydrogen. Green: chlorine. Two lines: a double bond.
Controls
A real chain has thousands of monomers. The double bond becomes a single bond in the chain.
Interpret a mass change
A shaped sample is 24 g before drying and 15 g after drying.
- Calculate mass lost: 24 − 15 = 9 g.
- Calculate fraction lost: 9 ÷ 24 = 0.375.
- Convert to percentage: 37.5%.
- Infer that much of the loss is water, while recognising that mass alone does not identify every substance.
How to make the reasoning visible: Drying data support water loss; they do not demonstrate new polymer-chain formation.
Process and property audit
Connect each stage to evidence and avoid claims the evidence cannot support.
| Stage | Observable change | Likely purpose | What it does not prove |
|---|---|---|---|
| Add acid | Curds appear | Separate casein | New chains formed |
| Filter | Solid retained | Remove liquid | Material is dry |
| Press and shape | Form changes | Create useful shape | Material is waterproof |
| Dry | Mass falls/material hardens | Remove water | All environmental impacts are low |
Which measurement could show water absorption?
Why must equal-sized samples be used when comparing drying time?
Use the table. Why do you filter the mixture after adding acid?
Check your understanding
Answer from memory, then check against the suggested answer.
What is casein?
What does filtration separate?
What is water absorption?
What is added to warm milk to make curds?
What is a curd?
Why might a dry casein cup fail when filled with water?
What evidence would compare two protective coatings fairly?
Design a simple test to see whether your milk plastic absorbs water.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A class makes a plastic-like material from milk and vinegar (an acid). Answer the parts to explain why this does not show that a new polymer was made.
What polymer does milk already contain?
(1)What does the vinegar do to the casein?
(1)What do filtering and drying do?
(1)So was a new polymer made? Why?
(1)Total for this question: 4 marks
Command word: Keep track of the casein from the milk to the final material.