DIY · Card 86 · Chemistry · Heat
Milk, sugar and vanilla in one bag; ice and a handful of salt in another. Ten minutes of shaking and you have real ice cream, made colder than a freezer by the strange way salt makes ice melt.
01The brief
Ice cream, no freezer required.
−21 °C
the coldest an ice-and-salt mixture can get
334 J
of heat soaked up by every gram of melting ice
1843
when Nancy Johnson patented the hand-cranked ice-cream freezer
Ice straight from the freezer melts at 0 °C, and at 0 °C ice cream won’t freeze: all that sugar keeps it liquid. Add salt to the ice, though, and something odd happens. The ice melts faster, the bag gets painfully cold, and the thermometer drops far below zero.
Harness that cold and you can make real ice cream in ten minutes, anywhere. It goes well with the density tower and oobleck, two more kitchen-table chemistry experiments.
02Kit
Two bags, a little milk and a lot of ice and salt.
Tick off what you have
0 of 5 ready
03Method
Bag the mix, salt the ice, then shake.
Step 1 of 8 · Minute 0
You need a small and a large zip-lock freezer bag, 120 ml (half a cup) of milk or half milk and half cream, 1 tablespoon of sugar, a few drops of vanilla, about 3 cups of ice and a third of a cup (about 100 g) of salt. Gloves or a tea towel help.
It stays runny.
Why
Not enough salt or ice, or not enough shaking.
Fix
Add more salt and ice, and keep shaking for a few more minutes. Make sure the small bag is buried in ice.
It tastes salty.
Why
Brine leaked into the small bag.
Fix
Double-bag the mix next time and rinse the bag before opening it.
It’s icy and crunchy.
Why
Big ice crystals formed while it froze slowly.
Fix
Use some cream instead of all milk, and shake harder and more steadily.
04Salt and cold
At the surface of melting ice, water molecules are constantly freezing on and breaking off. Salt dissolved in the meltwater gets in the way of the freezing, but not the melting, so the ice keeps melting even below 0 °C, and melting takes heat from its surroundings.
Add salt and watch the thermometer:
Salt in the brine
17 %
Brine reaches
-12.8 °C
Ice cream
freezes fast
Salt dissolves in the thin film of water on the ice and lowers its freezing point, so the ice keeps melting below 0 °C. Melting takes heat, which it pulls from everything around it, until the mixture settles at the brine’s new freezing point, here about -13 °C. Concentrations are simplified: real brine strength changes as the ice melts.
Freezing the ice cream means moving heat out of it and into the ice. Here is the bookkeeping:
where the heat comes from
Heat to remove
48 kJ
Ice that must melt
143 g
About
1.0 cups of ice
Cooling the mix down to its freezing point is the easy part. Turning its water into ice takes far more: 334 joules for every gram, the same 334 joules that each gram of ice in the big bag absorbs as it melts. So your ice cream is made by trading: for every gram of mix that freezes, roughly a gram of bag ice must melt, plus extra to cool it down and to make up for the warmth of your hands and the room.
05The big idea
Melting
steals heat.
It takes a surprising amount of energy to turn ice into water without warming it at all: 334 joules per gram, enough to heat that gram of water by 80 °C. When ice melts, that energy has to come from somewhere nearby.
That is why a drink with ice stays cold, why sweat cools you down as it evaporates, and why salted ice can freeze your ice cream: the melting ice drains heat out of everything it touches.
06The math
1 · Freezing-point drop
ΔT = K · m
K for water is 1.86 °C·kg/mol; m the dissolved particles per kg.
2 · Melting heat
Q = m · L
L = 334 J per gram for ice.
3 · Cooling heat
Q = m · c · ΔT
c the heat capacity, ΔT the temperature fall.
One tablespoon of sugar, 12 g or 0.035 mol, in about 85 g of water in the milk gives 0.41 mol/kg: a drop of 1.86 × 0.41 ≈ 0.8 °C. With the milk’s own sugars and salts, the mix starts to freeze at around −1 to −2 °C, and needs to get colder still to firm up.
Cooling 250 g of mix from 20 °C to −2 °C removes about 250 × 3.4 × 22 ≈ 19 kJ; freezing half its water, about 88 g, removes 88 × 334 ≈ 29 kJ more. That melts about 48,000 ⁄ 334 ≈ 140 g of ice: a good mugful.
07Real life
From gritted roads and sea ice to fish that never freeze.
Salting roads
Gritters spread salt on winter roads so ice melts even below 0 °C. It works well down to about −10 °C; in harder frosts, roads need other chemicals or sand for grip.
Sea ice
Seawater, at about 3.5 % salt, doesn’t freeze until about −1.9 °C. As it freezes, most of the salt is pushed out, so old sea ice can be melted for drinking water.
Antifreeze
Car radiators carry water mixed with antifreeze. Like salt, it lowers the freezing point, so the engine’s coolant stays liquid in the coldest winters.
Fish that don’t freeze
Antarctic fish swim in water below 0 °C. Special antifreeze proteins in their blood stop ice crystals from growing inside their bodies.
Hand-cranked freezers
Old-fashioned ice-cream makers have a metal can of cream turning in a bucket of ice and salt. The paddle inside scrapes and stirs, exactly like your shaking.
Liquid-nitrogen ice cream
Some shops freeze ice cream with liquid nitrogen at −196 °C. It freezes so fast that the ice crystals stay tiny, making extra-smooth ice cream (and plenty of fog).
1843
For centuries, ice cream was a luxury: cream had to be stirred by hand in a pot sitting in ice and salt, for hours. In 1843 Nancy Johnson of Philadelphia patented an ‘artificial freezer’ that made it far easier.
Her design was a tub of ice and salt surrounding a can of cream, with a hand crank turning a paddle inside to stir and scrape. Hand-cranked freezers based on the same idea were used in homes for the next hundred years, and your shaken bag is its pocket-sized descendant.
08Is and isn’t
Salt makes ice colder by itself.
Salt lets the ice melt at lower temperatures. The melting soaks up heat, and that is what makes the mixture so cold.
Salt water can’t freeze.
It can; it just needs to be colder. Seawater freezes at about −1.9 °C, and strong brine at down to −21 °C.
More salt always means colder.
Only up to a point. Beyond about 23 % salt the brine can’t get any colder than −21 °C; extra salt just stays undissolved.
You need a freezer to make ice cream.
People made ice cream with ice and salt for centuries before electric freezers, and the bag method is the same idea.
09Go further
A
Measure the cold
Put a thermometer in the ice before and after adding salt. How far does it fall?
B
Salt versus sugar
Try the big bag with sugar instead of salt. Does the ice get as cold?
C
Flavour lab
Try cocoa, strawberry purée or mashed banana in the mix. Which freezes fastest?
D
Milk versus cream
Make two batches, one with milk and one with cream. Which is smoother, and why?
E
Melting race
Sprinkle salt on one ice cube and leave another plain. Which melts first?
10Questions
Seal milk, sugar and vanilla in a small zip bag, then put it inside a large bag half full of ice mixed with about a third of a cup of salt. Seal the big bag and shake it, wrapped in a towel, for 5 to 10 minutes until the mix thickens into ice cream.
Salt lowers the temperature at which ice can melt. The ice keeps melting below 0 °C, and melting soaks up heat, so the ice-and-salt mixture gets much colder, down to around −10 to −20 °C: cold enough to freeze the sweetened milk.
Not really. Plain melting ice stays at 0 °C, but the sugary mix only starts to freeze at about −2 °C, and needs to be colder still to set. Without salt it just becomes cold milk.
With enough salt, as low as about −21 °C, the coldest temperature at which salt water can stay liquid. Adding more salt than that does nothing further.
Coarse rock salt or ice-cream salt is traditional, but table salt works too and dissolves faster. Anything that dissolves in the meltwater will lower its freezing point.
Safety · Salted ice can reach −20 °C and cause cold burns: hold the bag in a towel or wear gloves, and don’t touch the ice for long. Check for milk allergies first, and never eat ice cream that has salty brine in it.
All DIY cards →Keep wandering
A few more pieces in the same spirit — math, design, and slow attention.
How to Make a Density Tower: Stacking Liquids in a Glass
Honey, washing-up liquid, water, oil and rubbing alcohol, poured one by one into a glass, settle into five sharp layers. Which goes where is decided by one number, and one of them only stays put thanks to the oil.
Open
How to Make Oobleck: The Liquid That Turns Solid
Cornflour and water, stirred together in a bowl. Stir slowly and it pours like cream; punch it and it cracks like a brick. The grains inside are doing something physicists only pinned down in the last fifteen years.
Open
How to Make a Pizza Box Solar Oven That Really Cooks
An empty pizza box, some foil, black paper and cling film. Point it at the Sun and in half an hour the chocolate melts and the marshmallows go gooey.
Open
How to Get an Egg Into a Bottle Without Touching It
Two days in vinegar take the shell away, a warm bottle does the rest. The egg goes through a mouth narrower than itself, pushed by ten tonnes of air per square metre.
Open
How to Blow Up a Balloon With Hot Water: Charles’s Law
Put a balloon on an empty bottle, stand the bottle in hot water, and the balloon lifts itself up. The air didn’t get any more of itself. It just needs more room, and from that one fact you can find absolute zero.
Open
How to Make a Water Clock That Times Exactly One Minute
Two plastic cups, a pinhole and three-quarters of a cup of water. Tune the hole until it runs dry in exactly sixty seconds, and you have built the oldest kind of clock there is.
Open