🧂 Does more salt make ice water colder?

My hypothesis

If I change how much salt goes in the ice water, then how cold the ice water gets after 10 minutes will ______, because ______.

🔊 You fill in the blanks. Think about what salt does to the temperature at which water freezes. A guess is a good because: the test finds out.

Change, measure, keep the same

Scientists call the thing you change the independent variable and the thing you measure the dependent variable. The things you keep the same are the controls: if one of them changed too, you wouldn't know which change made the difference. Try each of the 4 levels 3 times.

What you need

• 🧊 Ice cubes, all from the same tray: about 24 cubes for each round
• 🥛 Clear plastic cups, all the same size: 4, the same size
• 🧊 Cold water from the fridge: half a cup for each cup
• 🥛 A measuring cup
• 🧂 Salt: 6 tablespoons for each round
• 🥄 Measuring spoons
• 🥄 A spoon
• 🌡️ A thermometer (digital, or the red-liquid kind)
• ⏱️ A stopwatch or a kitchen timer
• 🩹 Masking tape, to mark lines
• 🧽 A sponge or cloth, for wiping up

Steps

1. Label four cups with tape: no salt, 1, 2 and 3 tablespoons.🔊
2. Put six ice cubes in each cup, then half a cup of cold water.🔊
3. Read the thermometer in the no-salt cup and write it down: that is your starting number.🔊
4. Add the salt each label says, then start the timer.🔊
5. Stir each cup ten times every two minutes, the same way each time.🔊
6. At 10 minutes, hold the thermometer in each cup for 30 seconds, then read it.🔊
7. Write each cup's temperature in your table, rinsing the thermometer between cups.🔊
8. Pour everything away, dry the cups, and do it twice more with fresh ice.🔊
9. Draw a line graph: salt along the bottom, temperature up the side.🔊

Every step is one you can do yourself, with a grown-up nearby. Write each number down straight away, in the table below, so nothing is lost.

Your results table

From 4th grade, add the tries for each level and divide by how many there were: that's the mean, and it goes in the last column. Then draw a line graph, because the thing you change is a number that goes up.

Why it works

🔊 Salt dissolved in water lowers the temperature at which it freezes, so the ice keeps melting below 0 °C and the salty ice water gets colder than plain ice water. In a university physics demonstration salt lowers it by as much as 15 °C.

Going further (7th and up): Change a second thing: try the same amounts of salt in cups packed with crushed ice, and graph both lines on one chart.

Board layout

This project at your grade

From 4th grade up, the table gets a mean column. See every project for 3rd grade, or print a blank results table.

What a judge will ask

• What did you change, and what did you measure? I changed how much salt goes in the ice water and measured how cold the ice water gets after 10 minutes, in degrees.
• What did you keep the same, and why does that matter? I kept the same number of ice cubes in every cup and the same amount of cold water in every cup the same, so the only difference was how much salt goes in the ice water.
• How many times did you test it? 3 times at each of my 4 levels.
• If you did it again, what would you change or test next? I'd try the same amounts of salt in cups packed with crushed ice, and graph both lines on one chart.

Make it yours

Keep this project on this device and Peabody counts down to your fair day, with a plan for each day. Nothing is sent anywhere.

Try next

• 🥶 Does salty water take longer to freeze?4th–8th grade · 1 day of testing · free🧪 Water, ice and mixing · 🧂 Kitchen things
• 🧊 What melts ice fastest: salt, sugar or sand?2nd–6th grade · 1 day of testing · free🧪 Water, ice and mixing · 🧂 Kitchen things