🥶 Does salty water take longer to freeze?

My hypothesis

If I change how much salt is in the water, then how long the water takes to freeze all the way through will ______, because ______.

🔊 You fill in the blanks. Think about what the salt does to the water's tiny bits when they try to lock together into ice. 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

• 🥛 Clear plastic cups, all the same size: 12, the same size
• 🚰 Water from the tap
• 🧂 Salt
• 🥛 A measuring cup
• 🥄 Measuring spoons
• 🥄 A spoon
• 🍽️ A baking tray or big plate
• ⏱️ A stopwatch or a kitchen timer
• 🩹 Masking tape, to mark lines

Steps

1. Label twelve clear cups with tape: three each for no salt, a quarter teaspoon, half a teaspoon and 1 teaspoon.🔊
2. Pour a quarter cup of tap water into every cup.🔊
3. Stir the right amount of salt into each salty cup until it disappears. Never drink the salty water.🔊
4. Stand all twelve cups on a tray, spaced apart, and slide the tray into the freezer.🔊
5. Start the timer. Every 30 minutes, take the tray out for a moment and press a spoon into the middle of each cup.🔊
6. A cup is frozen all the way through when the spoon will not push in.🔊
7. Write the minutes when each cup was first frozen all the way through.🔊
8. If a cup is still runny at 240 minutes, write 240.🔊
9. Find the mean time for each amount of salt, and draw a line graph with salt along the bottom.🔊

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

🔊 Pure water freezes at 0 °C. Salt dissolved in water gets in the way of the water locking into ice, so salty water has to get colder than 0 °C before it freezes. The more salt there is, the colder it must get, so it usually takes longer in the same freezer.

Going further (7th and up): Change a second thing: swap the salt for sugar, spoon for spoon, and draw both lines on one graph.

Board layout

This project at your grade

Every grade this project fits works out a mean. See every project for 4th 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 is in the water and measured how long the water takes to freeze all the way through, in minutes.
• What did you keep the same, and why does that matter? I kept a quarter cup of water in every cup and the same kind of cup the same, so the only difference was how much salt is in the 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 swap the salt for sugar, spoon for spoon, and draw both lines on one graph.

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

• 🧊 What melts ice fastest: salt, sugar or sand?2nd–6th grade · 1 day of testing · free🧪 Water, ice and mixing · 🧂 Kitchen things
• 🧂 Does warm water hold more salt than cold?5th–8th grade · 1 day of testing · free🧪 Water, ice and mixing · 🧂 Kitchen things