Ice-Melting Experiment — Salt Water vs. Fresh Water
GED Science Practice — Experimental design, data tables & drawing valid conclusions
Read the passage
Salt water causes marine ecosystems to be very different from freshwater ecosystems. Organisms in both types of ecosystems are affected by the freezing and melting of ice. An experiment was conducted to test the hypothesis that ice melts faster in salt water than in fresh water. Ice cubes made of fresh water were placed into cups containing either fresh water or salt water. Various data were recorded, and the energy released by each melting ice cube was calculated from the data. The table shows the results for each cup.
Cup
Initial Mass of Water (g)
Final Mass of Water (g)
Change in Mass (g)
Initial Temp (°C)
Final Temp (°C)
Change in Temp (°C)
Time to Melt (s)
Energy Released (J)
Fresh water (not stirred)
50
83
33
45
33
−12
120
11,022
Salt water (not stirred)
45
81
36
48
43
−5
480
12,024
GED question: What statement describes one or more needed changes to this experiment that would allow the experimenter to draw a valid conclusion? You may use the calculator.
○ Salt water should have been used to make the ice cubes for the cup of salt water.
○ The time for ice cubes to melt should have been measured in minutes.
● At the beginning, both cups should have contained the same mass of water at the same temperature.
○ The energy released should have been measured, not calculated.
Work through each tab to understand why the highlighted answer is correct and why the others are wrong.
Ice-melting experiment — animated diagram
Watch how an ice cube melts differently in fresh water vs. salt water
What is this experiment about?
An experiment was conducted to test the hypothesis that ice melts faster in salt water than in fresh water. Ice cubes made of fresh water were placed into cups containing either fresh water or salt water. Various data were recorded, and the energy released by each melting ice cube was calculated from the data.
The key question: What changes are needed to allow the experimenter to draw a valid conclusion?
To draw a valid conclusion, an experiment must only have one variable that is different between the two groups. Everything else must be the same — this is called controlling the variables.
The hypothesis
Ice melts faster in salt water than in fresh water. The experiment is designed to test this claim.
Independent variable
The type of water (salt vs. fresh) — this is the ONE thing being tested. Everything else must be identical.
Controlled variables
Initial mass of water, initial temperature, size of ice cube — these must be the SAME in both cups for a fair test.
Dependent variable
The time for ice to melt and the energy released — these are the measured outcomes that change as a result of the experiment.
Ice-Melting Experiment — Data Table
Results for each cup. Yellow cells highlight values that differ between the two cups.
Cup
Initial Mass of Water (g)
Final Mass of Water (g)
Change in Mass (g)
Initial Temp (°C)
Final Temp (°C)
Change in Temp (°C)
Time to Melt (s)
Energy Released (J)
Fresh water (not stirred)
50
83
33
45
33
−12
120
11,022
Salt water (not stirred)
45
81
36
48
43
−5
480
12,024
Notice the yellow cells. The initial mass of water (50g vs 45g), the initial temperature (45°C vs 48°C), AND the melt time all differ. For a valid experiment, only the type of water should differ — not the starting conditions.
Visualizing the data
The fresh water cup melted in 120s — the salt water cup took 480s. That's 4× longer — opposite of the hypothesis!
What does the data show vs. what it means
Melt time: 120s vs 480s
The ice melted 4× SLOWER in salt water. This contradicts the hypothesis — but the experiment has design flaws that prevent a valid conclusion.
Temperature drop: −12°C vs −5°C
The fresh water cup cooled more (−12°C). The salt water cup only cooled by −5°C. These different starting temps affect results.
Mass: 50g vs 45g
The fresh water cup had 5g MORE water. More water = more thermal energy available to melt ice — regardless of salt content.
Energy: 11,022J vs 12,024J
The salt water cup released ~1,002J more energy. But since conditions weren't equal, this difference can't be attributed to salt alone.
Why can't we draw a valid conclusion from this experiment?
For a valid experiment testing one variable (salt vs. fresh water), ALL other conditions must be identical between the two cups. Let's check each condition:
Condition
Fresh Water Cup
Salt Water Cup
Controlled correctly?
Type of water
Fresh water
Salt water
✓ This is the variable being tested
Type of ice cube
Fresh water ice
Fresh water ice
✓ Same — correctly controlled
Initial mass of water
50 g
45 g
✗ DIFFERENT — not controlled!
Initial temperature
45°C
48°C
✗ DIFFERENT — not controlled!
Two variables changed at once — both the initial mass AND the initial temperature were different. This means we cannot tell whether any difference in melting time was caused by the salt, the extra mass, or the higher temperature.
Breaking down each answer choice
"What statement describes one or more needed changes to this experiment that would allow the experimenter to draw a valid conclusion?"
WRONG
"Salt water should have been used to make the ice cubes for the cup of salt water." This would actually introduce another variable — different types of ice. The passage states fresh water ice cubes were used in both cups, which is correct. Changing the ice cube type doesn't fix the actual problem.
WRONG
"The time for ice cubes to melt should have been measured in minutes." Measuring time in minutes vs. seconds is a unit conversion — it doesn't change the experiment or fix any design flaw. 120 seconds = 2 minutes; 480 seconds = 8 minutes. The same problem exists regardless of units.
CORRECT
"At the beginning, both cups should have contained the same mass of water at the same temperature." This directly fixes both flaws. If both cups start with the same mass (e.g., 50g) AND the same temperature (e.g., 45°C), then the only difference is the type of water — allowing a valid conclusion about whether salt affects melting speed.
WRONG
"The energy released should have been measured, not calculated." Calculating energy from measured data is a valid scientific method. The problem is not how energy was determined — it's that the starting conditions were unequal. Measuring energy directly wouldn't fix the unequal mass and temperature issue.
Explore the data — interactive calculator
Use this calculator to verify calculations from the table. Select a cup and a measurement to see the math.
Ice-melting data calculator
Select options above to see the calculation.
Try this: Select "Compare both cups" + "Melt time difference" to see how much longer the ice took to melt in salt water. Then select "Mass change" to see why the experiment is flawed.
What would a valid experiment look like?
Same initial mass
Both cups should start with the same amount of water — e.g., both 50g. Then the only mass difference would come from the ice melting.
Same initial temperature
Both cups should start at the same temperature — e.g., both at 45°C. This ensures each cup has the same thermal energy available.
Same ice cube
Both cups use the same type and size of fresh water ice cube — this was already done correctly in the experiment.
One variable differs
The ONLY difference should be: one cup has fresh water, the other has salt water. Then any difference in melt time can be attributed to the salt.
The correct answer — explained fully
Correct answer: "At the beginning, both cups should have contained the same mass of water at the same temperature."
Why: The experiment has two uncontrolled variables — initial mass (50g vs. 45g) and initial temperature (45°C vs. 48°C). Both must be equalized so the only difference between cups is whether the water is fresh or salt. Only then can any difference in melting time be validly attributed to the type of water.
GED strategy — experimental design questions
Step 1: Identify the hypothesis — what is being tested? (Here: does salt affect ice-melting speed?)
Step 2: Identify the independent variable — the ONE thing that should differ (Here: salt vs. fresh water)
Step 3: Check all other conditions in the data table — are they the same in both groups?
Step 4: Find what changed that shouldn't have — those are the flaws to fix
Step 5: Eliminate answer choices that don't fix the actual flaws
Key rule: In a controlled experiment, you can only change ONE variable at a time. If two conditions differ between groups, you can't tell which one caused the observed difference in results.
Why the other answers are wrong
Salt water ice cubes
Wrong — using different ice cubes adds another variable. The experiment correctly used the same fresh water ice in both cups.
Measure time in minutes
Wrong — changing units doesn't fix anything. 120s = 2min, 480s = 8min. The flaw is unequal starting conditions, not the unit of measurement.
Measure energy directly
Wrong — calculating energy from data is scientifically valid. The flaw is the unequal starting conditions, not the method of determining energy.
Same mass + temperature
CORRECT — this fixes both flaws at once. Equalizing starting conditions means the only remaining difference is the type of water.
