LF-20 · TEACHER LESSON PLAN

Salt water egg float lab

Why an egg sinks in plain water but floats in salt water

PhysicsGrades 3–720–25 minHazard low; raw egg and spillsNGSS MS-PS1-1

01 //

Driving question

START THE LESSON WITH THISHow does the amount of salt dissolved in water change whether an egg floats or sinks?

02 //

Learning objectives

Students prepare equal 300 mL water samples with 0, 3 and 6 tablespoons of salt, dissolving each salt amount fully before testing.
Students classify the egg as sinking or floating in each prepared amount and measure its height above the surface when it floats.
Students bracket the tested salt-amount interval where the egg begins to float without claiming an exact threshold between untested amounts.

03 //

Standards

Dimension	Alignment
Performance expectation	MS-PS1-1Develop models to describe the atomic composition of simple molecules and extended structures. (Partial alignment: this is the performance expectation the lab lists, but the activity investigates a physical property of matter (salt-water density and concentration) rather than building atomic models.)
Practices	SEP 3Plan and carry out investigations.
Core idea	PS1.AEach pure substance has characteristic physical and chemical properties (for any bulk quantity under given conditions) that can be used to identify it.
Crosscutting	Cause and effectCause and effect relationships are routinely identified, tested, and used to explain change.

Standards text paraphrased from nextgenscience.org. Alignment is partial; this hands-on activity does not cover every part of the performance expectation. CCC wording not independently verified.

03A //

State standards crosswalk

Use the row as a planning crosswalk, then confirm local district pacing and grade placement. “Partial” labels describe a skill connection, not full mastery of the complete standard.

State	Verified standard	How this lesson aligns	Official source
California	CA NGSS 5-PS1-3	Limited. Students measure a material property, but the California assessment boundary explicitly excludes density; this egg buoyancy test does not identify materials using the listed properties.	Open standard ↗
Texas	Science.5.6(A)	Partial. The standard explicitly includes relative density through sinking and floating, but this investigation uses spoon-measured salt and does not calculate the egg or solution density.	Open standard ↗
Florida	SC.8.P.8.4	Partial. Density is a relevant physical property, but students observe an egg in prepared solutions rather than measure mass and volume to compare densities.	Open standard ↗
New York	NYSSLS MS-PS1-7	Limited. The standard uses density evidence to identify matter; this activity neither measures density nor identifies an unknown sample, and its threshold is only bracketed.	Open standard ↗
Pennsylvania	STEELS 3.2.2.A	Limited. Students test an observable sinking or floating behavior, while this grade-two standard focuses on classifying materials by observable properties and does not address solution density.	Open standard ↗

04 //

Materials and prep timeline

PER GROUP

One similar raw egg for the three sequential tests
Three clear cups, each with 300 mL room-temperature water
Table salt: 0, 3 and 6 tablespoons
Measuring spoons and stirring spoon
Labels, balance if available, tray and towel

TEACHER ONLY

Check that the egg is intact before testing; keep it away from food and provide hand/surface-cleaning supplies.
Use stable cups deep enough to submerge the egg; set the station on a tray.
Provide one student sheet per student; no samples are tasted.

CLASS-SIZE KIT PLANNER

One per-group kit contains every item in the team materials list above. Groups are balanced around the listed 3–4 students target.

Students	Groups	Per-group kits	Student sheets	Suggested group split
20	6	6 complete kits	20 copies	2 groups of 4 and 4 groups of 3
25	7	7 complete kits	25 copies	4 groups of 4 and 3 groups of 3
30	8	8 complete kits	30 copies	6 groups of 4 and 2 groups of 3

0–4 MINPredict the threshold

Predict which tested salt amounts may make the egg float. Explain that the result will bracket a range, not necessarily an exact threshold.

4–10 MINPrepare equal samples

Measure 300 mL water into each cup; add 0, 3 and 6 tablespoons of salt and stir the salted samples until fully dissolved.

10–17 MINTest each solution

Lower the intact egg gently with a spoon into each cup, rinse between tests and record sink/float. Measure height above the surface only when it floats.

17–21 MINFind the tested interval

Identify the greatest tested salt amount where the egg sinks and the smallest where it floats; that pair brackets the threshold.

21–25 MINExplain and clean

Connect dissolved salt to solution density, discuss measurement limits and wash hands and surfaces.

04A //

Lab figures and data visuals

Reviewed article images support the lesson flow. Use the tables and student sheet to record actual observations; pictured patterns are not target results.

An egg sinks in plain water and floats higher as dissolved salt increases the density of equal-volume water samples. — **Dissolved salt raises the water's density.**An egg sinks when it is denser than plain water and floats when the salt solution becomes denser than the egg.

A holographic density and buoyancy comparison shows an egg sinking in plain water and floating in denser salt water. — **The egg's density stays the same as salt is added to water.**Dissolved salt adds mass to a similar water volume, raising the solution density and the buoyant support on the egg.

AI-generated classroom illustration titled Salt Makes Water Denser, showing an egg sinking in fresh water, suspended in a moderate salt solution, and floating higher in saltier water. — **Watch for the concentration where the egg begins to rise.**Compare the observed egg position across measured salt amounts. The exact threshold varies with the egg, so record what your sample does rather than treating the illustration as a guaranteed outcome.

04B //

Printable shopping list

TEACHER PREP · Salt water egg float lab

Shopping and staging checklist

Multiply the per-group kit contents by the class-size planner. The teacher section includes teacher-provided materials and setup or safety notes.

PER-GROUP KITS

One similar raw egg for the three sequential tests
Three clear cups, each with 300 mL room-temperature water
Table salt: 0, 3 and 6 tablespoons
Measuring spoons and stirring spoon
Labels, balance if available, tray and towel

TEACHER-PROVIDED MATERIALS + SETUP RULES

Check that the egg is intact before testing; keep it away from food and provide hand/surface-cleaning supplies.
Use stable cups deep enough to submerge the egg; set the station on a tray.
Provide one student sheet per student; no samples are tasted.

QUANTITIES BY CLASS SIZE

Material	Per group kit	20 students	25 students	30 students
Intact raw eggs	1 egg	6 eggs	7 eggs	8 eggs
Clear cups	3 cups	18 cups	21 cups	24 cups
Room-temperature water	900 mL	5.4 L	6.3 L	7.2 L
Table salt	9 tablespoons total	54 tablespoons	63 tablespoons	72 tablespoons
Measuring and stirring spoons	1 set	6 sets	7 sets	8 sets
Tray, labels and towels	1 station set	6 station sets	7 station sets	8 station sets
Student data sheets	1 per student	20 copies	25 copies	30 copies

Prepare 6 kits for 20 · 7 kits for 25 · 8 kits for 30 students.

05 //

Lesson flow

Minutes	What happens
0–4	Engage Ask how increasing dissolved salt may change an egg's position. Record a prediction before preparing or testing samples.
4–10	Explore Measure equal 300 mL water volumes into three labeled cups. Add 0, 3 and 6 tablespoons of salt; stir until every measured salt sample is dissolved.
10–17	Gently lower the same intact egg into each solution with a spoon, rinsing it between cups. Record sinks/floats and measure height above the water only when it floats.
17–21	Explain Find the highest tested salt amount where the egg sank and the lowest where it floated. Report these as the interval bracketing the threshold.
21–25	Evaluate Explain that dissolved salt raises water density; the exact transition depends on the egg and measured preparation. Wash hands, cups and the work surface.

06 //

Discussion questions

1. What does a sink result in plain water and a float result at 3 tablespoons tell you?

SAMPLE ANSWERUnder those specific conditions, the transition lies somewhere above the 0-tablespoon sample and at or below the 3-tablespoon sample. It does not establish an exact amount between them.

2. Why must the salt dissolve fully?

SAMPLE ANSWERUndissolved crystals at the bottom are not evenly mixed through the solution, so the cup would not have the intended prepared salt condition.

3. Why keep each water volume and cup size the same?

SAMPLE ANSWERThe question compares salt amount. Equal water volumes and similar cups help ensure a different solution recipe is the main planned change.

4. Why might another class find a different threshold?

SAMPLE ANSWEREgg size, shell and trapped air vary, and spoonfuls can contain different salt masses. The conclusion should describe the tested amounts and conditions.

07 //

Common misconceptions

STUDENTS OFTEN SAY“An egg floats at exactly 1.5 tablespoons because it floated at 3.”

BETTER IDEAThe tested results only bracket the transition; test intermediate amounts to narrow the interval.

STUDENTS OFTEN SAY“Salt crystals on the bottom make the whole cup equally dense.”

BETTER IDEAThe measured salt must dissolve and mix through the water before the egg is tested.

STUDENTS OFTEN SAY“The same spoon measure is always the same salt concentration.”

BETTER IDEASalt crystal size and how a spoon is filled vary. For a closer concentration estimate, weigh the salt and record the water volume.

08 //

Differentiation

SUPPORT

Use labeled cups and pre-measured 0, 3 and 6 tablespoon salt portions.
Show a three-state card: sinks, floats at surface, or suspended; record the actual state before measuring height.
Offer the frame: “The egg sank at __ tbsp and floated at __ tbsp, so the tested transition lies between those amounts.”

EXTEND

Weigh the salt and calculate mass per 300 mL to describe the solution more precisely than spoon count.
Test one or more intermediate amounts between the highest sink and lowest float, using fresh equal-volume samples.
Repeat the threshold series with similar eggs and compare the intervals rather than claiming one exact universal threshold.

09 //

Assessment rubric

Criterion	Beginning	Developing	Secure
Prepared samples	Does not label salt amounts or water volumes.	Labels the amounts but omits equal volume or complete dissolving.	Uses equal 300 mL samples labeled 0, 3 and 6 tbsp and dissolves the salt before testing.
Egg classification	Leaves sink/float results blank or guesses.	Records results but does not distinguish floating height from sinking.	Records actual sink/float result and measures above-surface height only when the egg floats.
Threshold interval	Claims an exact untested salt amount.	Identifies a broad transition but omits the bracketing results.	Uses the greatest tested sink amount and lowest tested float amount to state an interval.
Density explanation and safety	Explains only by egg weight or tastes a sample.	Names salt or density but omits a relevant control or safety step.	Connects dissolved salt with density, notes egg/preparation variation and handles raw egg safely.

10 //

Vocabulary

Solution – A mixture in which a substance such as salt is dissolved in water.
Concentration – How much dissolved substance is present in a given amount of solution.
Dissolve – To spread a substance's particles through a liquid to form a solution.

Density – Mass divided by volume; how much mass occupies a given amount of space.
Threshold – A boundary or range where an observed result changes from one state to another.
Buoyancy – The upward support a liquid provides to an immersed object.

11 //

Safety, cleanup and troubleshooting

BEFORE STUDENTS TOUCH ANYTHING

Raw eggs can carry Salmonella. Do not eat or taste any sample; wash hands, cups, and the work surface after handling, and keep the egg away from food-preparation areas.
Use stable cups on a tray and wipe spilled water or salt solution promptly.
Inspect the egg for cracks before use and lower it gently with a spoon so it does not strike the cup bottom. Stop if the shell breaks and clean up with adult help.

Troubleshooting: Six tablespoons is close to the most salt that 300 mL of room-temperature water can dissolve, so some may stay on the bottom; keep stirring, record any salt that does not dissolve, and do not treat that cup as a fully prepared sample. If salt remains visible in the other cups, keep stirring until dissolved before testing. If the egg cracks, do not continue with it; ask an adult to clean the cup and area, wash hands and use an intact egg if the activity can safely continue.