Lab / experiment Grade 8 Science

Leaving the Salt Behind: An Evaporation and Water-Cycle Lab

A grade 8 science lab where students evaporate salt water, measure the salt left behind, and scatter-plot class data to model the water cycle and distillation.

Grade: Grade 8
Subject: Science
Material: Lab / experiment
Time: 2 sessions, ~60 min total
Prep: Moderate prep
Materials: Shallow dish or pie tin (1 per group), Table salt, Warm tap water, Balance that reads in grams, Graduated cylinder or measuring cup (100 mL), Spoon for stirring, Marker and labels, Warm, sunny windowsill or safe warm spot, Graph paper for the class scatter plot

Standards

MS-ESS2-4 NGSS

Develop a model to describe the cycling of water through Earth's systems driven by energy from the sun and the force of gravity.

How this material aligns: Students model the sun-driven part of the water cycle by evaporating water and showing the salt stays behind, the state-change pathway of the hydrologic cycle this standard requires students to model.

All materials for MS-ESS2-4 →

8.SP.A.1 Common Core Math

Construct and interpret scatter plots for bivariate measurement data to investigate patterns of association between two quantities, describing clustering, outliers, and positive, negative, linear, or nonlinear association.

How this material aligns: In the analysis students scatter-plot salt added against residue recovered for the whole class and describe the positive, roughly linear association between the two quantities.

All materials for 8.SP.A.1 →

Download Word file

This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-23.

Objectives

Students will evaporate a salt-water sample and measure the mass of salt residue left behind.
Students will scatter-plot class data for salt added versus residue recovered and describe the association.

Vocabulary

Evaporation: Evaporation happens when liquid water, often warmed by the sun, turns into an invisible gas called water vapor that rises into the air. It is how water leaves oceans, lakes, and puddles.
Water vapor: Water vapor is water in the form of an invisible gas mixed into the air. There is always some water vapor around you, even when the sky is completely clear.

Also: residue, distillation, scatter plot

Teacher overview

The ocean is salty, yet rain is fresh. This lab shows why. Students dissolve a measured mass of salt in water, let the water evaporate with warmth from the sun or the room, and then measure the salt left behind. Because only the water leaves as vapor, the salt stays in the dish, which is the same idea behind both the natural water cycle and the distillation step in desalination. Each group tests a different amount of salt, so the class can pool its results into a scatter plot.

Leaving the Salt Behind: An Evaporation and Water-Cycle Lab — Grade 8 Science

Name: ____________________________ Date: ______________

Procedure

Set up the sample. Put a label on your dish with your group name. Place the empty dish on the balance and record its mass in grams. This is your empty-dish mass.

Make the salt water. Measure 100 mL of warm tap water. Stir in the amount of salt your teacher assigns your group, which will be somewhere between 5 and 25 grams. Stir until the water looks clear and no grains remain.

Pour the sample. Pour the salt water into your labeled dish. You do not need to weigh it again; you have recorded the salt you added.

Let it evaporate. Set the dish in a warm, sunny spot where it will not be bumped. Safety note: do not taste or drink any water or residue in this lab, and wash your hands after handling the salt water. Leave the dish until the next session, when the water has fully evaporated. This may take one to three days.

Measure the residue. Once the dish is completely dry, place it on the balance and record the mass of the dish plus the dry residue. Subtract the empty-dish mass to find the mass of salt residue left behind.

Data & Analysis

Record your group's numbers, then add every group's salt-added and residue-recovered values to a class table for the scatter plot.

Measurement	Value (grams)
Empty-dish mass
Salt added
Dish plus residue mass
Residue recovered (dish plus residue − empty dish)

What mass of salt residue did your group recover after all the water evaporated?
Suppose a group dissolved 10 grams of salt in their water. If no salt is lost, how many grams of residue should they recover?
Where did the water go? Explain using the words evaporation and water vapor.
Using the class table, make a scatter plot with salt added (grams) on the x-axis and residue recovered (grams) on the y-axis. Describe the association: is it positive or negative, and roughly linear or not?
Does your scatter plot support the idea that salt does not leave with the evaporating water? Explain using the pattern you see.
A solar still uses the sun to evaporate salty water and then collects the vapor as it cools into liquid. Based on your results, explain why that collected water would be fresh.
The ocean is salty but rain is fresh. Use what happened in your dish to explain why.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key

Answer: The mass the group recorded.
Accept the residue mass the group measured, which equals dish-plus-residue mass minus empty-dish mass. It should be close to the salt the group added.
Answer: 10 g.
Salt does not evaporate, so if none is lost the residue mass should be about equal to the salt added, which is 10 grams.
Answer: The water evaporated into vapor.
The water absorbed energy from the sun and warm air and changed into water vapor, which spread into the air and left the dish. The salt could not evaporate, so it stayed behind.
Answer: Positive, roughly linear.
The plot should show residue increasing as salt added increases, forming a roughly straight, upward pattern close to the line where residue equals salt added.
Answer: Yes.
Because the residue recovered rises about one-to-one with the salt added, the salt clearly stayed in the dish while the water left, supporting the idea that salt does not evaporate with the water.
Answer: Only water evaporates.
In a solar still the salt stays behind just as it did in the dish, so the vapor that rises and then cools back into liquid contains no salt, which makes the collected water fresh.
Answer: Evaporation leaves the salt in the ocean.
When the sun evaporates ocean water, only the water rises as vapor and the salt stays in the sea. That vapor cools and falls as fresh rain, the same separation students saw in their dishes.

Differentiation

Support

Pre-measure the salt for each group and pre-record the empty-dish mass so students focus on the evaporation and the final measurement. Provide a partly labeled scatter plot with the axes and scale already drawn.

Extension

Have students calculate how close their recovered residue was to the salt they added, describe any difference as a possible loss or measurement error, and suggest one change to the procedure that would reduce that error.

ELL supports

Provide picture steps for the procedure showing weigh, dissolve, pour, wait, and measure. Pre-teach "evaporation," "residue," and "water vapor" with simple images, and let students record the association in item 4 by circling "positive" or "negative" and "linear" or "not linear."