Lab / experiment Grade 9 Science

Salt, Conductivity, and Dissolved Solids: A Grade 9 Water Lab

A grade 9 lab where students test how dissolved salt changes water's electrical conductivity, then link the result to dissolved solids and reverse osmosis.

Grade: Grade 9
Subject: Science
Material: Lab / experiment
Time: 50 min
Prep: Moderate prep
Materials: Conductivity tester per group: a 9-volt battery, an LED with a current-limiting resistor, and two insulated wires with bare tips (teacher may pre-build), Five clear plastic cups per group, Distilled water, about 1 liter per group, Table salt, about 4 teaspoons per group, Quarter-teaspoon measuring spoon, Stir stick, Paper towels for drying the wire tips, Marker for labeling cups, Brightness comparison card, 1-to-5 scale, one per group

Standards

HS-PS1-3 NGSS

Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.

How this material aligns: Students gather evidence that salt water conducts current while distilled water barely does, inferring that dissolved ions carry charge, the bulk-property-to-particle-force reasoning this standard requires.

All materials for HS-PS1-3 →

HSN-Q.A.3 Common Core Math

Choose a level of accuracy appropriate to limitations on measurement when reporting quantities.

How this material aligns: Analysis question 7 asks students to judge what a whole-number brightness card can and cannot support and name a more precise instrument, the accuracy-to-measurement matching this standard requires.

All materials for HSN-Q.A.3 →

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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 test how added salt changes water's electrical conductivity using a simple tester.
Students will describe the pattern between dissolved salt and brightness on a 1-to-5 scale.
Students will connect conductivity to dissolved solids and to reverse-osmosis treatment.

Vocabulary

Reverse osmosis: Reverse osmosis cleans water by pushing it through a membrane with holes so tiny that salt and most other dissolved things cannot fit through. It can even turn seawater into fresh water.

Also: conductivity, ion, dissolved solids, distilled water

Teacher overview

Pure water barely conducts electricity, but water with dissolved salt conducts it well, because the salt splits into charged particles called ions that carry current. Treatment plants use that fact every day: a conductivity meter gives a fast estimate of how many dissolved solids are in the water. This lab lets students see the effect with a simple battery-and-LED tester, comparing distilled water against water with rising amounts of salt. Because the tester only gives a rough brightness rating, students also practice judging what a whole-number scale can and cannot tell them.

Salt, Conductivity, and Dissolved Solids: A Grade 9 Water Lab — Grade 9 Science

Name: ____________________________ Date: ______________

Procedure

Step 1. Label five clear cups A through E. Fill each cup about half full with distilled water, using the same amount in every cup.

Step 2. Leave Cup A as distilled water only. This is your control. Add a quarter teaspoon of salt to Cup B, a half teaspoon to Cup C, one teaspoon to Cup D, and two teaspoons to Cup E. Stir each cup until the salt dissolves.

Step 3. Set up the conductivity tester. Connect the two bare wire tips so that dipping both into water completes the circuit through the water and lights the LED. Safety note: use only the 9-volt battery tester. Never test any wall outlet or household power, and do not taste any of the water.

Step 4. Dip both wire tips into Cup A, holding them the same distance apart each time. Rate how brightly the LED glows using the 1-to-5 card, where 1 is no glow and 5 is brightest. Record the rating.

Step 5. Rinse and dry the wire tips, then repeat Step 4 for Cups B, C, D, and E, keeping the tips the same distance apart each time. Record each brightness rating in the data table.

Step 6. Compare your ratings with another group's before you answer the analysis questions.

Data & Analysis

Cup	Salt added	Brightness rating (1-5)
A	none (distilled)
B	quarter teaspoon
C	half teaspoon
D	one teaspoon
E	two teaspoons

Which cup glowed brightest, and which cup gave little or no glow? Use your data.
Describe the pattern between the amount of salt in a cup and the brightness of the LED.
Distilled water barely lights the tester. Explain why adding salt lets the water carry more current.
Suppose one cup rated 5 and another rated 3 on the card. Find the difference in rating, and state which cup holds more dissolved salt.
A treatment operator uses a conductivity meter to estimate dissolved solids quickly. Use your results to explain why conductivity is a useful stand-in for dissolved solids.
Reverse osmosis removes dissolved ions from water. If water leaving an RO membrane lights the tester far more dimly than the water entering it, what does that tell you about how the membrane is working?
The card only allows whole-number ratings from 1 to 5. Name one instrument that would give a more precise reading, and explain why whole-number ratings limit the conclusions you can draw.
Predict what would happen to the brightness if you kept adding salt to Cup E, well past two teaspoons. Would the brightness keep rising the same way, level off, or drop? Justify your prediction.

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: Cup E glowed brightest; Cup A gave little or no glow.
Cup E has the most salt, so it conducts best, while distilled Cup A has almost no ions to carry current. Accept the pattern from the student's own data.
Answer: More salt gave a brighter glow.
As the salt increased from Cup A to Cup E, the brightness rating rose. Accept any answer that states brightness increased with salt, matching the group's data.
Answer: Salt splits into ions that carry electric charge through the water.
When salt dissolves, it separates into charged particles (ions). Those moving ions carry current between the wire tips, so more dissolved salt means more ions and a brighter light. Distilled water has very few ions, so it barely conducts.
Answer: 2 rating points.
The difference is 5 minus 3 = 2 rating points. The cup rated 5 conducts more, so it holds more dissolved salt.
Answer: Conductivity rises with dissolved solids, so it estimates them quickly.
The lab shows brightness (a stand-in for conductivity) rising with the amount of dissolved salt. Because more dissolved solids raise conductivity, a quick conductivity reading estimates the amount of dissolved solids without a slow lab test. Accept any answer linking conductivity to dissolved solids.
Answer: The membrane is removing most of the dissolved ions.
A much dimmer glow after the membrane means far fewer ions are left, so the reverse-osmosis membrane is successfully removing dissolved salts. Accept any answer connecting the dimmer light to fewer dissolved ions passing through.
Answer: A conductivity meter; whole numbers hide small differences.
A conductivity meter reports a precise number instead of a 1-to-5 estimate. Whole-number ratings cannot show small differences between cups, so two cups that differ slightly might get the same rating, which limits how finely the results can be compared.
Answer: The brightness would level off rather than keep rising the same way.
Accept a prediction that the increase slows and levels off, reasoning that the tester and the water reach a limit to how much current they carry. Accept a "keep rising" answer only if the student gives sound reasoning, and credit any prediction justified with the observed pattern.

Differentiation

Support

Build one tester as a class and demonstrate Cup A and Cup E first, so students see the two extremes before rating the middle cups. Provide a simplified data table with only Cups A, C, and E for students who need fewer trials.

Extension

Have groups test tap water and bottled water against their salt cups, then estimate where each falls on the brightness scale and discuss what that suggests about their dissolved solids.

ELL supports

Provide picture cards for "ion," "conductivity," and "distilled water," and a labeled 1-to-5 brightness scale with a shaded example for each number. Allow students to record ratings using the numbered card alone rather than written descriptions.