# Which Water Conducts

Laplace’s Mini World · Plate 91 · Matter, Light & Sound · Matter

> The same two rods, a battery and a small lamp: in distilled water the lamp stays dark; stir in salt and it lights.

- Interactive model: https://world.lapla.si/en/conductivity/
- Embed: https://world.lapla.si/embed/en/conductivity/
- 中文: https://world.lapla.si/zh/conductivity/
- Updated: 2026-11-04

## 01 Distilled Water, a Dark Lamp

Two carbon rods stand in distilled water, wired through a small lamp to a 6 V battery. The lamp stays dark. The water seems to block nothing, yet the current does not pass.

Pure water conducts very little, but not nothing. At 25 °C its conductivity is only 0.055 µS⁠/⁠cm. Only a very few water molecules split into hydrogen and hydroxide ions: about two in every billion.

Distilled water standing in air takes up carbon dioxide, which raises its conductivity to about 1 µS⁠/⁠cm. In this page’s model such water has about 360 000 Ω between the two rods; the current is about 0.012 mA, and the filament stays cold.

*Pure water 0.055 µS⁠/⁠cm (25 °C) · standing in air about 1 µS⁠/⁠cm · the page’s model: about 0.012 mA, a dark lamp*

## 02 Stir in Salt

Salt goes into the same distilled water. The grains dissolve as they sink; the filament glows red, then lights.

In this page’s model, with 6 g of salt in every 100 g of the solution (20 °C), the conductivity is about 81 mS⁠/⁠cm, more than 80 000 times the distilled water’s. The resistance between the rods falls to about 4.4 Ω, the current rises to about 270 mA, and the lamp gives about half its rated light.

Fine bubbles rise from the rods: hydrogen at the negative one, chlorine and oxygen at the positive. While a current passes through brine the electrodes keep reacting; industry uses this to split brine into chlorine, hydrogen and caustic soda (see Electrolysis of Brine).

*6% brine (20 °C) about 81 mS⁠/⁠cm · about 270 mA · the lamp about half lit (the page’s model)*

## 03 Ions Carry the Charge

Table salt is a crystal of sodium and chloride ions. In water, water molecules gather round, pull the ions apart and surround them: NaCl → Na⁺ + Cl⁻. The charged ions move freely through the water.

With the battery connected, one rod is positive and the other negative. The positive sodium ions drift towards the negative rod, the negative chloride ions towards the positive. As the Royal Society of Chemistry’s teaching notes put it, it is the movement of the positive and negative ions that conducts the electricity in solution.

In the wires it is electrons that move, but electrons do not swim from the rods into the water. At each rod’s surface they change hands by chemical reactions: the negative rod hands electrons to water and hydrogen comes off; the positive rod takes electrons from chloride ions and water, and chlorine and oxygen come off. The circle beside the bench magnifies a patch of water a few nanometres across between the rods; the ions drift there far faster than they really do, so the direction shows.

*NaCl → Na⁺ + Cl⁻ · positive ions to the negative rod, negative ions to the positive · electrons in the wires, ions in the water*

## 04 Slow Ions, a Quick Lamp

Ions drift slowly. In a field of 1 V⁠/⁠cm a sodium ion moves only about 5.2 µm a second, a chloride ion about 7.9 µm; a sodium ion takes about 32 minutes to cover 1 cm.

Yet the lamp lights the moment the circuit closes. There are ions everywhere between the rods; once the field is there they all start drifting together, and nothing waits for one ion to cross from one rod to the other.

Hydrogen ions are much faster, about 36 µm a second, seven times a sodium ion. In the Royal Society of Chemistry’s words, the hydronium ion need not move at all: its extra hydrogen is passed along neighbouring water molecules like a parcel. So hydrochloric acid conducts far better than salt water of the same strength.

*At 1 V⁠/⁠cm: Na⁺ 5.2 · Cl⁻ 7.9 · H⁺ 36 µm⁠/⁠s · Na⁺ takes about 32 minutes for 1 cm*

## 05 Strong, Weak and None

Dissolve 2 g of each in 98 g of water and try them in turn. In this page’s model: hydrochloric acid about 296 mA, the lamp near its rated light; salt water about 203 mA, the lamp at about 8% of it; acetic acid about 9.5 mA, the filament dark; sugar water about 0.012 mA, the same as distilled water.

The difference is how much of what dissolves becomes ions. Salt and hydrogen chloride in dilute solution become almost all ions: strong electrolytes. Acetic acid splits only a little: a weak electrolyte; at 0.1 mol⁠/⁠L about 1.33% of it is ionised, and in vinegar with 5 g of acetic acid per 100 mL only about 0.46%. Sucrose stays as whole molecules in water: a nonelectrolyte.

So vinegar does conduct; its current is just too small to heat this lamp’s filament: about 14 mA, about a twentieth of 6% salt water’s. At the same number of moles, salt gives about 216 times as many ions as vinegar.

*All at 2% (the page’s model): hydrochloric acid 296 mA · salt water 203 mA · acetic acid 9.5 mA · sugar water 0.012 mA*

## 06 More Salt, Brighter, Up to a Point

More salt, a brighter lamp. But each extra spoonful adds less conductivity than the one before. At 20 °C, from 0.5% salt to 25%, the salt per kilogram of water rises 66 times; the conductivity only from 8.2 to 222 mS⁠/⁠cm, 27 times.

The more crowded the ions, the more they hold one another back: the conductivity each mole of salt brings falls from 95 at 0.5% to 39 at 25% (in mS⁠/⁠cm per mol per kg of water). At 20 °C, 100 g of water dissolves at most about 35.9 g of salt; the rest lies on the bottom.

The lamp has its own limit. In this page’s model the lamp gives about 74% of its rated light at 10% salt, and only its rated light at 25%: the current stops near 300 mA, held back by the filament’s own resistance. Hydrochloric and sulfuric acid differ: their conductivity peaks at one strength and falls beyond it.

*Brine (20 °C): 0.5% 8.2 · 5% 70.1 · 25% 222 mS⁠/⁠cm · 66 times the salt, 27 times the conductivity*

## 07 Measuring Water

A laboratory uses a conductivity meter, not a lamp. It measures the conductance between two electrodes and turns it into conductivity using the electrodes’ size and spacing (the cell constant), in µS⁠/⁠cm or mS⁠/⁠cm.

The meter uses alternating current. Direct current would draw the ions steadily to the electrodes, where they react and change the solution; alternating current rocks them back and forth where they are. Readings are also corrected to 25 °C, since conductivity rises about 2% for every degree.

Some figures: pure water 0.055 µS⁠/⁠cm; distilled water standing in air about 1; tap water anywhere from 50 to 800, Singapore’s averaging 232 in 2025; seawater about 53 mS⁠/⁠cm at 25 °C, about 230 times Singapore’s tap water. The EU drinking water directive sets 2 500 µS⁠/⁠cm (20 °C) as an indicator; China’s GB 5749-2022 has no conductivity limit, only one on dissolved solids, 1 000 mg⁠/⁠L.

*Pure water 0.055 µS⁠/⁠cm · distilled about 1 · tap water 50⁠–⁠800 · seawater about 53 mS⁠/⁠cm (25 °C)*

## 08 Wet Hands and Nerves

OSHA, the US workplace safety agency, puts it this way: pure water is a poor conductor, but small amounts of impurities such as salt or acid make water conduct; dry skin has a fairly high resistance, and moist or wet skin acts as a conductor. NIOSH gives the figures: dry, the body’s resistance may be as high as 100 000 Ω; wet or broken skin may drop it to 1 000 Ω. So keep wet hands away from sockets and appliances.

British school safety sheets class battery work below 30 V as a low electrical hazard, and say to keep water away from conductors live above 30 V. This page’s tester runs on a 6 V battery; for a battery’s own voltage and resistance, see Electromotive Force.

Nerves signal with ions too: a nerve impulse lets sodium ions into the cell, and the voltage across its membrane flips (see Nerve Impulse).

*The body’s resistance: up to 100 000 Ω dry · down to 1 000 Ω wet (NIOSH) · battery work below 30 V*

## 09 1834 and 1884

In 1834 Faraday published a set of new words: electrode, anode and cathode, electrolyte, ion, with anion for what goes to the anode and cation for what goes to the cathode. Anode, cathode, anion, cation and ion were suggested by William Whewell of Cambridge in a letter of 5 May that year. Faraday’s ions were what the current set free at the electrodes.

From 1873 to 1876 Friedrich Kohlrausch measured electrolytes with alternating current, using a telephone in place of a galvanometer, and showed that a solution has a definite resistance: Ohm’s law holds for it too.

In 1884 Svante Arrhenius argued in his doctoral thesis that an electrolyte dissolved in water is already split, to a varying degree, into positive and negative ions, without waiting for a current to split it. Uppsala gave the thesis only a fourth class. In 1903 he received the Nobel Prize in Chemistry for his theory of electrolytic dissociation.

*1834 Faraday’s names · 1873⁠–⁠1876 Kohlrausch measures with AC · 1884 Arrhenius’s dissociation · 1903 Nobel Prize*

## 10 Laid Out Piece by Piece

Eleven pieces on the bench: the lit and the dark tester (each with a holder of four AA cells and two leads), five 250 mL beakers (sugar water, distilled water, tap water, vinegar, dilute hydrochloric acid), a jar of salt, a dish of sugar, a bottle of vinegar and a pocket meter. Each tester’s wooden bridge carries a lamp holder, two terminals and two carbon rods, the lamp and the rods in series with the cells.

*Testers · cells · carbon rods · beakers · salt · sugar · vinegar · pocket meter*

## 11 Choose the Liquid

Under the lit tester is distilled water; into it goes one thing, salt, sugar, acetic acid or hydrochloric acid, as much as you choose. The lamp, the current and the ions in the magnifier follow.

This is the page’s model: a 6 V battery, a 3.8 V 0.3 A lamp, two carbon rods 5 mm across and 48 mm apart, wetted for 26 mm; conductivities from the CRC Handbook’s table at 20 °C; a back voltage at the rods of 1.7 V. Hydrochloric acid goes only to 2%, the table’s last row.

*The page’s model · conductivity from the 20 °C table · the lamp’s light from its rerating rule*

**Try this**: In the lab, take acetic acid at its strongest: how bright is the lamp, and what is the current? Then try hydrochloric acid: what does it give at the same 2%?

## What the model leaves out

The bench is this page’s model, not a copy of any one set. The beakers follow DURAN’s 250 mL low form (70 mm across, 95 mm high), the rods a school catalogue’s 5 mm by 100 mm. The lamp is drawn larger than life: the small lamps schools use are 11 mm across; its electrical values are those of an apparatus maker’s 3.8 V 0.3 A lamp. In the circuit the battery gives 6 V with no internal resistance; the back voltage at the rods is 1.7 V for every liquid, the Encyclopædia Britannica’s (1911) figure for acidified water between platinum plates; brine needs at least 2.19 V in theory, so the lamp in salt water would really be a little dimmer than the model shows. The resistance between the rods is that of two parallel rods in an unbounded liquid, without the beaker’s wall or the rods’ ends, so the real resistance is higher. The lamp’s current and light follow a lamp maker’s rerating rule (current as voltage to the 0.55, light to the 3.5), less exact further from the rating. Conductivities come from the 20 °C table, scaled in proportion below its first row (acetic acid as the square root). The sinking salt, the bubbles and the drifting ions are drawn; in the magnifier the ions drift far faster than they do, and the shell of water molecules round each ion is left out.

## Easy to get wrong

It is not the water that conducts, and electrons do not swim in from the wires: in solution ions carry the charge; electrons move only in the wires and change hands by chemical reactions at the electrodes’ surfaces. Pure water conducts too, only very weakly. “Fully ionised” is said of dilute solutions; in strong brine the ions hold one another back and conduct less well. Vinegar does conduct; its current is just too small to light this lamp. However much sugar dissolves, it gives no ions. A lamp’s light does not follow its current: at about seven tenths of the current it gives only about a tenth of its light. Wet hands are dangerous because sweat and tap water hold ions, and wet skin has a far lower resistance.

## Common questions

### Why does pure water conduct so little?

Only a very few water molecules split into hydrogen and hydroxide ions: about two in every billion.

See: [01 Distilled Water, a Dark Lamp](https://world.lapla.si/en/conductivity/#pure)

### Why does salt water conduct electricity?

Table salt is a crystal of sodium and chloride ions. In water, water molecules gather round, pull the ions apart and surround them: NaCl → Na⁺ + Cl⁻. The charged ions move freely through the water.

See: [03 Ions Carry the Charge](https://world.lapla.si/en/conductivity/#ions)

### What are strong and weak electrolytes?

Salt and hydrogen chloride in dilute solution become almost all ions: strong electrolytes. Acetic acid splits only a little: a weak electrolyte; at 0.1 mol⁠/⁠L about 1.33% of it is ionised, and in vinegar with 5 g of acetic acid per 100 mL only about 0.46%.

See: [05 Strong, Weak and None](https://world.lapla.si/en/conductivity/#kinds)

### Why does a conductivity meter use alternating current?

The meter uses alternating current. Direct current would draw the ions steadily to the electrodes, where they react and change the solution; alternating current rocks them back and forth where they are.

See: [07 Measuring Water](https://world.lapla.si/en/conductivity/#meter)

### Why are wet hands dangerous near electricity?

NIOSH gives the figures: dry, the body’s resistance may be as high as 100 000 Ω; wet or broken skin may drop it to 1 000 Ω. So keep wet hands away from sockets and appliances.

See: [08 Wet Hands and Nerves](https://world.lapla.si/en/conductivity/#body)

## Sources

1. [OpenStax: Chemistry 2e, 11.2 (electrolytes and nonelectrolytes, strong and weak; a solution conducts if it holds ions, compared by a lamp’s brightness in the circuit)](https://openstax.org/books/chemistry-2e/pages/11-2-electrolytes)
2. [OpenStax: Chemistry 2e, 11.4 (NaCl → Na⁺ + Cl⁻; sucrose a nonelectrolyte)](https://openstax.org/books/chemistry-2e/pages/11-4-colligative-properties)
3. [OpenStax: Chemistry 2e, 14.3 and appendix H (acetic acid Ka = 1.8 × 10⁻⁵; about 1.3% ionised at 0.10 mol⁠/⁠L)](https://openstax.org/books/chemistry-2e/pages/14-3-relative-strengths-of-acids-and-bases)
4. [OpenStax: Chemistry 2e, 17.7 (electrolysis: hydrogen at the negative electrode, chlorine and oxygen at the positive; brine needs at least 2.186 V)](https://openstax.org/books/chemistry-2e/pages/17-7-electrolysis)
5. [OpenStax: Biology 2e, 35.2 (a nerve impulse lets sodium ions into the cell)](https://openstax.org/books/biology-2e/pages/35-2-how-neurons-communicate)
6. [IAPWS: G1-90, the electrolytic conductivity of water (pure water 0.0550 µS⁠/⁠cm at 25 °C)](https://iapws.org/technical-guidance/release/conduct.download)
7. [Mettler Toledo: Conductivity Guide (the cell constant; measuring with AC; temperature coefficients; CO₂ from the air adds about 1 µS⁠/⁠cm; ions’ molar conductivities)](https://www.tau.ac.il/~chemlaba/Files/conductivity_guide_EN%20(2).pdf)
8. [N. Jaffrezic-⁠Renault (2008): Sensors 8(4), table 1 (ions’ molar conductivities at infinite dilution)](https://pmc.ncbi.nlm.nih.gov/articles/PMC3673432/)
9. [W. J. Hamer and H. J. DeWane (1970): NSRDS-⁠NBS 33 (the conductance of hydrochloric acid; Kohlrausch’s square-⁠root law)](https://nvlpubs.nist.gov/nistpubs/Legacy/NSRDS/nbsnsrds33.pdf)
10. [Emerson (Rosemount): Conductance Data for Commonly Used Chemicals (the peak conductivities of hydrochloric, sulfuric and acetic acid)](https://www.emerson.com/documents/automation/manual-conductance-data-for-commonly-used-chemicals-rosemount-en-68896.pdf)
11. [N. P. Fofonoff and R. C. Millard (1983): UNESCO Technical Papers in Marine Science 44 (PSS-⁠78’s temperature ratio)](https://vliz.be/imisdocs/publications/ocrd/122840.pdf)
12. [TEOS-⁠10: gsw\_C3515 (standard seawater 42.914 mS⁠/⁠cm at 15 °C)](https://www.teos-10.org/pubs/gsw/html/gsw_C3515.html)
13. [US FDA: CPG Sec. 525.825 (vinegar: 4 g of acetic acid per 100 mL)](https://www.fda.gov/regulatory-information/search-fda-guidance-documents/cpg-sec-525825-vinegar-definitions-adulteration-vinegar-eels)
14. [Royal Society of Chemistry: Education in Chemistry, Shocking revelations 2 (hydronium ions pass the charge on like a parcel)](https://edu.rsc.org/download?ac=518618)
15. [Royal Society of Chemistry: Electrolysis practical videos 14⁠–⁠16 (6 V DC, two carbon electrodes; the movement of positive and negative ions conducts in solution)](https://edu.rsc.org/practical/electrolysis-practical-videos-14-16-students/4011983.article)
16. [CLEAPSS: Student Safety Sheet 10 (battery work below 30 V: a low electrical hazard)](https://science.cleapss.org.uk/resource/All-in-one-Student-Safety-Sheets.pdf)
17. [United Scientific: Conductivity of Solutions Apparatus CSA001 (6 V DC, a 3.8 V 0.3 A miniature lamp)](https://image-resize-v3.s3.amazonaws.com/1218/products/142667/0P1AxZkg_csa001-conductivity-of-solutions-apparatus.pdf)
18. [JKL Components: Lamp Voltage Re-⁠Rating (a miniature lamp’s current goes as voltage to the 0.55, its light to the 3.5)](https://www.jkllamps.com/home/?p=2061)
19. [Encyclopædia Britannica, 11th edition (1911): Conduction, Electric (Kohlrausch, 1873⁠–⁠1876, AC and a telephone)](https://en.wikisource.org/wiki/1911_Encyclop%C3%A6dia_Britannica/Conduction,_Electric/Liquids)
20. [Encyclopædia Britannica, 11th edition (1911): Electrolysis (a steady current only above about 1.7 V)](https://en.wikisource.org/wiki/Page:EB1911_-_Volume_09.djvu/242)
21. [EURAMET.QM-⁠S12 (2020): electrolytic conductivity at pure water level (CO₂ from the air adds around 1 µS⁠/⁠cm)](https://bipm.org/documents/20126/44473623/EURAMET.QM-S12.pdf/79cecfba-d241-384e-c091-d267eb7875aa)
22. [US EPA: Volunteer Stream Monitoring 5.9, Conductivity (distilled water 0.5⁠–⁠3 µS⁠/⁠cm)](https://archive.epa.gov/water/archive/web/html/vms59.html)
23. [California State Water Resources Control Board: Electrical Conductivity/Salinity Fact Sheet (tap water 50⁠–⁠800 µS⁠/⁠cm)](https://www.waterboards.ca.gov/water_issues/programs/swamp/docs/cwt/guidance/3130en.pdf)
24. [Singapore PUB: Drinking Water Quality 2025 (conductivity 232 on average, 98⁠–⁠625 µS⁠/⁠cm)](https://www.pub.gov.sg/-/media/PUB/PDF/SG-Drinking-Water-Quality.pdf)
25. [Directive (EU) 2020⁠/⁠2184, annex I part C (conductivity 2 500 µS⁠/⁠cm at 20 °C)](https://eur-lex.europa.eu/eli/dir/2020/2184/oj)
26. [GB 5749-2022, standards for drinking water quality (dissolved solids 1 000 mg⁠/⁠L; no conductivity limit)](https://faolex.fao.org/docs/pdf/chn222304.pdf)
27. [OSHA 3075: Controlling Electrical Hazards (pure water conducts poorly, impurities make it conduct; wet skin conducts)](https://www.osha.gov/sites/default/files/publications/osha3075.pdf)
28. [NIOSH 98-131: Worker Deaths by Electrocution (the body’s resistance up to 100 000 Ω dry, down to 1 000 Ω wet)](https://webarchive.library.unt.edu/eot2008/20090116090514id_/http://www.cdc.gov/niosh/docs/98-131/overview.html)
29. [M. Faraday (1834): Experimental Researches in Electricity, Seventh Series, §§ 662⁠–⁠665 (electrode, anode, cathode, electrolyte, ion)](https://www.gutenberg.org/cache/epub/14986/pg14986.txt)
30. [Epsilon: the Faraday letters, W. Whewell to M. Faraday, 5 May 1834 (anode, cathode, anion, cation, ion)](https://epsilon.ac.uk/view/faraday/letters/Faraday0716)
31. [NobelPrize.org: Svante Arrhenius, Biographical (the thesis of 1884; the Nobel Prize in Chemistry 1903)](https://www.nobelprize.org/prizes/chemistry/1903/arrhenius/biographical/)
32. [H. A. M. Snelders: Arrhenius, Svante August, Dictionary of Scientific Biography (the thesis got only a fourth class)](https://www.encyclopedia.com/people/science-and-technology/chemistry-biographies/svante-august-arrhenius)
33. [HORIBA: LAQUAtwin EC-⁠11⁠/⁠22⁠/⁠33 manual (164 × 29 × 20 mm; two-⁠electrode AC; 2%/°C compensation)](https://cdn.shopify.com/s/files/1/0529/9529/3345/files/LAQUAtwin_EC-11__EC-22___EC-33_Manual_April_2017.pdf)
34. [DWK: DURAN low-⁠form beakers (250 mL: 70 mm across, 95 mm high)](https://tienda.dicsa.es/beakers-glass-duran-low$B$einfo252_lang_uk.htm)
35. [Philip Harris: Carbon Electrodes (5 mm across, 100 mm long)](https://philipharris.co.uk/product/chemistry/gas-preparation-and-analysis/electrolysis/carbon-rod/b8a37242)

## Data

Conductivities of salt, hydrochloric and acetic acid solutions from the CRC Handbook of Chemistry and Physics (91st ed.); pure water from IAPWS G1-90; single-⁠ion conductivities, the cell constant, AC measurement and temperature coefficients from Mettler Toledo’s Conductivity Guide and N. Jaffrezic-⁠Renault (Sensors, 2008); hydrochloric acid from NBS NSRDS-⁠33 and Emerson; electrolytes, Ka, Kw and electrolysis from OpenStax Chemistry 2e; seawater from UNESCO’s PSS-⁠78 and TEOS-⁠10; vinegar from the US FDA; the lamp’s rerating rule from JKL Components. The currents, the lamp’s light, the drift speeds and the ratios are computed here; the tester is the page’s own model. The demonstration after the Royal Society of Chemistry (electrolysis practicals and Education in Chemistry), CLEAPSS Student Safety Sheets and United Scientific’s CSA001 apparatus; water after EURAMET, the US EPA, the California Water Boards, Singapore’s PUB, Directive (EU) 2020⁠/⁠2184 and GB 5749-2022; safety after OSHA 3075 and NIOSH 98-131; nerves after OpenStax Biology 2e; history after Faraday’s Experimental Researches, the Faraday⁠–⁠Whewell letters (Epsilon), the Encyclopædia Britannica (1911), the Dictionary of Scientific Biography and NobelPrize.org. The bench is this page’s model: 250 mL low-⁠form beakers of 70 × 95 mm (after DURAN’s table), carbon rods of 5 × 100 mm (after Philip Harris’s catalogue), a 3.8 V 0.3 A lamp drawn larger than life, a pocket meter of 164 × 29 × 20 mm (after HORIBA’s LAQUAtwin manual); not a copy of any one set. Material presets and the model kit as on the site’s other model plates (public/looks/CREDITS.md).

## Licence CC BY-NC 4.0

The text, pictures and models on this page are licensed CC BY-NC 4.0: free to use and adapt for teaching and other non-commercial purposes, with credit to world.lapla.si. Third-party material listed above keeps its own licence.

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## Cite this plate

Laplace’s Mini World, Plate 91, “Which Water Conducts”, https://world.lapla.si/en/conductivity/, CC BY-NC 4.0, updated 2026-11-04.