A bulb barely lights in distilled water, but it glows once salt is dissolved. As salt dissolves in water it splits into charged particles called ions.
An atom that loses electrons becomes a positive ion (cation), and one that gains electrons becomes a negative ion (anion); a substance that splits into ions when dissolved in water is an electrolyte.
Apply a voltage in the diagram and the cations are drawn toward the negative electrode and the anions toward the positive one; the higher the voltage, the more ions move and the faster they go.
When ions reach an electrode they gain or lose electrons and turn into different substances; splitting substances with an electric current this way is electrolysis.
What Are Ions?
⚡ Atoms That Lost or Gained Electrons
①An atom that loses electrons becomes a positively charged cation, e.g. Na → Na⁺ + e⁻
②An atom that gains electrons becomes a negatively charged anion, e.g. Cl + e⁻ → Cl⁻
③Because ions are charged, a voltage makes them move toward the electrode of opposite charge
④The movement of ions is how current flows through a solution
Electrolysis
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Cations and anions moving to opposite electrodes when a voltage is applied to an electrolyte solution
6 V
🔬 How Electrolysis Works
①Put two electrodes in an electrolyte solution and apply a voltage
②Cations (+) are drawn to the negative electrode and gain electrons (reduction)
③Anions (−) are drawn to the positive electrode and lose electrons (oxidation)
④Example: in electrolysis of copper(II) chloride solution, red copper coats the negative electrode and chlorine gas forms at the positive electrode
⑤A higher voltage gives a larger current and a faster reaction
Electrolytes and Non-electrolytes
💧 Substances Whose Solutions Conduct
①Electrolytes split into ions in water, so their solutions conduct: sodium chloride, copper(II) chloride, hydrochloric acid
②Non-electrolytes form no ions in water, so their solutions do not conduct: sugar, ethanol
③Pure water has very few ions and barely conducts
④Solid salt does not conduct because its ions cannot move; it conducts once dissolved
Passing a current through an electrolyte solution breaks substances down
🎯 Common Test Points
①Cations (+) move to the negative electrode
②Anions (−) move to the positive electrode
③An electrolyte is a substance that splits into ions in water
④Electrons are gained at the negative electrode (reduction) and lost at the positive electrode (oxidation)
⑤Electrolysis of water: hydrogen at the negative electrode and oxygen at the positive, in a 2 : 1 volume ratio
Examples and Unit Review
Example 1
When sodium (Na) and chlorine (Cl) atoms become ions, does each lose or gain electrons? Write equations showing which ions form.
1
Sodium loses one electron and becomes a cation: Na → Na⁺ + e⁻.
2
Chlorine gains one electron and becomes an anion: Cl + e⁻ → Cl⁻.
▸ Na → Na⁺ + e⁻ (cation); Cl + e⁻ → Cl⁻ (anion)
Chlorine takes the electron sodium gives up, forming Na⁺ and Cl⁻, which attract each other to make salt (sodium chloride).
Example 2
When water is electrolyzed, which gas forms at each electrode, and what is the volume ratio of the two gases?
1
Hydrogen gas forms at the negative electrode and oxygen gas at the positive electrode.
2
A water molecule, H₂O, has twice as many hydrogen atoms as oxygen atoms, so hydrogen : oxygen is 2 : 1 by volume.
▸ Hydrogen at the negative electrode, oxygen at the positive; volume ratio hydrogen : oxygen = 2 : 1
Pure water barely conducts, so a little sodium hydroxide is dissolved in it before the experiment to let current flow.
Unit Test
Which statement about ions and electrolysis is correct?
①When an atom loses an electron it becomes an anion
②In electrolysis cations move to the anode
③An electrolyte dissolves in water, splits into ions, and lets current flow
④Sugar water is an electrolyte that conducts current well
⑤Electrolyzing water gives a hydrogen-to-oxygen volume ratio of 1 : 2
▸ ③ An electrolyte dissolves in water, splits into ions, and lets current flow
1
Losing an electron makes a cation (① wrong), and in electrolysis cations move to the cathode (② wrong).
2
Sugar water is a non-electrolyte that forms no ions (④ wrong), and electrolyzing water gives hydrogen : oxygen = 2 : 1 (⑤ wrong); an electrolyte dissolves in water, splits into ions, and lets current flow (③ correct).