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Ions and conductivity of water

  1. Sep 3, 2015 #1
    I have studied that Ions conduct electricity in solutions due to their ability to move freely.BUT EVEN THE WATER MOLECULES MOVE FREELY, but they don't conduct electricity.What's so special in ions? I have studied the entire electrolysis process , in which the ions are attracted to the electrodes. But kindly elaborate , what goes on inside during conduction.
    Please consider that I am a grade 8 student.
  2. jcsd
  3. Sep 3, 2015 #2
    Water is mostly electrically neutral. (It has a dipole, like a magnet, but still has a net charge of 0.)

    Ions (by definition) have an electrical charge. Some are positive and some negative. So when the ions move, the charge moves with them.

    Salts have both charges. For example table salt is NaCl. The sodium (Na) gives an electron to the chlorine (Cl) becoming Na+ and Cl-when mixed with water.

    So pure water is an insulator, but adding impurities like salt makes it conductive.
  4. Sep 3, 2015 #3
    Howcome moving ions conduct electricity and moving molecules don't? Could you give a detailed explanation of how are ions able to conduct electricity?
  5. Sep 3, 2015 #4
    Ions have a net charge, molecules do not.
  6. Sep 3, 2015 #5
    How do the charges help in conduction, please kindly elaborate the whole process
  7. Sep 3, 2015 #6
    Electricity is not just that which happens in a wire.

    Electricity is the movement of charge. ( And also the presence of net charge is also a part of electricity )

    For the wire, the movement of charge is with the negatively charged electrons.
    I assume you already know that part of electricity.

    Ions are either positively or negatively charged. ( If it was not charged by a lack of or a surplus of an electron, it would not be an ion, but neutral just like your water molecule ). So to be an ion, it has to have a charge.
    Thus when ions move, the charge of the ion also moves. And moving charge is electricity.
  8. Sep 3, 2015 #7
    Charge is a basic aspect of reality. We don't know why. We just notice that some subatomic particles have charge and others don't. The electron is the most common charge used because it is lightweight and travels easily. Ions are lots heavier, but still charged. So usually we think of electricity as electrons but any charge will do.

    Charge is a conserved quantity. It can't be created or destroyed but can change form.

    Like charges repel each other. Opposite charges attract. This attraction/repulsion is called the electric force and is quite strong.

    Moving charges are called electric current. (They create a magnetic field. I don't think that's relevant to this discussion though.) This current is often called electricity. (The word "electricity" includes lots of other ideas like that magnetic field we are ignoring.)

    Atoms (and molecules) normally have an equal number of protons (with + charges) and electrons (with - charges), so their total charge is zero. Sometimes though an atom or molecule gives up an electron. It then has a net charge of +1. We call such an atom or molecule an ion. Similarly some atoms or molecules can accept an extra electron giving them a net charge of -1. They are also called ions. Basically any atom or molecule that's got a net charge is called an ion.

    So moving molecules are electricity -- as long as they are also ions.

    BTW, saying "Ions conduct electricity" really means that moving ions are a form of electricity. But ions are much larger and heavier than electrons so they can't move in wires like electrons can.
  9. Sep 3, 2015 #8
    Alright, then consider an electrolysis setup. The electrons start up from the battery and reach the cathode, right? Thereafter, positive ions move to the cathode and get deposited there, and they say anode becomes positive but I don't understand how, now the negative ions move towards the anode, Is this what is happening? And do explain how the anode became positive?
  10. Sep 3, 2015 #9
    Consider an electrolysis setup where a battery is connected to two electrodes. The electrodes suddenly become positively charged and negatively charged, how?
  11. Sep 3, 2015 #10
    The electric charge is very powerful. This forces things to try to remain neutral (all things being equal).

    From above, we have two types of ions, positive ions which are called cations and negative ions called anions.

    A cathode is defined as an electrode that attracts cations (+ ions) while an anode attracts anions.

    In electrolysis (of water) the cathode has a strong enough potential (built up electric charge, powered by the battery) to rip the hydrogen out of the water molecule forming H+ cations which stick to the cathode due to the large negative charge there (which is caused by too many electrons in the metal). The hydrogen then grabs one of the extra electrons and becomes hydrogen gas H, which is unstable until it forms H2 which is stable.

    Meanwhile at the anode, there aren't enough electrons, so the leftover oxygen anions O-2 are attracted to the positive charge and surrender the leftover electrons becoming O gas. Since this is also unstable, two of them join together into O2 gas.
  12. Sep 3, 2015 #11


    Staff: Mentor

    Chemical reactions at the surface of the electrode move charge to or from the electrolyte.
  13. Sep 3, 2015 #12
    But sir , the ions come later right? When the electrodes have already got charged. How did they have potential difference in the first place?
  14. Sep 3, 2015 #13


    Staff: Mentor

    No, there are already ions in the electrolyte to begin with.

    How much chemistry have you taken?
  15. Sep 3, 2015 #14
    Not much, But the electrolyte forms ions only after the passage of electric current. How did we obtain anode and cathode in the beginning? How did we derive the potential difference between electrodes to start up?
    Last edited: Sep 3, 2015
  16. Sep 3, 2015 #15


    Staff: Mentor

    No, this is not true. The ions are in the electrolyte with or without a current. The difference is that without a current they move only randomly and isotropically, via diffusion, but when they have a net movement then that is current.
  17. Sep 3, 2015 #16
    Okay, so how did we get the potential difference between the electrodes?
  18. Sep 3, 2015 #17


    User Avatar
    Gold Member

    You connected the battery.
    The battery has a potential difference between its electrodes. The conducting wires you use to join it to your electrolytic cell make your electrodes have the same potential difference as the battery electrodes.

    I'm not sure why you said, "The electrodes suddenly become positively charged and negatively charged". Anything you connect to a battery changes "suddenly" when it is connected. When you connect a bulb to a battery, it "suddenly" lights up, but we don't usually comment on the suddenness: when it is connected, it lights when it is not connected, it doesn't light. Your electrodes are the same.
  19. Sep 3, 2015 #18


    Staff: Mentor

    Chemical reactions at the surface of the electrode. See post 11 where I already answered this.

    Do you want to know the details of such chemical reactions? It didn't sound like you had a solid background in chemistry yet.
    Last edited: Sep 3, 2015
  20. Sep 3, 2015 #19
    Are you sure you are answering the OP question?
    She is talking about electrolysis and not about a voltaic cell.
    The electrodes are connected to a battery. The potential difference between the electrodes exists even before you deep them in the electrolyte and is due to the battery.
    Even if you deep them in non-ionic, non-reactive liquid (like oil), the potential difference is still there.
    Last edited: Sep 3, 2015
  21. Sep 3, 2015 #20
    Thank You Merlin 3189 ,Mr.DaleSpam and nasu !
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