Water Auto‑Ionization, pH Scale, and Acid‑Base Definitions

Name: Acid Base Introduction
Uploaded: 2015-05-07T14:52:36+00:00
Duration: 18 min 36 s
Channel: Saylor University
Description: Summary and key takeaways on Acid Base Introduction: Summary & Key Takeaways, covering Water Auto‑Ionization If you just leave water to itself it auto‑ionizes.

Saylor University

May 07, 2015

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18 min video

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2 min read

YouTube video ID: fWmpjayHH0o

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If you just leave water to itself it auto‑ionizes. Two water molecules interact; one hydrogen atom (a proton) transfers to the other, producing a hydronium ion (H₃O⁺) and a hydroxide ion (OH⁻). At 25 °C the concentration of each ion in pure water is 10⁻⁷ M, giving a neutral pH of 7. Hydrogen ions are essentially protons that usually attach to a water molecule to form hydronium.

pH Scale

Chemists for whatever reason don't like dealing with negative exponents so they defined the pH. The pH value is the negative base‑10 logarithm of the hydrogen‑ion concentration:

[ \text{pH}= -\log_{10}[H^+] ]

This logarithmic scale compresses the wide range of possible hydrogen‑ion concentrations into a convenient numeric range, making it easier to compare acidity without handling very small numbers.

Arrhenius Definition

Arrhenius is always… you're always dealing with water, everything's in an aqueous solution. In this framework, acids increase the concentration of hydrogen ions in water, while bases increase the concentration of hydroxide ions. Strong acids—such as HCl, HBr, HI, HNO₃, H₂SO₄, and HClO₄—dissociate completely in a one‑way reaction, donating their proton to water to form hydronium and a conjugate anion. Strong bases, exemplified by Group 1 metal hydroxides like LiOH and NaOH, also dissociate completely, releasing free hydroxide ions.

Brønsted‑Lowry Definition

Lewis cares about electrons; Brønsted‑Lowry cared about protons. This definition broadens the scope beyond aqueous solutions: acids are proton donors and bases are proton acceptors. Because it does not require water as the medium, reactions such as acetic acid donating a proton to ammonia are comfortably described within this model.

Lewis Definition

The Lewis definition is the broadest of the three. Acids are electron‑pair acceptors and bases are electron‑pair donors, so it applies to reactions that involve no protons at all. A classic example is the interaction between boron trifluoride (BF₃) and a fluoride anion, where BF₃ accepts an electron pair from the fluoride ion. This framework captures a wide variety of acid‑base chemistry, including many non‑protic processes.

Takeaways

Water continuously reaches an equilibrium where two molecules produce hydronium and hydroxide ions, giving a hydrogen ion concentration of 10⁻⁷ M at 25 °C and a neutral pH of 7.
The pH scale is defined as the negative base‑10 logarithm of the hydrogen ion concentration, allowing chemists to avoid working directly with very small numbers.
The Arrhenius definition classifies acids as substances that increase hydrogen ion concentration and bases as substances that increase hydroxide ion concentration in water, with strong acids and bases dissociating completely in one‑way reactions.
The Brønsted‑Lowry definition expands acid‑base concepts to any proton donor and acceptor, enabling description of reactions that occur outside aqueous media, such as acetic acid reacting with ammonia.
The Lewis definition further broadens the framework by defining acids as electron acceptors and bases as electron donors, covering reactions like the interaction of boron trifluoride with a fluoride ion that involve no protons.

Frequently Asked Questions

Why does water auto‑ionize?

Water molecules constantly collide; when one donates a proton to another, the donor becomes hydroxide (OH⁻) and the acceptor becomes hydronium (H₃O⁺). This proton transfer establishes the auto‑ionization equilibrium, resulting in equal concentrations of H₃O⁺ and OH⁻ (10⁻⁷ M each) at 25 °C.

What distinguishes the Lewis acid‑base definition from the Arrhenius and Brønsted‑Lowry definitions?

The Lewis definition describes acids as electron‑pair acceptors and bases as electron‑pair donors, so it applies to reactions that do not involve protons, such as BF₃ accepting an electron pair from a fluoride ion. Arrhenius focuses on H⁺ or OH⁻ production in water, while Brønsted‑Lowry centers on proton transfer.

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Helpful resources related to this video

If you want to practice or explore the concepts discussed in the video, these commonly used tools may help.

General Chemistry Textbook For College Students Recommended

Provides comprehensive coverage of acid-base theories, equilibrium, and pH calculations to reinforce the lecture concepts.

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Digital Ph Meter For Water Testing

Allows the user to practically measure the pH of various solutions and observe the concepts of ion concentration in real-time.

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Molecular Model Kit For Chemistry Students

Helps visualize the structure of molecules like hydronium and hydroxide, making the proton transfer process easier to understand.

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Periodic Table Of Elements Wall Chart

Provides a quick reference for the elements involved in strong acids and bases, such as Group 1 metals and halogens.

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Full Transcript YouTube

in the last video we learned that if you
just leave water to itself it auto
ionizes so let me see I have two
molecules of water there's some
probability and it's an equilibrium that
one of the hydrogens on one of the
waters bumps off and just joins the
other one so you could end up with a
hydronium ion it's an ion because it has
an extra hydrogen so I'll put a plus
charge there and obviously everything
here is in an aquous solution I could
write aquous everywhere AQ
could write AQ it's kind of obvious
water is obviously in an aquous solution
it's obviously dissolved in water and
then plus a hydroxide ion this is the
water molecule that lost its its its
hydrogen atom or its proton because we
know that a hydrogen atom or at least a
hydrogen ion if you get rid of its
electron all you have left is a proton
and that's also an aquous solution and
we learned that people really care I
mean we learned about the equilibrium
constant of this reaction and we also
learned that people really care about
the concentration of this right here of
the hydronium or sometimes and let me
write this again because this is
actually what happens and I went over
this in the last video but sometimes the
same exact Auto ionization reaction will
be written like this H2O dis
disassociating in itself really into a
hydrogen ion in an aquous solution plus
a
hydroxide anion in an aquous solution or
a negative ion these are the same thing
and I write this way because this is
actually the state that happens that you
don't actually have these protons just
independently they actually do join onto
another water molecule and form a
hydronium ion but these are the same
these are the same exact concept what we
learned in the last video that people
really care about the concentration of
you could pick one of these two they
normally write this one right here so
people really care about the
concentration of
hydrogen of your hydrogen ions and we
learned in the last video that just pure
water at 25° C which is room temperature
so let me write that at
25° Celsius the concentration of of of
your of your hydrogen protons because
that's really what they are or hydrogen
ions is 10 Theus 7 molar and then we
even said hey you know people don't like
to chemists for whatever reason don't
like dealing with negative exponents so
they defined the
pH the pH
is equal to the minus log base 10 of
your hydrogen
concentration of your hydrogen
concentration and of course that's equal
to minus log base 10 of 10us 7 for pure
water at
25° which is equal to seven fair enough
and you could probably imagine if people
took the trouble of constructing this pH
thing and saying it's a negative log
they must really care about what the
hydrogen concentration of water is and
it does matter to in a in a whole bunch
of especially actually biological
systems all the time people worry about
the pH of this or that and obviously I
talked in the last video about pH
balance Cosmetics they they're
essentially all sorts of bad things
might happen to your skin if the pH of
the Cosmetic is different than your skin
I don't know if that's a fact but I
guess that's the that's the theory
behind it so there's obviously things in
the world that change the Hy hen
concentration that changed this
concentration so when we learned about
lalia principle we said hey you know if
we if we maybe if we add more hydrogen
on this side the equilibrium will change
and this concentration will go up and so
we would have more hydrogen
concentration I guess on the other side
you could add more hydroxide and then
you you would you would have more o
concentration maybe a little less
hydrogen
concentration so if you if you care
about things that are going to do that
to this reaction you probably want to
have a name for them and the name for
them is acids and bases and I'm sure
you've heard both of these words before
so acids let me write them both
down acids acid and
base now just to you know over
complicate your life a little bit more
there are actually multiple definitions
of an acid and multiple definitions of a
base and they kind of become more and
more Broad and and what they and what
they imply the the definition that
you're probably going to use the most
and it's definition that I always use in
my head whenever someone says an acid is
it
increases hydrogen
concentration when you place it in water
when you in
water in water and a base is something
that
increases that increases your hydroxide
concentration in water this is the
definition that you're probably going to
see in 90% especially in a first year
chemistry class and this is called the
urenus acid or base let me hope I get
his spelling right you should never take
any of the my spelling words at face
value you should look them up because
I'm clearly not a master speller arenus
arenus with an H these are arenus acid
orous base so for example if I were to
take if I were to let me give you an
example if I were to take
some if I were to take
some Hy hydrochloric acid that's
actually what it's called but you could
say hydrogen chloride in an aquous
solution it actually disassociates
completely this is not an equilibrium
reaction so let me put it in this nice
well let me use a bolder color this nice
oneway Arrow which says hey this isn't
one of these wishy-washy reactions that
go in both directions this is a this is
a strong acid it completely
disassociates in water into let me put
the hydrogen in a different color it
completely disassociates in water into a
hydrogen ion in
aquous
solution
plus chloride plus just a chloride
negative ion in an aqu solution and they
just float in there so you can imagine
if you place hydrogen chloride in water
you're going to increase it its overall
hydrogen concentration and the same
exact reaction could have been written
like this just to hit the point home
hydrogen chloride and an
aquous aquous solution and then you
could say
plus h let me do in different color plus
H2O it's a one-way
reaction it's a one-way reaction and
then you end up with and I think you
understand you have
H obviously not all three of them are
h3o it's too dark
h3o plus it donated its hydrogen proton
to that to this water molecule and then
you have plus the chloride annion in an
aquous solution and these this these
acids that completely dissociated in
future videos we're going to look at
ones that don't that are a little bit
more wishy-washy that don't completely
disassociate but these acids that
completely disassociate are called
strong acids and you know strong isn't
just you know just a nice word that they
use in chemistry it literally means when
someone says a strong acid something
that completely dissociates into water
it's a one-way reaction and all of the
strong acids I mean you could probably
even guess based on on their chemical
makeup but hydrochloric acid is one of
them you also have you know hydrogen
bromide another strong acid hydrogen
iodide hydrogen iodide you have nitric
acid I think I showed you that in a
previous all of these when you when you
put them into water they're just going
to this little H is going to pop off
join another water molecule form
hydronium and then this molecule right
here in these cases these are well in
the case of chloride or these are these
are halogens but they're going to go and
form negative ions what is left over
nitric acid then you have sulfuric acid
you've probably heard of that very
strong
acid sulfuric acid and then perchloric
acid these are the strong
acids o4 and these are good to know
because if you see them on a chemistry
exam you know that these are going to
completely disassociate and remember you
know we we keep using this word acid
what does acid mean it means that these
are strong acids these completely
disassociate and if you if you like
Arias's definition of what an acid is
they increase your hydrogen
concentration because you're obviously
when you throw this in some water all of
these all of these new hydronium ions
are going to be formed and they're going
to overall increase your your
concentration of them and that's why
they're called an acid and now if you
went on the on the strong base side
borenius is definition right by aras's
definition a base is something that
creates is something that creates
hydroxide ions or anion in water so
pretty much anything if you look at the
periodic
table anything here any of your group
one elements and group one elements your
Alkali earth metals your Alkali earth
metals that's bonded with hydroxide if
you put them in water the hydroxide pops
off so let me just ask you with lithium
or
sodium so for example these are strong
bases so if I take some if I take some
lith ium let me do nice
lithium
hydroxide in an aquous
solution aquous solution this is a this
completely disassociates completely
disassociates no equilibrium here it's a
it's a this is a strong base so what's
it going to disassociate to well the
little hydroxide ion is going to pop off
that's going to be a minus it's in water
still Plus
a
lithium plus a lithium ion right there
so this will always disassociate in
water the same thing as if you have if
you have
sodium if you have
sodium sodium is going to do the same
thing sodium hydroxide and aquous
solution one-way reaction you produce
hydroxide and some sodium cation and so
you imagine when you put these in water
this concentration is going to go up
it's just like what we did in the
chitalia principle where you know we
said oh what if you add some of this or
some of that to an equilibrium reaction
well how do you add it well in this case
you can add a strong acid or a strong
base now everything I've done so far
this is the arous definition where an
acid increases your hydrogen
concentration a base increases your
hydroxide concentration now that is
you're what you're going to see 90% of
the time but there's a slightly broader
definition out there and that is the and
I don't know the correct you know I
always say just Bron dead
but if you don't have fancy fonts it's
spelled like this
bronzee
lry
acid acid or base and I'll say and but
you know if you if you have good fonts
there's usually this little cross cross
in the O so I don't I Brun dead
brownstead well brownstead Lowry acid
and all of these would also be bronstad
low acid but but the the broadening of
the definition is an
acid is a proton donor proton donor and
a
base a
base is a
proton
proton
acceptor acceptor so let's look at this
definition and in the context of
everything we've just done so far so if
you by the Bron stead Lowry definition
what is a proton donor here well this
hydrochloric acid it is let me look at
this reaction right here this
hydrochloric acid is donating a proton
to this water molecule right the proton
a hydrogen atom is just a proton that's
an important thing to remember because
it had a hydrogen ion because if you get
rid of its electron it has no Neutron
it's just a proton sitting in sitting in
space so this hydrochloric acid is
donating a proton to this water molecule
to make a hydronium molecule and it gave
it away so this is a bronze dead lowy
acid as well just as it was an arenus
acid let's see the brownstead ver Lowry
version of a base okay a base is someone
who proton
acceptor so this guy he was he had this
nice little relationship with this
hydroxide ion this the hydroxide ion got
disassociated so you can kind of and it
becomes a little bit funy fun right here
you can kind of say hey this guy
accepted accepted this positive charge
or he was and and you know it's a little
bit wishy-washy here because this this
isn't uh you know it's not like this
someone gave it a proton you can kind of
think it as he actually gave away
electrons but you know if you just look
at the the final prodct okay he's got
he's got a positive charge when
everything was done so you say okay
maybe that's a bronze dead Lowry uh a
bronze dead Lowry B
now you're saying okay well why did
people even make the the trouble of
defining a bronze dead Lowry base when
you know all of the arenus acids and
bases could also be bronze Lowry and
well that's because arenus is always
You're always dealing with water
everything's in an aquous solution but I
drew an example here of a bron dead lowy
base it doesn't have to be an aquous
solution so if you have acetic acid
right here that's what's in vinegar plus
ammonia there's there doesn't have to be
an aquous solution what happens this
hydrogen gets donated to the ammonia ion
to make ammonium so this becomes
positive becomes negative it donated
this electron this proton right and
arenius has nothing to say for this he
he because everything he deals with is
hydronium and water but the Bron stead
Lowry definition Works in this situation
now the last kind of or the broadest
definition although you know there are
all these definitions but you're going
to be 90% of the time good if you just
know the arenus definition and just know
that Bron Lowry and and what I'm about
to say right now also exists and that's
Lewis acid and
bases Lewis acid and bases the acid now
Lewis cares about electrons bronstad
Larry cared about protons so Lewis
instead of saying an acid is a proton
donor they a LS acid says it's an
electron acceptor
electron
acceptor and the base is an
electron electron donor now let's look
at it in the context of everything we've
done so far so El if if if this is
really an acid it should be a electron
acceptor and the way you can kind of
think of it is you had hydrochloric acid
before when this guy gave away his when
he gave away this this atom or this this
proton right there he kind of kept his
electron so he was kind of an electron
acceptor it's a little bit it's a little
bit gray area there I mean it's not like
he took an electron from somebody else
but this would be considered still a LS
acid and if you think about a LS base a
leis base is what it's an
electron
donor right a leis base is an
electron
donor so if you have it right here loose
base is an electron donor this lithium
hydroxide when it goes into water
there's a couple of ways to think of it
you can say hey it's donating this o
which has these two extra these two
extra these two extra or or these this
negative charge so it's an electron
donor I don't like it's kind of squishy
that way so I'm not a big fan of it of
that definition I in my mind arenus
makes the most sense and it's kind of
the purest thing are you creating hyd
hydronium or not or are you creating
hydroxide or not but just to just to
show that the Lewis the Lewis acid base
definition is the broadest definition
here's a a case of a Lewis acid base
reaction that is that is that that would
not be considered either a bronze St
lowy or an arenus base because this
doesn't have to happen in water and what
you have here is boron Tri fluoride with
a fluoride annion it has a negative
charge it has this little extra electron
here in this situation this fluoride
right here or this this this FL this
Florine
annion can donate these two electrons so
we could let's say that this this this
one right here is this one right here
and what it does is it
donates these two electrons to this
Boron complex right here and so if it's
the electron donor what is it the
Florine is a base so this is the
ls Lewis
base and then the electron acceptor is a
LS acid so this right here is the ls
acid Lewis acid it's good to know that
these definitions exist and especially
if you don't get so you don't get
confused in the future but for a first
year chemistry class if you know the
urenus definition really well that's the
one that's going to that's you'll you'll
do very very well and frankly in my mind
that's the easiest one to contemplate
and that's the one that's going to
matter in most of the the reactions
where you deal with the changes in PH to
water because that's what all the acids
and bases do especially in arenus case
anyway see you in the next video and
we'll actually do some math figuring out
the changes in PH due to some acid

Help & FAQ

Biology 1A Lecture 2

Saylor University

Jun 13, 2026

pH Scale

Arrhenius Definition

Brønsted‑Lowry Definition

Lewis Definition

Takeaways

Frequently Asked Questions

Why does water auto‑ionize?

What distinguishes the Lewis acid‑base definition from the Arrhenius and Brønsted‑Lowry definitions?

Who is Saylor University on YouTube?

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