Why does the solubility of Group 2 hydroxides in water increase down the group?

Firstly, we must consider the thermodynamics involved

It is known that a reaction is spontaneous due to thermodynamic favourability if the Gibbs Free Energy is negative. This is further broken down to entropy and enthalpy terms.

Upon dissolution of a salt, e.g. a hydroxide, the entropy increases (a favourable term) in all cases as the solid is separated into ions, which are free to move in solution.

However, in order for the ions to separate, the lattice must be broken. This requires breakage of the electrostatic forces of attraction between oppositely charged ions. Thus, energy must put in equal tot he lattice enthalpy - this is a very large positive term and so is unfavourable.

The above statements have only considered forced between the ions in the lattice, however, upon dissolution, the free ions interact with solvent molecules. In our example, we are using water. The polar water molecules can interact favourably with both types of ions and this introduces a further enthalpy term, known as the hydration enthalpy. If this interaction is favourable enough (large negative enthalpy) to overcome the energy required to break the lattice, the salt will dissolve.

Now we can consider the group 2 hudroxides and since the anion is identical in each case, we will only examine the cations. The early hydroxides, e.g. CaOH, are comprised of smaller cations (with a larger charge density) and thus have a very large lattice enthalpy. Interactions with water molecules are not great due to their relative sizes and so the hydration enthalpy is not large enough to overcome the electrostatic forces within the lattice and so the salt does not dissolve.

The opposite holds true for larger cations, e.g. Barium, and so later hydroxides are more readily dissolved in water

Note: HSAB theory has not been discussed as it is not taught at A Level (tot he best of my knowledge)

Eleni N. GCSE Biology tutor, A Level Biology tutor, GCSE Chemistry tu...

5 months ago

Answered by Eleni, an A Level Chemistry tutor with MyTutor

Still stuck? Get one-to-one help from a personally interviewed subject specialist


£24 /hr

Sarah T.

Degree: PGCE (Masters) - Durham University

Subjects offered: Chemistry, Psychology+ 1 more


“About me: In 2015 I graduated from Durham University. Whilst there I had the most amazing time studying Psychology, Chemistry and Education. Science has always interested me and I find it fascinating how we can use scientific methods ...”

£20 /hr

Maria F.

Degree: Natural Sciences (Bachelors) - York University

Subjects offered: Chemistry, Physics+ 1 more


“I am a second year BSc Natural Sciences undergraduate at the University of York, specialising in Physics and Biology. I’ve always been fascinated by science, finding out how everything works and how the universe fits together, especia...”

MyTutor guarantee

£20 /hr

Alex C.

Degree: Natural Sciences (Bachelors) - University College London University

Subjects offered: Chemistry, Physics+ 3 more

Further Mathematics
.BMAT (BioMedical Admissions)

“Hi, I'm a first year undergraduate at University College London studying Natural Sciences. I've always had a passion for science and think that tutoring is a great way to bring that across and inspire it in others.  In the past I've t...”

MyTutor guarantee

About the author

Eleni N.

Currently unavailable: for new students

Degree: Chemistry (Masters) - Edinburgh University

Subjects offered: Chemistry, Physics+ 2 more


“About Me: I am a Chemistry student at the University of Edinburgh, but I didn't always start out on this path. I spent my first two years at university persuing a  Biochemistry degree and I took every opportunity to do more science sub...”

You may also like...

Other A Level Chemistry questions

Why does a salt not conduct electricity in its solid state?

What is the difference in kinetics between a 0th, 1st and 2nd order reaction?

The recommended daily allowance of methionine for an adult is 15 mg per kg of body mass. Tuna contains 755 mg of methionine per 100 g portion. Calculate the mass, in grams, of tuna that would provide the RDA of methionine for a 60 kg adult.

What is entropy?

View A Level Chemistry tutors


We use cookies to improve our service. By continuing to use this website, we'll assume that you're OK with this. Dismiss