Curious which salts dissolve better in acid than in pure water? Here is what happens, where it matters, how to predict it, when it helps in labs and industry, who uses it, and why pH controls solubility. Acid protonates basic anions, shifting equilibria and pulling more solid into solution. Use these simple rules and data to make fast, correct calls.
Why Acidity Changes Salt Solubility
When a salt contains a basic anion, added H plus removes that anion from solution by protonation. The solid responds by dissolving more to replace the lost ions.
This is Le Chatelier in action. Carbonates, hydroxides, oxides, phosphates and sulfides all react with H plus to form weak acids or neutral molecules. For example, MCO3 plus 2 H plus gives M2 plus, H2O and CO2 gas. Acid reduces the free basic anion, so the dissolution equilibrium shifts toward more dissolved ions.
Gas release strengthens the pull. Carbonate and sulfide salts often dissolve faster in acid because CO2 or H2S leaves the liquid and drives the reaction forward.
Which Anions Make Salts More Soluble in Acid
Look first for a weak base as the anion. If that anion has a conjugate acid with a pKa in the acid or near neutral range, acid will boost solubility.
Good signals include pKa values such as H2CO3 pKa1 about 6.35, H3PO4 pKa1 about 2.15 and HS minus to H2S pKa1 about 7.0 at 25 C. These values mean H plus can protonate CO3 2 minus, PO4 3 minus and S 2 minus in common lab acids. We then see more dissolution and, for carbonate and sulfide, gas evolution. The lower the pKa of the conjugate acid, the stronger the pull.
Oxides and hydroxides behave in the same way. They neutralize acid to form water, which removes the basic species and increases solubility.
Be careful with toxic gases. Sulfides can release H2S and cyanides can release HCN in acid, so work in a hood with proper training and monitoring.
Common Salts That Dissolve Better in Acid
Here are well known salts whose solubility increases in acidic solutions compared to neutral water. These examples are widely used in teaching labs and industry.
- Calcium Carbonate CaCO3 and Magnesium Carbonate MgCO3: acid produces CO2 gas, water and soluble Ca2 plus or Mg2 plus.
- Magnesium Hydroxide Mg(OH)2 and Aluminum Hydroxide Al(OH)3: acid neutralizes OH minus to water, raising dissolved metal ions.
- Calcium Phosphate Ca3(PO4)2: stepwise protonation to HPO4 2 minus and H2PO4 minus increases solubility at low pH.
For CaCO3, Ksp is about 3.3 × 10 to the minus 9 at 25 C, yet it dissolves quickly in 0.1 M HCl because CO2 escapes. For Mg(OH)2 with Ksp about 5.6 × 10 to the minus 12, even mild acid removes OH minus and dissolves the solid. Whenever the anion changes to a weakly dissociated acid or a gas, expect a large solubility boost in acid.
Salts That Do Not Gain Much Solubility in Acid
Not all salts respond. Chlorides like AgCl and many sulfate salts of large cations show little change in common mineral acids.
Barium sulfate is the classic case. Ksp is about 1.1 × 10 to the minus 10 at 25 C, and it stays very insoluble in hydrochloric acid and nitric acid. Medical imaging uses BaSO4 as a contrast agent by mouth because it does not dissolve in stomach acid.
The common ion effect can also stop dissolution. Adding sulfate lowers BaSO4 solubility, and adding chloride lowers AgCl solubility. Unless an acid reacts with the anion to a weaker species, simply lowering pH may not help.
Quick Steps To Predict Solubility Shift With pH
Use this short checklist before you reach for the acid bottle.
- Identify the anion and ask if it is a weak base, such as CO3 2 minus, PO4 3 minus, OH minus, O 2 minus or S 2 minus.
- Check the conjugate acid pKa values. If pKa1 is below about 8, acid will likely increase solubility.
- Look for gas forming reactions. CO2 from carbonates and H2S from sulfides strongly drive dissolution.
- Watch for common ions and complexation. Added anions that match the solid can reduce solubility instead.
Temperature also matters because Ksp and acid dissociation change with heat. Ionic strength and background electrolytes can shift activity and thus measured solubility. Test a small sample when the matrix is complex or unknown. Predict with data, then confirm with a quick bench trial.
Data Table: Ksp, pKa and Acid Reaction Clues
Use these values at 25 C as a guide. They show why certain salts are more soluble in acidic solutions than in water.
| Salt | Ksp | Conjugate Acid pKa1 | Key Acid Reaction | Effect of Acid |
|---|---|---|---|---|
| Calcium Carbonate CaCO3 | ≈ 3.3 × 10^-9 | H2CO3 ≈ 6.35 | CO3 2 minus + 2 H plus → H2O + CO2 gas | Large increase, fast dissolution |
| Magnesium Hydroxide Mg(OH)2 | ≈ 5.6 × 10^-12 | H2O pKa neutralization | OH minus + H plus → H2O | Large increase with any mineral acid |
| Iron(II) Sulfide FeS | Very low, varies | H2S ≈ 7.0 | S 2 minus + 2 H plus → H2S gas | Strong increase, releases H2S |
| Calcium Phosphate Ca3(PO4)2 | ≈ 1 × 10^-26 | H3PO4 ≈ 2.15 | PO4 3 minus + H plus → HPO4 2 minus, H2PO4 minus | Increase at low pH |
| Barium Sulfate BaSO4 | ≈ 1.1 × 10^-10 | HSO4 minus ≈ 1.99 | SO4 2 minus poorly protonated | Little change in most acids |
Carbonate and hydroxide rows show direct neutralization to weak species, so solubility jumps. The sulfate row shows weak response because SO4 2 minus is not easily protonated by typical acids. The anion chemistry explains the trend every time.
Real Uses in Labs, Industry and Nature
Scale removal relies on this chemistry. Acid cleaning of CaCO3 deposits in pipes and heat exchangers works because CO2 forms and the solid dissolves. In the stomach, antacids like Mg(OH)2 neutralize HCl and become more soluble as they react. Ocean acidification data show surface pH has fallen from about 8.2 to 8.1 since the 1800s, which increases CaCO3 dissolution in marine systems.
Wastewater plants adjust pH to precipitate or dissolve species as needed, for example dissolving carbonates before reprecipitating as targeted solids. Soil science uses acid to mobilize phosphate from calcium phosphate in calcareous soils.
Analytical labs confirm carbonate by adding acid and detecting CO2 release, classic in field kits and teaching labs. When gas bubbles appear, you are watching solubility shift with pH in real time.
FAQ
Which salts are more soluble in acidic solutions than in water?
Salts with basic anions such as carbonates, hydroxides, oxides, phosphates and sulfides dissolve better in acid. Acid protonates the anion to a weaker species or a gas, which pulls the solid into solution.
Why does acid increase the solubility of carbonate salts?
H plus converts CO3 2 minus to carbonic acid, which breaks down to CO2 and water. The loss of CO3 2 minus and CO2 escape shift the equilibrium toward more dissolved Ca2 plus or Mg2 plus.
Does barium sulfate dissolve more in acid?
No, BaSO4 remains very insoluble in hydrochloric acid and nitric acid, which is why it is safe for GI imaging. SO4 2 minus is not easily protonated by these acids, so the Ksp limit dominates.
How can I predict if a salt will dissolve better in acid?
Check if the anion is a weak base and look up the conjugate acid pKa values. If protonation gives a weakly dissociated acid or a gas, expect higher solubility at low pH.
Do temperature and ionic strength change solubility in acid?
Yes, Ksp and acid dissociation constants vary with temperature, and background ions change activities. Run a small test at your actual conditions for the most reliable result.
Are there safety risks when dissolving salts in acid?
Yes, sulfides can release H2S and cyanides can form HCN in acid, both very hazardous. Work in a fume hood with proper detectors and follow your safety rules.
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