What is the strongest simple acid in water?

Perchloric acid (HClO4) is generally considered the strongest simple acid in aqueous solutions. However, superacids like Fluoroantimonic acid are over a billion times stronger than pure sulfuric acid.

What is the strongest base in water?

Cesium hydroxide (CsOH) and other heavy alkali hydroxides are the strongest common bases in water. Superbases like lithium diisopropylamide (LDA) exist but are used in non-aqueous environments.

Why is Hydrofluoric acid (HF) considered a weak acid?

Despite fluorine's high electronegativity, HF is a weak acid compared to HCl, HBr, and HI because the H-F bond is extremely strong and requires a lot of energy to break, preventing complete ionization in water.

Master Ionic Equilibrium! Explore our complete, categorized list of the 100 most common Acids and Bases, complete with pK_a / pK_b values, strengths, and 30 mind-blowing chemistry facts.

100 Most Common Acids & Bases (With Strengths & pK_a)

Understanding the strength of various acids and bases is the cornerstone of Physical and Organic Chemistry. Whether you are dealing with buffer solutions, titrations, or organic reaction mechanisms, knowing which compound will act as a strong acid or a weak base is crucial.

We have compiled the ultimate cheat sheet: 100 of the most important Acids, Bases, and Amphoteric compounds, neatly categorized by their dissociation strength (pK_a and pK_b). Bookmark this guide for your JEE, NEET, and Board Exam preparations!

📘 Ionic Equilibrium Notes 📄 Download Formula Sheets 📇 Flashcard Tool

🔥 Part 1: Important Acids & Their Strengths

1. Strong Acids (Completely Ionized in Water)

Extremely low pK_a (< -1)

No.	Formula	Acid Name	Strength / pK_a Note
1	HCl	Hydrochloric acid	Strong
2	HBr	Hydrobromic acid	Strong
3	HI	Hydroiodic acid	Strong
4	HNO₃	Nitric acid	Strong
5	HClO₄	Perchloric acid	Very Strong
6	HClO₃	Chloric acid	Strong
7	H₂SO₄	Sulfuric acid	1st proton strong (pK_a ≈ -3)
8	HBrO₄	Perbromic acid	Strong
9	HIO₄	Periodic acid	Strong
10	H₂SeO₄	Selenic acid	Strong

2. Moderately Strong Acids

Partially ionized, pK_a ≈ 0 to +3

No.	Formula	Acid Name	Strength / pK_a Note
11	H₃PO₄	Phosphoric acid	pK_a1 ≈ 2.1 (weak for further protons)
12	H₂SO₃	Sulfurous acid	pK_a1 ≈ 1.9 (weak)
13	HF	Hydrofluoric acid	Weak compared to HCl (pK_a ≈ 3.2)
14	HNO₂	Nitrous acid	pK_a ≈ 3.3
15	HCN	Hydrocyanic acid	Weak (pK_a ≈ 9.2)
16	HClO₂	Chlorous acid	pK_a ≈ 1.9
17	HClO	Hypochlorous acid	pK_a ≈ 7.5 (weak oxidizing)
18	H₂CO₃	Carbonic acid	pK_a1 ≈ 6.3 (weak)
19	H₂C₂O₄	Oxalic acid	pK_a1 ≈ 1.3 (strongest weak acid)
20	H₂CrO₄	Chromic acid	Moderate

3. Weak Organic Acids (Carboxylic & Phenolic)

pK_a ≈ 3 to 10

No.	Formula	Acid Name	Strength / pK_a Note
21	HCOOH	Formic acid	pK_a ≈ 3.7
22	CH₃COOH	Acetic acid	pK_a ≈ 4.7
23	C₂H₅COOH	Propionic acid	pK_a ≈ 4.9
24	C₆H₅COOH	Benzoic acid	pK_a ≈ 4.2
25	C₃H₆O₃	Lactate / Lactic acid	pK_a ≈ 3.8
26	C₄H₆O₆	Tartaric acid	Diprotic, pK_a1 ≈ 3.0
27	C₆H₈O₇	Citric acid	Triprotic, pK_a1 ≈ 3.1
28	C₄H₆O₄	Succinic acid	Diprotic, pK_a1 ≈ 4.2
29	cis-C₄H₄O₄	Maleic acid	pK_a1 ≈ 1.9
30	trans-C₄H₄O₄	Fumaric acid	pK_a1 ≈ 3.0
31	C₆H₅OH	Phenol	pK_a ≈ 10.0 (very weak)
32	o-NO₂C₆H₄OH	o-Nitrophenol	pK_a ≈ 7.2 (stronger due to -NO₂)
33	p-NO₂C₆H₄OH	p-Nitrophenol	pK_a ≈ 7.1
34	CH₃C₆H₄OH	Cresol	pK_a ≈ 10.2
35	C₆H₄(OH)₂	Resorcinol	pK_a ≈ 9.3
36	C₆H₄(OH)₂	Hydroquinone	pK_a ≈ 10.0
37	(NO₂)₃C₆H₂OH	Picric acid (2,4,6-trinitrophenol)	pK_a ≈ 0.4 (Strong organic acid)
38	o-HOC₆H₄COOH	Salicylic acid	pK_a ≈ 2.9
39	C₆H₈O₆	Ascorbic acid (Vitamin C)	pK_a1 ≈ 4.2
40	C₅H₄N₄O₃	Uric acid	pK_a ≈ 5.4 (weak diprotic)

💧 Part 2: Important Bases & Their Strengths

4. Strong Bases (Completely Ionized in Water)

Very low pK_b

No.	Formula	Base Name	Strength Note
41	NaOH	Sodium hydroxide	Strong
42	KOH	Potassium hydroxide	Strong
43	LiOH	Lithium hydroxide	Strong
44	RbOH	Rubidium hydroxide	Strong
45	CsOH	Cesium hydroxide	Strong
46	Ca(OH)₂	Calcium hydroxide	Strong (but less soluble)
47	Sr(OH)₂	Strontium hydroxide	Strong
48	Ba(OH)₂	Barium hydroxide	Strong
49	Mg(OH)₂	Magnesium hydroxide	Sparingly soluble
50	TlOH	Thallium hydroxide	Strong

5. Weak Inorganic Bases & Organic Amines

Partially ionized, pK_b ≈ 3 to 10

No.	Formula	Base Name	Strength / pK_b Note
51	NH₃	Ammonia	pK_b ≈ 4.7
52	NH₄OH	Ammonium hydroxide	Weak
53	CaCO₃	Calcium carbonate	Acts as a weak base
54	Na₂CO₃	Sodium carbonate	Moderately strong base
55	NaHCO₃	Sodium bicarbonate	Weak amphoteric base
56	Al(OH)₃	Aluminium hydroxide	Weak / Amphoteric
57	Zn(OH)₂	Zinc hydroxide	Weak / Amphoteric
58	Pb(OH)₂	Lead hydroxide	Weak
59	Fe(OH)₂	Iron(II) hydroxide	Weak
60	Fe(OH)₃	Iron(III) hydroxide	Weak
61	CH₃NH₂	Methylamine	pK_b ≈ 3.4
62	C₂H₅NH₂	Ethylamine	Weak base
63	(CH₃)₂NH	Dimethylamine	pK_b ≈ 3.3 (stronger than 1°)
64	(CH₃)₃N	Trimethylamine	pK_b ≈ 4.2
65	C₆H₅NH₂	Aniline	Aromatic, pK_b ≈ 9.4
66	CH₃C₆H₄NH₂	o-Toluidine	pK_b ≈ 9.3
67	CH₃C₆H₄NH₂	p-Toluidine	pK_b ≈ 9.2
68	C₅H₅N	Pyridine	pK_b ≈ 8.8
69	C₉H₇N	Quinoline	Heterocyclic, pK_b ≈ 9.2
70	C₃H₄N₂	Imidazole	Moderate (conjugate acid pK_a ≈ 7.0)

6. Biochemical Bases & Amino Acids

No.	Name	Type / Strength Note
71	Adenine	Nucleobase, weak base (pK_b ≈ 4)
72	Guanine	Nucleobase, weak base
73	Cytosine	Nucleobase, moderate base
74	Thymine	Nucleobase, weak base
75	Uracil	Nucleobase, weak base
76	Histidine	Amino acid (Imidazole side chain, pK_a ≈ 6.0)
77	Lysine	Amino acid (-NH₂ group, strongly basic, pK_a ≈ 10.5)
78	Arginine	Amino acid (Guanidino group, strongest basic a.a., pK_a ≈ 12.5)
79	Tyrosine	Amino acid (Phenolic group, weak acid/base balance)
80	Cysteine	Amino acid (Thiol group, weak acid/base)

⚖️ Part 3: Amphoteric Compounds & Other Acids

7. Amphoteric Compounds (Act as Acid & Base)

No.	Formula	Compound Name & Behavior
81	H₂O	Water (Neutral, amphoteric self-ionizer)
82	NH₃	Ammonia (Can form NH₄⁺ or NH₂^-)
83	HCO₃^-	Bicarbonate ion (Can form H₂CO₃ or CO₃^2-)
84	HSO₄^-	Bisulfate ion
85	H₂PO₄^-	Dihydrogen phosphate
86	HPO₄^2-	Hydrogen phosphate
87	ZnO	Zinc oxide
88	Al₂O₃	Aluminium oxide
89	PbO	Lead(II) oxide
90	Cr(OH)₃	Chromium(III) hydroxide

8. Other Important Acids

No.	Formula	Acid Name	Strength Note
91	H₂S	Hydrosulfuric acid	Weak, pK_a ≈ 7.0
92	H₂Se	Hydroselenic acid	Stronger, pK_a ≈ 3.9
93	H₂Te	Hydrotelluric acid	Stronger, pK_a ≈ 2.6
94	HClO₄ derivs.	Perchloric acid derivatives	Very strong oxidizing acids
95	HBrO₃	Bromic acid	Moderate strong
96	HIO₃	Iodic acid	Moderate strong
97	H₂MnO₄	Manganic acid	Weak
98	HMnO₄	Permanganic acid	Strong oxidizing
99	HReO₄	Perrhenic acid	Very strong
100	H₂SiF₆	Fluorosilicic acid	Strong acid in Industry

💡 30 Interesting Facts About Acids & Bases

🧪 Acid Facts

1. Strongest Simple Acid: Perchloric acid (HClO₄) is often considered the strongest acid in aqueous solutions.

2. HF is Deceptively Weak: Hydrofluoric acid (HF) is weaker than HCl, HBr, and HI because of its very strong H–F bond which prevents complete ionization.

3. Superacids: These are stronger than 100% sulfuric acid. Example: Fluoroantimonic acid (HSbF₆) is over a billion times stronger than H₂SO₄.

4. Natural Fruit Acids: Citric acid (lemons), malic acid (apples), tartaric acid (grapes), and ascorbic acid (Vitamin C).

5. Ant Stings: Formic acid (HCOOH) is the painful chemical present in ant and bee stings.

6. Stomach Acid: The human stomach contains HCl, keeping the pH between 1.5 and 3.5 for digestion.

7. Acid Rain: Primarily caused by H₂SO₄ and HNO₃ formed from SO₂ and NO_x pollution in the atmosphere.

8. Picric Acid: 2,4,6-trinitrophenol is one of the strongest organic acids and is highly explosive.

9. Carbonic Acid: H₂CO₃ is a weak acid but is vital for maintaining human blood pH balance.

10. Vinegar: Simply a dilute aqueous solution of acetic acid (CH₃COOH, ~5–8%).

🧪 Base Facts

11. Strongest Aqueous Base: Cesium hydroxide (CsOH) and other heavy alkali hydroxides are extremely strong.

12. Superbases: Bases stronger than OH^-, like lithium diisopropylamide (LDA) or sodium hydride (NaH), used in organic synthesis.

13. Ammonia (NH₃): A weak base but industrially massive, used primarily for fertilizers and cleaning agents.

14. Baking Soda: Sodium bicarbonate (NaHCO₃) is a weak base used in baking and as a medical antacid.

15. Milk of Magnesia: A suspension of Magnesium hydroxide, used to safely neutralize excess stomach acid.

16. Soap is Basic: Soap solutions feel slippery and are basic because they are sodium salts of weak fatty acids.

17. Strongest Known Base: The ortho-diethynylbenzene dianion is considered the strongest base, existing only in special non-aqueous conditions.

18. Egg Whites: Are naturally slightly basic, with a pH of around 8.

19. Blood pH: Human blood is slightly basic, strictly maintained at a pH of ~7.35–7.45.

20. Amphoterism: Hydroxides and oxides like Al(OH)₃ and ZnO can act as both an acid and a base depending on the environment.

⚖️ General Facts

21. The pH Scale: Runs from 0 (strongly acidic) to 14 (strongly basic), with 7 as perfectly neutral at 25°C.

22. Negative pH: Yes, it exists! Extremely concentrated strong acids (like >1M HCl) can have a pH < 0.

23. Basic Extremes: Highly concentrated NaOH solutions can push the pH scale up to ~15.

24. Litmus Test: The classic indicator turns red in the presence of an acid, and blue in a base.

25. Universal Indicator: A blend of dyes that shows a full rainbow of colors to indicate specific pH values.

26. Water's Secret: At 25°C, pure water self-ionizes so that [H⁺] = [OH^-] = 10^-7 M.

27. Bronsted vs. Lewis: Bronsted-Lowry defines acids/bases by proton (H⁺) transfer. Lewis defines them by electron pair transfer.

28. The Conjugate Rule: Strong acids yield extremely weak conjugate bases (e.g., Cl^-). Strong bases yield extremely weak conjugate acids (e.g., Na⁺).

29. Extraterrestrial Acid: Venus has thick clouds of sulfuric acid (H₂SO₄) that rain down, but evaporate before hitting the scorching surface.

30. pK_a & Strength: The lower the pK_a, the stronger the acid. The lower the pK_b, the stronger the base.

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