Prof. Priya Mehta

M.Sc. Chemistry, 15 years teaching experience
Specialization: Organic Chemistry, Board Exam Expert
DAV Public School, Pune

Q1: If alcohol has -OH group like acids, why isn’t it acidic?

Great question! This confuses many students. Yes, both alcohol (C₂H₅OH) and acids like HCl have hydrogen, but the key difference is in how that hydrogen is bonded. In HCl, the hydrogen is bonded to a highly electronegative chlorine atom, making the H-Cl bond polar and easy to break in water, releasing H⁺ ions. In alcohol, the hydrogen in the -OH group is bonded to oxygen, which is also electronegative, but the C-O bond is much stronger and more stable. The hydrogen in alcohol’s -OH group doesn’t easily break away as H⁺ in water because the electron density distribution doesn’t favor ionization. Additionally, even if the -OH hydrogen could ionize slightly (which it does, but extremely weakly), the concentration of H⁺ ions would be so negligible that it wouldn’t show measurable acidic properties. This is why we classify alcohol as neutral, not acidic. In organic chemistry (Class 11-12), you’ll learn that alcohols can show very weak acidic character only under special conditions with very strong bases, but in aqueous solution at Class 10 level, they’re considered neutral.

Q2: Does dry HCl gas show acidic properties? Why or why not?

Excellent exam-focused question! No, dry HCl gas does NOT show acidic properties. This is a very important concept that often appears in board exams. Here’s why: Acidic properties require the presence of H⁺ ions (or H₃O⁺ ions). For HCl to produce these ions, it must dissociate, and this dissociation only occurs in the presence of water. Dry HCl gas consists of HCl molecules with intact H-Cl bonds. Without water, there’s no medium for ionization to occur. If you place dry litmus paper in dry HCl gas, the litmus paper will NOT change color. But the moment you introduce moisture (water), HCl dissolves and ionizes (HCl → H⁺ + Cl⁻), and immediately the litmus turns red. This is why we always say “acids show acidic character in aqueous solution” – the water is essential for ionization. This concept also explains why concentrated sulfuric acid (which has very little water) is less acidic than dilute sulfuric acid in terms of showing typical acidic reactions. Remember: No water = No ionization = No acidic properties, even for strong acids like HCl!

Q3: What’s the difference between H⁺ and H₃O⁺? Which one should I write in exams?

This is a common doubt, and I’m glad you asked! Both representations are correct, but they represent slightly different things. When an acid like HCl dissociates in water, it releases H⁺ (a hydrogen ion, which is basically just a proton). However, this bare proton doesn’t exist freely in aqueous solution – it immediately attaches to a water molecule to form H₃O⁺ (hydronium ion). So technically, H₃O⁺ is the more accurate representation of what actually exists in acidic solutions. However, for simplicity, we often write H⁺ in equations, and it’s understood that we mean H₃O⁺. For CBSE Class 10 board exams, both are acceptable! You can write either HCl → H⁺ + Cl⁻ OR HCl + H₂O → H₃O⁺ + Cl⁻, and you’ll get full marks. However, if you write H₃O⁺, it shows a deeper understanding of the chemistry involved, which examiners appreciate. My recommendation: use H⁺ for quick answers and H₃O⁺ when you have time to write the complete equation with water. The important thing is to show that ionization is occurring and hydrogen ions are being produced – that’s what gets you the marks!

Q4: Why does acetic acid (vinegar) conduct electricity but glucose doesn’t, even though both are organic compounds?

Fantastic observation! This question shows you’re thinking critically. The key difference is that acetic acid (CH₃COOH) is an acid, while glucose is not. Even though both are organic compounds containing carbon, hydrogen, and oxygen, their behavior in water is completely different. Acetic acid has a carboxyl group (-COOH) where the hydrogen is ionizable. When acetic acid dissolves in water, it partially dissociates: CH₃COOH ⇌ CH₃COO⁻ + H⁺. This produces ions (acetate ions and hydrogen ions), which can conduct electricity. That’s why vinegar (dilute acetic acid) conducts electricity and shows acidic properties. Glucose, on the other hand, has hydroxyl groups (-OH) where the hydrogen is non-ionizable and covalently bonded. When glucose dissolves, it remains as intact C₆H₁₂O₆ molecules – no ions are formed, so no conductivity, no acidic properties. This is a perfect example of how molecular structure determines chemical behavior. The presence of a carboxyl group (-COOH) makes acetic acid acidic, while glucose with only hydroxyl groups (-OH) remains neutral. Remember this distinction – it’s very important for understanding organic chemistry in higher classes!

Q5: Can we say that all compounds that conduct electricity in aqueous solution are acids?

No, this is a common misconception that I need to clear up! Not all compounds that conduct electricity are acids. Electrical conductivity simply means that ions are present in the solution – but those ions don’t have to be H⁺ ions. Let me explain with examples: Acids like HCl conduct electricity because they produce H⁺ and Cl⁻ ions. But bases like NaOH also conduct electricity because they produce Na⁺ and OH⁻ ions. Salts like NaCl conduct electricity because they produce Na⁺ and Cl⁻ ions. So conductivity is a property of all electrolytes (substances that produce ions in solution), not just acids. To identify specifically an acid, you need to check for H⁺ ion production, which can be done through litmus test (turns blue litmus red), pH test (pH < 7), or testing for typical acid reactions like reacting with metals to produce H₂ gas. So the correct statement would be: "All acids conduct electricity in aqueous solution, but not all compounds that conduct electricity are acids." This is an important distinction that often appears in MCQs and assertion-reason questions in board exams!

Q6: If I add HCl to alcohol, will the mixture show acidic properties?

Great practical question! Yes, if you add HCl to alcohol, the mixture will show acidic properties – but only because of the HCl, not because of the alcohol! Here’s what happens: When you mix HCl with alcohol, the HCl can still dissociate (though less effectively than in water) to produce some H⁺ ions, giving the mixture acidic character. The alcohol itself still doesn’t contribute any H⁺ ions – it remains neutral. Think of it like this: if you mix sugar (neutral) with lemon juice (acidic), the mixture tastes sour because of the lemon juice, not because the sugar became acidic! However, there’s an important point to note: HCl dissociates much better in water than in alcohol because water is a better ionizing solvent. So HCl in pure alcohol will show weaker acidic properties compared to HCl in water. This is why in chemistry, we always specify “aqueous solution” when talking about acids – water is the ideal medium for ionization. In your board exam, if asked about acidic character, always discuss aqueous solutions unless the question specifically mentions otherwise. This shows you understand that the solvent matters for ionization!