What is the pH Scale and How Does it Classify Acids and Bases?
Acids and Bases is essential in chemistry and helps students understand various practical and theoretical applications related to this topic.
What is Acids and Bases in Chemistry?
An acid is a substance that donates hydrogen ions (H⁺) when dissolved in water, while a base is a substance that accepts hydrogen ions or releases hydroxide ions (OH⁻). This concept appears in chapters related to acid-base theories, pH scale, and titration methods, making it a foundational part of your chemistry syllabus.
Molecular Formula and Composition
There is no single molecular formula for all acids and bases, as these terms describe a wide group of compounds. For example, hydrochloric acid (HCl) is an acid; sodium hydroxide (NaOH) is a base. Both can be simple molecules or ions and are categorized under Arrhenius, Bronsted-Lowry, or Lewis definitions based on their structure and behavior.
Preparation and Synthesis Methods
Acids can be prepared by dissolving non-metal oxides (like SO2 or CO2) in water, reacting some salts with strong acids, or by industrial synthesis, such as the manufacturing of sulfuric acid (H2SO4) using the Contact Process. Bases are typically prepared by dissolving metal oxides or hydroxides in water, reacting metals with water (to produce alkalis like NaOH), or through other industrial methods like the electrolysis of brine for sodium hydroxide production.
Physical Properties of Acids and Bases
Acids are usually sour in taste, turn blue litmus paper red, and conduct electricity in solution. Bases are bitter, feel slippery or soapy, turn red litmus blue, and also conduct electricity when aqueous. Most acids have a pH less than 7, while most bases have a pH greater than 7. Strong acids and bases are highly corrosive and should be handled with care.
Chemical Properties and Reactions
Acids react with metals to produce hydrogen gas, with bases to form salts and water (neutralization), and with carbonates or bicarbonates to release carbon dioxide. Bases react with acids, ammonium salts, and certain metal ions. Both acids and bases participate in various redox and displacement reactions. For example, mixing HCl and NaOH yields NaCl (salt) and H2O (water).
Frequent Related Errors
- Confusing acids and bases with neutral molecules or unrelated compounds.
- Forgetting that some compounds can act as both acids and bases (amphoteric behavior).
- Mistaking strong/weak with concentrated/dilute solutions.
- Ignoring that some base reactions are not water-soluble (not all bases are alkalis).
Uses of Acids and Bases in Real Life
Acids and bases are widely used in industry and at home. Acetic acid is used in vinegar, hydrochloric acid is used for cleaning, and sulfuric acid is key in battery production. Sodium hydroxide and ammonia are used in soap, detergent manufacturing, and household cleaning. Students often encounter acids and bases in food chemistry, medicine (antacids), and agriculture (soil treatment). These examples show their importance in daily life.
Relevance in Competitive Exams
Students preparing for NEET, JEE, and Olympiads should be familiar with acids and bases, as this topic frequently appears in MCQs, conceptual questions, and calculation-based problems involving definitions, reaction types, pH calculations, and applications.
Relation with Other Chemistry Concepts
Acids and bases are closely related to concepts such as neutralization reactions, pH scale, acid and base strengths, and salt formation. These relations help students make connections between theoretical chemistry and practical laboratory results.
Step-by-Step Reaction Example
1. Consider the reaction between hydrochloric acid and sodium hydroxide:2. Write the balanced chemical equation:
3. State reaction conditions: Both reactants must be in an aqueous solution, at room temperature.
4. Explain the process: H⁺ ions from HCl combine with OH⁻ from NaOH to form water, and Na⁺ joins Cl⁻ to form sodium chloride.
5. Final Answer: This is a classic neutralization reaction.
Lab or Experimental Tips
Remember: acids turn blue litmus red, bases turn red litmus blue. Use universal indicators to identify solutions. Vedantu educators advise always wearing gloves and eye protection when handling strong acids or bases in the lab.
Try This Yourself
- Write the IUPAC name of acetic acid.
- Decide if Na2CO3 is acidic, basic, or neutral in solution.
- Give two real-life uses of sodium hydroxide.
Final Wrap-Up
We explored acids and bases—their structure, properties, reactions, and real-life importance. For more in-depth explanations, online quizzes, and exam-prep tips, check out the live classes and notes available on Vedantu.
| Examples of Acids | Examples of Bases |
|---|---|
| Hydrochloric acid (HCl) | Sodium hydroxide (NaOH) |
| Sulfuric acid (H2SO4) | Potassium hydroxide (KOH) |
| Acetic acid (CH3COOH) | Calcium hydroxide (Ca(OH)2) |
| Nitric acid (HNO3) | Ammonia (NH3) |
| Citric acid (C6H8O7) | Magnesium hydroxide (Mg(OH)2) |
Explore Related Topics
- pH of Acids and Bases
- Neutralization
- Difference Between Acid and Base
- Lewis Acid and Base
- Acid Strength
FAQs on Acids and Bases Explained: Definitions, Properties & Examples
1. What is the fundamental definition of an acid and a base in chemistry?
In chemistry, an acid is typically defined as a substance that increases the concentration of hydrogen ions (H⁺) when dissolved in water. A base is a substance that increases the concentration of hydroxide ions (OH⁻) in water. Acids generally have a sour taste and a pH less than 7, while bases are often bitter, feel slippery, and have a pH greater than 7.
2. What are the key properties that distinguish acids from bases?
The key properties to distinguish them are:
Taste: Acids taste sour (e.g., lemon juice), while bases taste bitter (e.g., baking soda).
Litmus Test: Acids turn blue litmus paper red. Bases turn red litmus paper blue.
Reaction with Metals: Acids react with active metals like zinc and magnesium to produce hydrogen gas. Bases generally do not react with metals in this way.
Feel: Bases feel slippery or soapy to the touch, whereas acids do not have this characteristic.
3. Can you provide some examples of common acids and bases found in everyday life?
Certainly. Common examples include:
Acids: Acetic acid (in vinegar), citric acid (in lemons and oranges), carbonic acid (in fizzy drinks), and lactic acid (in yogurt).
Bases: Sodium bicarbonate (baking soda), ammonia (in cleaning products), magnesium hydroxide (in antacids), and sodium hydroxide (in drain cleaners).
4. What is a neutralisation reaction and what does it produce?
A neutralisation reaction is a chemical reaction between an acid and a base. When they react, their acidic and basic properties are cancelled out, or neutralised. The primary products of this reaction are always a salt and water. For example, when hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), it produces sodium chloride (NaCl), a salt, and water (H₂O).
5. How does the pH scale work to indicate acidity or basicity?
The pH scale is a measure of how acidic or basic a water-based solution is. It ranges from 0 to 14. A pH of 7 is neutral (like pure water). A pH value less than 7 indicates acidity, with lower numbers signifying stronger acids. A pH value greater than 7 indicates basicity (or alkalinity), with higher numbers signifying stronger bases. The scale is logarithmic, meaning a change of one pH unit represents a tenfold change in acidity or basicity.
6. What is the main difference between a strong acid and a weak acid?
The main difference lies in their degree of ionisation in water. A strong acid, like hydrochloric acid (HCl), completely dissociates or ionises in water to release all its hydrogen ions (H⁺). A weak acid, like acetic acid (CH₃COOH), only partially ionises, meaning only a small fraction of its molecules release hydrogen ions at any given time. This is why a strong acid is more corrosive and has a lower pH than a weak acid of the same concentration.
7. How do common acid-base indicators like phenolphthalein and litmus work?
Acid-base indicators are special substances that change colour at different pH values. They work by reacting with H⁺ or OH⁻ ions in the solution. Litmus, for instance, is red in acidic solutions (pH < 7) and blue in basic solutions (pH > 7). Phenolphthalein is another common indicator that is colourless in acidic solutions but turns pink in basic solutions (specifically above pH 8.2).
8. Why are the Brønsted-Lowry and Lewis theories considered more advanced than the Arrhenius theory?
The Arrhenius theory is limited because it only defines acids and bases in aqueous (water) solutions. The Brønsted-Lowry theory is more general, defining an acid as a proton (H⁺) donor and a base as a proton acceptor, which applies to non-aqueous solutions too. The Lewis theory is the most general, defining an acid as an electron-pair acceptor and a base as an electron-pair donor. This allows substances without hydrogen, like Boron trifluoride (BF₃), to be classified as acids.
9. What is the importance of buffer solutions in real-world applications like biology?
A buffer solution is a mixture that can resist significant changes in pH upon the addition of small amounts of an acid or a base. They are vital in biological systems. For example, human blood is a buffer solution that maintains a stable pH of about 7.4. This is crucial because the enzymes that regulate metabolic processes can only function within a very narrow pH range. Without this buffering capacity, our cells would not be able to operate correctly.
10. Why is it dangerous to add water directly to a concentrated acid?
It is extremely dangerous because the reaction of a strong acid with water is highly exothermic, meaning it releases a large amount of heat. If you add water to acid, the small volume of water heats up so rapidly that it can flash boil, splashing the concentrated acid out of the container. The correct and safe procedure is to always add the acid slowly to the water, which allows the larger volume of water to absorb the heat more safely.
