Acids – GCSE Chemistry

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Introduction

Comparison chart showing acids and bases with their pH values.

Acids: Hand holding a pH colour chart and test tube used to check if a solution is acidic or basic.

What are Acids and Base?

ACIDS

  • An Acid is a substance that releases H+ ions in water. Acids usually have sour taste and can turn blue litmus paper into red.

Examples:

Acids: Images showing different types of acids including hydrochloric acid, acetic acid, sulphuric acid, carbonic acid, citric acid, and salicylic acid.

Base

  • A base is a substance that releases OH⁻ (hydroxide) ions in water. Bases usually have a bitter taste, feel soapy, and turn red litmus paper blue.

Examples:

Images of common bases including sodium hydroxide, calcium hydroxide, potassium hydroxide and magnesium hydroxide.

Uses of Acids

  • The carbonic acid gives fizz to soda drinks.
  • Acetic acid(vinegar) is used for pickling.
  • HCl is used for concrete cleaning and for pool PH adjustment.
  • Acids are also used in batteries and metal works.

Uses of Bases

  • The soap and detergent products contain Sodium hydroxide(base).
  • Ammonia is common household cleaner and fertilizer raw material.
  • Calcium Hydroxide(slaked lime) is used to soften water, to deal with acidic soil and is used in cement.

What is pH Scale and How it is measured?

  • pH Scale is a numerical scale that is used to measure how acidic or basic a substance is. This scale’s value ranged from 0 to 14-

Diagram explaining the pH scale showing acidic, neutral and alkaline values with examples like lemon juice, vinegar, baking soda and pure water.

Fact: Bee stings are acidic and has pH between 5 and 5.5

Formula to find the pH of any solution

  • The pH of any substance is a measure of how much acidic or a basic that substance is. The formula to measure pH is –

The pH formula showing pH equals negative log of hydrogen ion concentration.

Where, [H+] is the concentration of Hydrogen ions in (Moles per Liter or M)

Steps to calculate PH

  • Find the H+ concentration, for strong acids like HCl (Hydrochloric acid). The concentration of acid equals the concentration of H+ ions.
  • For weak acids and bases you may need to use an equilibrium expression or the Ka (Acid Dissociation constant).

Example:

Calculating PH Values

  • HCl (Hydrochloric Acid) is a strong acid(dissolves completely)

Diagram showing the dissociation reaction of hydrochloric acid into hydrogen ions and chloride ions.

  • This reaction is responsible for the acidic nature of Hydrochloric Acid

Diagram showing hydrogen reacting with chlorine to form hydrochloric acid.

The concentration of Hydrogen ions is same as the concentration of Hydrochloric acid.

  • If [HCl] = 0.001M

Example calculation showing how to find the pH of an acid using the negative log of hydrogen ion concentration.

Concept of Dilute and Concentrated Solution

Acids: Diagram comparing dilute and concentrated acid solutions with illustrations and examples.

Acids: Diagram showing dilute and concentrated solutions with different amounts of solute in beakers.

Dilute or Concentrated ≠ Acidic or Basic

  • The terms Dilute and Concentrated describe the amount of solute dissolved in a given quantity of solvent, not the chemical strength or nature of the solute.
  • In brief: A Small amount of solute in a large amount of solvent.
  • Concentrated Solution: A Large amount of solute in a smaller amount of solvent. These terms tell us how much solute is present, but not how weak or strong the solute is in terms of chemical behaviour.

Acids: Diagram showing the difference between concentration and strength for acids and alkalis with examples.

Examples to Clarify

  • Dilute Acid: Small amount of Hydrochloric acid in large amount of water. Still an acid but weak and not dangerous.
  • Concentrated Acid: A large amount of HCl in little amount of water. Still an acid but stronger and much corrosive.
  • Dilute Base: A small amount of Sodium Hydroxide(NaOH) in water. Still a base but mild.

Examples of Acid and Base Reaction

  • The reaction between Acid and base is called Neutralisation reaction. Often when an acid and base react salt and water is formed.

Examples

  • Hydrogen Bromide reacts with Potassium Hydroxide to form water and Potassium Bromide. Potassium Bromide is formed from the K+ ions from the base(KOH) and OH– ions from the acid HBr.

Neutralisation reaction showing hydrobromic acid reacting with potassium hydroxide to form water and potassium bromide.

  • Hydrochloric acid reacts with Ammonia to form Ammonium Chloride(salt). Ammonium Chloride is made up of NH4+ cations from the base NH3 and Cl– anions from the acid HCl.

Neutralisation reaction showing hydrochloric acid reacting with ammonia to form ammonium chloride.

Some Uses of Acid Base Reactions

Acids: Images showing acid use examples: treating soil and acid in the stomach causing indigestion.

  • Let us discuss some specific types of Acid Base reaction in which type of base used changes and acid remains the same –

Diagram listing three types of acid reactions: Acids with metal hydroxide, metal oxide, and metal carbonate.

1. Acid and Metal Hydroxide:

  • When an Acid reacts with metal hydroxide, a salt and water are formed. These are a type of double displacement reactions.

Examples

  • Hydrochloric acid reacts with Sodium Hydroxide to form Water and Sodium Chloride.

Neutralisation reaction showing hydrochloric acid reacting with sodium hydroxide to form water and sodium chloride.

  • Here, Hydrochloric Acid reacts with Aluminium Hydroxide to from Water and Aluminium Chloride.

Balanced chemical equation showing hydrochloric acid reacting with aluminium hydroxide to form water and aluminium chloride.

Properties

  • It is an Exothermic reaction.
  • The reaction do not produce any gas.
  • Both soluble and insoluble metal hydroxides can react with acids.

2. Acid and Metal oxide:

  • When an Acid reacts with metal oxide, a salt and water are formed.

Examples

  • Reaction of Hydrogen Bromide and Magnisium oxide gives us Water and Magnisium Bromide.

Balanced chemical equation showing hydrobromic acid reacting with magnesium oxide to form water and magnesium bromide.

  • Hydrochloric Acid reacts with Aluminium Oxide and forms Water and Aluminium Chloride.

Balanced chemical equation showing hydrochloric acid reacting with aluminium oxide to form water and aluminium chloride.

Properties

  • It is an Exothermic reaction.
  • The reaction do not produce any gas.

3. Acid and Metal Carbonates:

  • When an Acid reacts with metal carbonate, Carbon Dioxide and Water are formed.

Examples

  • Nitric Acid reacts with Sodium Carbonate and forms Nitrate, Carbon dioxide and water.

Balanced chemical equation showing nitric acid reacting with sodium carbonate to form sodium nitrate, carbon dioxide and water.

  • Sulfuric acid reacts with Sodium carbonate to form Nitrate, Carbon dioxide and water.

Balanced chemical equation showing sulphuric acid reacting with calcium carbonate to form calcium sulphate, carbon dioxide and water.

Properties

  • The reaction is mildly Exothermic.
  • In the reaction of acid and carbonates the CO2 gas is produced. The gas can be tested by passing it through limewater which turns milky/cloudy if there is CO2.

Formation of Water from its ions

  • We know that Water(a compound) consists of two elements, Hydrogen and Oxygen (constituents).

Diagram showing hydrogen reacting with oxygen to form water, with molecular structures and partial charges.

Activation Energy:

  • The energy required to reach at the stage of breaking bonds within Hydrogen and Oxygen Molecules.

Bond Breaking:

  • The energy required to break the bonds present in reactants.

Energy Release:

  • Energy is released in the form of Water and Light, and that is why Hydrogen is used as fuel in rocket

Frequently Asked Questions

Solution:

Acids donate H+ ions on the other hand bases donate OH– ions when dissolved in a solution.

Solution:

It is the reaction of Acid and Base and is called neutralisation reaction as the ions from acid and base combine and form water.

Solution:

Maintaining the PH level is crucial for proper functioning of many systems in our everyday life including human body.

Solution:

pH = – log+10[H+]

Solution:

The concept of dilute and concentrated solution tells us how much strong an acidic or basic solution is.

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