Strong and Weak Acids

In aqueous solutions, acids ionise to produce hydrogen ions (H⁺). An aqueous solution just means ‘dissolved in water’, so water is the solvent. The strength of an acid depends on how much it ionises in water.

Strong acids

In aqueous solutions, strong acids completely ionise to form H⁺ ions, making the solution highly acidic. Below are some examples of strong acids:

  • Hydrochloric acid: HCl (aq) → H⁺ (aq) + Cl (aq)
  • Sulfuric acid: H2SO4 (aq) → 2H⁺ (aq) + SO4²⁻ (aq)
  • Nitric acid: HNO3 (aq) → H⁺ (aq) + NO3 (aq)

As you can see, all of the acid molecules fully ionise in water.

Weak acids

In aqueous solutions, weak acids only partially ionise, so the solution is less acidic than with strong acids. Below are some examples of weak acids:

  • Ethanoic acid: CH3COOH (aq) H⁺ (aq) +CH3COO (aq)
  • Carbonic acid: H2CO3 (aq) H⁺ (aq) + HCO3 (aq)

We can identify a weak acid by the reversible reaction symbol (⇌). This symbol means that the reaction is in a state of equilibrium, where the amount of product and reactant is constant. This means that only a small proportion of the acid molecules ionise in water, resulting in a relatively low concentration of H+ ions.

Acidic Dilution and Concentration

Concentration is a measure of how much solute is dissolved in a solution. A higher concentration means there are more solute particles in a given volume of the solution. On the other hand, a lower concentration means there are fewer solute particles.

  • Dilute solutions have a relatively small amount of solute dissolved in the solvent. For example, if you add a pinch of salt to a glass of water and stir, you’ll have a dilute saltwater solution.
  • Concentrated solutions have a relatively large amount of solute dissolved in the solvent. An example of a concentrated solution would be adding several spoonfuls of salt to the same glass of water.
Write the alt text for this image (In British English) and don't use bullet pointsChatGPTA comparative illustration of dilute and concentrated solutions. On the left, a beaker labelled "Dilute Solution" shows a smaller number of solute particles (represented by blue and white circles) dispersed in a larger volume of solvent. Below it reads, "Smaller amount of solute in more solvent". On the right, a beaker labelled "Concentrated Solution" depicts a larger number of solute particles in a lesser volume of solvent. Below it reads, "Larger amount of solute in less solvent". Arrows point to the solvent and solute in both beakers for clarification.

Concentration can also change if more solvent is added to a solution. Adding more solvent to a solution dilutes it by reducing the concentration of solute. In contrast, if no additional solvent is added and more solute is added instead, the solution becomes more concentrated.

Hydrogen Ion Concentration

In aqueous solutions, the concentration of hydrogen ions (H⁺) can be measured using the pH level.

  • A higher concentration of hydrogen ions (H⁺) in a solution results in a lower pH level
  • A lower concentration of hydrogen ions (H⁺) in a solution results in a higher pH level

Strong acids fully ionise, producing a higher concentration of H⁺ ions, while weak acids only partially ionise. Therefore, the more concentrated the solution of an acid, the lower the pH level will be.

As the pH scale decreases by one unit, the concentration of H⁺ ions increases by a factor of 10. For example, a solution with a pH of 1 has a concentration that is 10 times greater than a solution with a pH of 2, and 100 times greater than a solution with a pH of 3.

A visual representation of the pH scale alongside the concentration of H+ ions. On the left, a vertical column displays the pH scale ranging from 0 to 14, with each level coloured differently from red at the most acidic end to dark blue at the most alkaline end. On the right, a gradient triangle depicts the concentration of H+ ions corresponding to each pH level, starting from 1 at the top, becoming progressively smaller by a factor of ten, to 0.0000000000001 at the bottom. Both the pH scale and the H+ concentration triangle are labelled at the top.

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