Understanding Acids, Bases, and the pH Scale

Acids, bases, and the pH scale are fundamental concepts in GCSE Chemistry, and a clear understanding of them will set you up well for exams. This guide will break down what you need to know about acids, bases, the pH scale, and how to handle simple calculations. By the end, you’ll feel more confident in understanding how pH changes and what that means for the concentration of hydrogen ions in solutions.

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What Are Acids and Bases?

Acids are substances that release hydrogen ions (H⁺) when dissolved in water. The strength of an acid depends on how many hydrogen ions it can release in solution. Examples of common acids are hydrochloric acid (HCl) and sulfuric acid (H₂SO₄).

Bases, on the other hand, accept hydrogen ions. When dissolved in water, some bases release hydroxide ions (OH⁻). A base that dissolves in water is called an alkali. Examples of common alkalis are sodium hydroxide (NaOH) and potassium hydroxide (KOH).

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The pH Scale

The pH scale is a numerical scale used to measure the acidity or alkalinity of a solution. Ranging from 0 to 14, it tells us how many hydrogen ions are present:

• pH 7 is neutral (pure water).
• pH below 7 is acidic.
• pH above 7 is alkaline.

The pH scale is logarithmic, which means that each pH unit represents a tenfold difference in hydrogen ion concentration. For example:

• A solution with pH 3 has ten times more hydrogen ions than a solution with pH 4.
• A solution with pH 3 has 100 times more hydrogen ions than a solution with pH 5.

This logarithmic nature can make calculations simpler once you get the hang of it!

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Calculating Hydrogen Ion Concentrations and pH

One useful formula for hydrogen ion concentration is:

$$\text{pH} = -\log_{10}[\text{H}^+]$$

Where $[\text{H}^+]$ represents the concentration of hydrogen ions in moles per litre (mol/L). Knowing that the scale is logarithmic, you can easily work out changes in hydrogen ion concentration without needing a calculator.

For example:

• If the pH drops from 5 to 4, the concentration of hydrogen ions has increased by 10 times.
• If the pH increases from 3 to 5, the concentration of hydrogen ions has decreased by 100 times (10 times for each pH unit).

Neutralisation and Salt Formation

Understanding neutralisation is essential for GCSE chemistry, especially as it leads to the formation of salts, a common topic in exams. When acids are neutralised, they react with alkalis (like soluble metal hydroxides) or bases (such as insoluble metal hydroxides and metal oxides) to produce salts and water. For example, if hydrochloric acid reacts with sodium hydroxide, the products are sodium chloride and water.

In neutralisation reactions between an acid and an alkali, the hydrogen ions ($H^+$) in the acid react with the hydroxide ions ($OH^-$) in the alkali to form water ($H_2O$). This reaction helps balance the solution, reducing acidity or alkalinity to create a neutral solution.

Metal carbonates can also neutralise acids. In these reactions, the products include a salt, water, and carbon dioxide gas. For instance, if hydrochloric acid is added to calcium carbonate, calcium chloride, water, and carbon dioxide gas are produced.

Using pH Indicators and the pH Scale

To measure the approximate pH of a solution, universal indicators or wide-range indicators are frequently used. These indicators provide a colour change that helps you identify the pH range of the solution. The pH scale itself, ranging from 0 to 14, is used to determine whether a solution is acidic (below 7) or alkaline (above 7), with pH 7 being neutral.

Make sure you can:

• Describe the use of a universal indicator or wide-range indicator to measure the approximate pH of a solution.
• Use the pH scale to identify whether solutions are acidic or alkaline.

Higher Tier Students: Strong vs. Weak Acids

For higher-tier students, understanding the difference between strong and weak acids is crucial. A strong acid is completely ionised in an aqueous solution, meaning it fully dissociates into ions when dissolved in water. Common examples of strong acids include hydrochloric acid, nitric acid, and sulfuric acid. Because they ionise completely, they are very effective at donating hydrogen ions, making the solution highly acidic.

In contrast, a weak acid is only partially ionised in an aqueous solution. This means that only a fraction of the acid molecules dissociate into ions, which results in a lower concentration of hydrogen ions. Examples of weak acids include ethanoic acid (found in vinegar), citric acid, and carbonic acid. These acids produce a less intense acidic solution compared to strong acids of the same concentration.

Practice Questions

1. If a solution has a pH of 4, the concentration of hydrogen ions is $1 \times 10^{-4}$ mol/dm³. If the pH decreases to 3, what will be the new concentration of hydrogen ions?

   • Answer: The concentration of hydrogen ions will increase by 10 times, so the new concentration is $1 \times 10^{-3}$ mol/dm³.

2. A solution has a hydrogen ion concentration of $1 \times 10^{-6}$ mol/dm³, which corresponds to a pH of 6. If the pH changes to 4, what is the new concentration of hydrogen ions?

   • Answer: The concentration increases by 100 times (10 times per pH unit), so the new concentration is $1 \times 10^{-4}$ mol/dm³.

3. A solution with a pH of 7 has a hydrogen ion concentration of $1 \times 10^{-7}$ mol/dm³. If the pH increases to 8, what is the concentration of hydrogen ions?

   • Answer: The concentration of hydrogen ions decreases by 10 times, so the new concentration is $1 \times 10^{-8}$ mol/dm³.

4. A solution’s pH is 2, with a hydrogen ion concentration of $1 \times 10^{-2}$ mol/dm³. If the pH changes to 5, what is the new concentration of hydrogen ions?

• Answer: The concentration of hydrogen ions decreases by 1,000 times, so the new concentration is $1 \times 10^{-5}$ mol/dm³.

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Tips for Answering GCSE Questions on Acids, Bases, and pH

• Memorise the pH scale: Remember that 7 is neutral, and each unit change represents a tenfold difference.
• Understand the logarithmic nature of pH: Each pH step means a 10x change in H⁺ concentration.
• Practise with example questions: Start by practising with questions like those above to get comfortable with the scale’s exponential nature.

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