Acids and Alkalis

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infoWhy this? Acids, alkalis, and bases occur in domestic, biological, environmental, and industrial settings, where their properties can be useful or hazardous. Using pH, indicators, equations, and controlled investigations enables us to identify solutions, explain neutralisation and metal–acid reactions, and distinguish strength from concentration.

scheduleWhy now? This unit builds on primary study of material properties and Year 8 work on chemical reactions, formulae, and word equations. It establishes the acid–base concepts and practical techniques that are developed further in Year 9 Chemical Changes.

An acid is a substance that releases hydrogen ions (`\mathrm{H^+}`) when dissolved in water.
An alkali is a soluble base that releases hydroxide ions (`\mathrm{OH^-}`) when dissolved in water.
Examples of common acids include hydrochloric acid (`\mathrm{HCl}`), sulphuric acid (`\mathrm{H_2SO_4}`), nitric acid (`\mathrm{HNO_3}`), ethanoic (acetic) acid (`\mathrm{CH_3COOH}`), and citric acid (`\mathrm{C_6H_8O_7}`).
Examples of common alkalis include sodium hydroxide (`\mathrm{NaOH}`), potassium hydroxide (`\mathrm{KOH}`), calcium hydroxide (`\mathrm{Ca(OH)_2}`), and ammonia solution (`\mathrm{NH_4OH}`).
All acids contain hydrogen atoms in their formula and release `\mathrm{H^+}` ions in solution.
The strength of an acid describes the proportion of acid particles that ionise in water; strong acids ionise fully, while weak acids ionise only partially.
The concentration of an acid describes the amount of acid dissolved per unit volume of solution.
The pH of an acid solution depends on both the acid's strength and its concentration, so pH alone does not identify acid strength.
The rate of reaction of an acid with metals or carbonates is faster for stronger acids at the same concentration.
The pH scale measures how acidic or alkaline a solution is, ranging from 0 (most acidic) to 14 (most alkaline), with 7 being neutral.
Indicators are substances that change colour depending on the pH of the solution.
Universal indicator follows the pH colour scale, with red, orange, and yellow showing acidic solutions, green showing neutral solutions, and blue and purple showing alkaline solutions.
Litmus is red in acid and blue in alkali; methyl orange is red in acid and yellow in alkali; phenolphthalein is colourless in acid or neutral solution and pink in alkali.
A neutralisation reaction occurs when an acid reacts with an alkali (or base) to form a salt and water.
The general word equation for neutralisation is `\text{Acid} + \text{Alkali} \rightarrow \text{Salt} + \text{Water}`.
Neutralisation reactions are used in everyday life, such as in antacids for treating indigestion and in the treatment of acidic soils.
When a metal reacts with an acid, the products are a salt and hydrogen gas.
The general word equation for this reaction is `\text{Metal} + \text{Acid} \rightarrow \text{Salt} + \text{Hydrogen}`.
Signs of a chemical reaction include gas bubbles (hydrogen gas when metals react with acids), temperature change, and colour change.
The independent variable is the variable you change in an experiment; the dependent variable is what you measure.
Bases are substances that neutralise acids; alkalis are bases that are soluble in water and have a pH above 7.
Stomach acid is a strong acid with a pH of about 1.

rocket_launchYou must be able to

Use the pH scale to determine whether a solution is acidic, neutral, or alkaline, and compare how acidic or alkaline solutions are.
Use different indicators, including universal indicator, litmus, methyl orange, and phenolphthalein, to categorise solutions as acidic, neutral, or alkaline and interpret their colour changes.
Write word equations for neutralisation reactions and reactions between metals and acids, naming the correct salt formed.
Describe and explain observations when acids react with metals, such as fizzing due to hydrogen gas.
Identify the independent and dependent variables in an investigation, such as testing the effectiveness of indigestion remedies.
Plan and explain a method for investigating the effectiveness of indigestion remedies, including how to collect reliable results and control variables.
Predict and justify which indigestion remedy will be most effective based on knowledge of neutralisation.
Represent experimental data appropriately, for example using a bar chart, and explain the choice of graph.
Draw conclusions from experimental results and link findings to the reactivity of acids, alkalis, or metals.
Predict and justify the final pH category and indicator colour when mixing a strong acid with a weak base by using the supplied relative amounts, concentrations, and reaction stoichiometry, rather than strength alone.
Explain why a pH probe gives a more accurate measurement of pH than an indicator.
Suggest ways to improve the reliability of experimental results, such as repeating measurements and controlling variables.

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