Acids, bases and pHIB MYP Sciences: Revision notes
Section 1
The pH scale and universal indicator
The pH scale runs from 0 to 14 and tells us how acidic or alkaline a solution is.
- pH below 7 is acidic (the lower the number, the stronger the acid)
- pH 7 is neutral, for example pure water
- pH above 7 is alkaline (the higher the number, the stronger the alkali)
Universal indicator is a mixture of dyes that changes colour across the whole pH range. Roughly: red (pH 0-2), orange (3-5), yellow (about 6), green (7), blue (about 9-10) and purple (11-14). Because it gives a range of colours, it tells you the pH value, not just whether a solution is acidic or alkaline.
Remember the colour order of universal indicator: red, orange, yellow, green, blue, purple, from acid to alkali.
Section 2
Litmus and other indicators
An indicator is a substance that changes colour depending on whether a solution is acidic or alkaline.
- Litmus turns red in acids and blue in alkalis. It is used as a solution or on paper.
- Methyl orange is red in acid and yellow in alkali.
- Phenolphthalein is colourless in acid and pink in alkali.
These indicators only have two colours, so they show whether a solution is acidic or alkaline but not its pH. Some natural substances, such as red cabbage juice, also work as indicators.
Litmus cannot tell you the pH. It only tells you acid or alkali.
Section 3
Acids
An acid is a substance with a pH below 7. Acids turn blue litmus red and have a sour taste. Acids are soluble and, in water, they release hydrogen ions (H⁺).
Common examples:
- hydrochloric acid, HCl (in the stomach)
- sulfuric acid, H₂SO₄ (car batteries)
- nitric acid, HNO₃
- citric acid in lemons, ethanoic acid in vinegar
A strong acid has a very low pH (for example 1), while a weak acid has a pH nearer 7 (for example 4 or 5).
Section 4
Bases and alkalis
A base is a substance that can neutralise an acid. Most bases are metal oxides or metal hydroxides, for example copper(II) oxide and magnesium hydroxide.
A base that dissolves in water is called an alkali. Alkalis have a pH above 7 and turn red litmus blue. Examples are sodium hydroxide, potassium hydroxide and calcium hydroxide (lime water). Alkalis feel soapy.
All alkalis are bases, but not all bases are alkalis, because many bases (such as copper(II) oxide) do not dissolve in water.
Do not say that all bases are alkalis. Only bases that dissolve in water are alkalis.
Section 5
Hazards of acids and alkalis
Concentrated acids and alkalis are corrosive, which means they can damage skin, eyes and some metals. The corrosive hazard symbol shows this. Dilute solutions are usually less hazardous but can still be irritants.
Safety precautions:
- wear eye protection and gloves
- add concentrated acid slowly to water, never the reverse
- wash splashes off with plenty of cold water
- tell the teacher straight away about any spills
Section 6
Neutralisation
Neutralisation is the reaction between an acid and a base (or alkali) in which the acid and base cancel each other out. As the base is added, the pH rises towards 7.
acid + base → salt + water
Example: hydrochloric acid + sodium hydroxide → sodium chloride + water
Uses of neutralisation:
- indigestion tablets contain bases that neutralise excess stomach acid
- farmers add lime to acidic soil
- toothpaste neutralises acids made by bacteria in the mouth
The end point is when the indicator shows the solution is neutral (green with universal indicator). If too much base is added, the solution becomes alkaline.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Acids, bases and pH
- Students at a school in Nairobi are sorting four household liquids using universal indicator solution. Lemon juice turned the indicator orange-red, tap water turned it green, a baking-soda solution turned it blue and oven cleaner turned it deep purple.Explain why universal indicator is more useful than litmus paper for comparing the lemon juice with a stronger acid.2 marks
- A technician at a school in Lisbon is preparing to dilute a bottle of concentrated hydrochloric acid and a bottle of sodium hydroxide solution for a practical lesson. Both bottles carry the corrosive hazard symbol.Suggest two safety precautions the technician should take when handling the concentrated solutions.2 marks
- Priya, a student in Mumbai, wants to compare how well three brands of indigestion tablet (P, Q and R) neutralise acid. Each tablet contains a base. She crushes one tablet at a time, adds it to 50 cm³ of dilute hydrochloric acid containing universal indicator, stirs, and keeps adding tablets until the indicator turns green. She records the number of tablets needed. Brand P needed 3 tablets, brand Q needed 5 tablets and brand R needed 2 tablets. She did each test once and concludes that brand R is the best.State a testable hypothesis for Priya's investigation, with a scientific reason, and identify the independent variable and the dependent variable.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).