The pH scale, indicators, strong and weak acids
Watch the whole lecture free
Make a free account to watch every lecture to the end: 2 worked examples, 6 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
That was the teaching part. The rest of this lecture is free with an account: 2 worked examples, 6 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
No password needed now: we email you a link to set one. Already have an account? Sign in, then come back to this page.
The teaching part (1 min) is free to watch. A free account opens the whole lecture: 2 worked examples, 6 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Two acids, one pH
- The pH scale
- Universal indicator
- Simple indicators
Read the transcript of the teaching part
Last time
Before we start: last time, in Oxides and making salts, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Two acids, one pH
Hydrochloric acid and vinegar are both acids. Yet you can sip vinegar on your chips, but you could not do that with hydrochloric acid. The difference is not the idea of acid, but how strong it is, and the pH scale measures it.
The pH scale
The pH scale runs from 0 to 14. It measures how acidic or alkaline a solution is. Below 7 is acidic, 7 is neutral, and above 7 is alkaline. The lower the pH, the more acidic. Pure water has a pH of 7.
Universal indicator
Universal indicator is a mixture that gives a different colour at each pH. Red is strongly acidic, orange and yellow are weakly acidic, and green is neutral. Blue is weakly alkaline and purple is strongly alkaline.
Simple indicators
Litmus is red in acid and blue in alkali. Methyl orange is red in acid, and yellow in alkali and in neutral solutions. Thymolphthalein is colourless in acid and neutral solutions, and blue in alkali.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Indicators and the pH scale · Strong and weak acids · pH and indicators · Strong and weak acids.
The whole chapter: Acids, Bases and Salts, Acids, Bases and Salts.
Chapter 7: Acids, bases and salts
- Properties of acids and bases, proton transfer 5 min
- The pH scale, indicators, strong and weak acids 6 min
- Neutralisation and oxides 6 min
- Making soluble and insoluble salts, hydrated salts 6 min
Also for IGCSE Chemistry 0620
Which topics come up most in IGCSE Chemistry 0620 past papers, counted from every paper since 2004.