The relative acidity or alkalinity of a solution is measured on the pH scale. The pH of a solution can be obtained simply by using universal indicator paper or solution and comparing the colour with a standard scale. Acidic, alkaline and neutral materials can be found in the home, often in the kitchen, bathroom or garage. Some are harmless and, for example, could be used in food preparation, while others are potentially harmful.
The pH scale ranges from 0 to 14, where:
- acidic solutions have a pH of less than 7
- neutral solutions have a pH of exactly 7
- alkaline solutions have a pH of more than 7
- Many chemicals that can be found in the home may be hazardous and should be labelled to warn of potential dangers. Describe two potential dangers from a chemical labelled with the following sign:
SignAnswerThe sign indicates an irritating substance.
For example, it can cause itching or mild inflammation/blistering/reddening of skin and may be harmful if breathed in, digested or absorbed through skin, BUT is not strong enough to be toxic.
- Some bottles may also be labelled with the sign below. What other danger does this represent?
SignAnswerThis sign indicates a corrosive substance.
For example, it may attack and destroy living tissue such as skin and eyes.
- Open the Yenka file Model 1. It is possible to find the pH of any solution by using universal indicator. This indicator is a mixture of several indicators and therefore has different colours for a range of pH values. Add a drop of indicator to each of the beakers, then by comparing their colour to the chart, list them in order of acidity (starting with the most acidic):
AnswerB, A, D, C.
- Using what you've learnt, try to complete the table below, making as accurate predictions as you can:
Substance Colour of universal indicator pH Description (e.g. strong acid) Salt water GreenAnswer7AnswerNeutral Lemon juiceAnswerorange - pale red 3AnswerWeak acid Bleach Dark purpleAnswer11-12AnswerStrong alkali Battery acidAnswerRed 1AnswerStrong acid Toothpaste Green-blue or dark greenAnswer8 Weak alkali Red wineAnswerYellow 6Answer(very) weak acid
- Which substance from the table above should be labelled with the warning label from Q.2 and why? Other than using battery acid, name two substances in the table which will neutralise each other.
Answer(a) Battery acid, it's a strong acid (sulphuric acid)
(b) Any with pH below 7 (acid) plus any with a pH above 7 (alkali) e.g. lemon juice and wine or toothpaste and bleach.
- Open Yenka file Model 2.
- Litmus solution is another indicator often used as a test to see if a solution is acidic, alkali or neutral. It is red in acidic solutions, blue in alkaline solutions and green in neutral solutions.
Add both solutions to the empty beaker. Is the resulting solution acidic, alkaline or neutral?AnswerNeutral
- What might you call the process of mixing an acid and an alkali?
- Using what you've learnt, explain why lime is sometimes added to garden soil or a field before crop planting. How might a gardener check whether lime is needed?
AnswerSoil is too acidic - lime is an alkali and neutralises the excess acid (in fact, often each plant/crop has an optimum pH range).
Can simply use damp universal pH indicator paper and press on the soil, but a better solution is to shake soil samples with an indicator solution, let them settle out, and compare the colour of the solution to a pH colour chart.
- Antacid medications like Alka-Seltzer, Rennies or Tums help control indigestion caused by excess acid in the stomach.
(a) Explain what sort of chemical must be in Alka-Seltzer and how it works.
(b) Which of the words 'strong' or 'weak' would help describe this chemical and why?Answer(a) Must be an alkali (base), to neutralise the acid
(b) Weak (alkali/base). A strong alkali would be dangerous to human tissue.
Indicators such as universal indicator or litmus solution can be used to measure how acid or alkaline a solution is. Neutralisation is an important reaction with various uses. Many useful chemicals in the home are acids and alkalis but there may be dangers associated with them.