acid and alkalis Flashcards
What are two colour indicators?
Two colours indicators are used to distinguish between acids and alkalis
What is the most common natural indicator?
Litmus, which is extracted from lichens
What are synthetic indicators?
Organic compounds that are sensitive to changes in acidity and appear different colours in acids and alkalis
Name two synthetic indicators frequently used in acid-alkali titrations.
- Phenolphthalein
- Methyl orange
Why is litmus not suitable for titrations?
The colour change is not sharp and it goes through a purple transition colour in neutral solutions
What is the colour of litmus in acid?
Red
What is the colour of litmus in alkali?
Blue
What colour does phenolphthalein turn in alkali?
Pink
What colour does methyl orange turn in alkali?
Yellow
What is the pH range of the pH scale?
0 – 14
What pH values indicate strong acids?
pH 0−3
What pH values indicate weak acids?
pH 4−6
What pH values indicate weak alkalis?
pH 8−10
What pH values indicate strong alkalis?
pH 11−14
What pH value is described as neutral?
pH 7
What is a universal indicator?
A wide range indicator that gives an approximate value for pH
How does universal indicator determine pH?
A few drops are added to the solution and the colour is matched with a colour chart
Why is universal indicator not suitable for acid-base titrations?
It does not provide a sharp colour change required to identify the end-point
What do acids produce when added to water?
Positively charged hydrogen ions (H+)
What do alkalis produce when added to water?
Negative hydroxide ions (OH–)
What is a neutralisation reaction?
When an acid reacts with an alkali
What is produced when H+ ions react with OH– ions?
Water
What is the net ionic equation for all acid-base neutralisations?
H+ (aq) + OH– (aq) ⟶ H2O (l)
Why is neutralisation important in agriculture?
It raises the pH of soils, as some crops cannot tolerate pH levels below 7
What bases are commonly added to soil to achieve neutralisation?
- Limestone
- Quicklime
True or False: All reactions of acids are neutralisations.
False
What is produced when a metal reacts with an acid?
A salt, but no water is formed
