Chemistry Edexcel iGCSE - Chapter 16 - Acids, Alkalis and Titrations

What is an acid?

A source of H⁺ ions, which they produce when dissolved in water.

What is an alkali?

A source of OH^- ions, which they produce when dissolved in water.

What is the pH of a neutral solution?

7

Are pHs bigger than 7 acid or alkali?

Alkali

Are pHs less than 7 acid or alkali?

Acid

Why isn't full range universal indicator very accurate?

Full-range universal indicators are less accurate because they cover a wide pH range, leading to less precision in detecting specific pH values due to their broad color changes that rely on human judgement to categorise.

What color is litmus paper in acid and alkali?

Acid - Red
Alkali - Blue

What color is methyl orange in acid and alkali?

Acid - red
Alkali - yellow

What color is phenolphthalein in acid and alkali?

Acid - colorless
Alkali - pink

What color is universal indicator in acid and alkali?

Acid - Red
Alkali - Blue

What color is litmus in neutral solutions?

Purple, a mix of blue and red

What color is universal indicator in neutral solutions?

Green

Formula for hydrochloric acid

HCl

Formula for nitric acid

HNO₃

Formula for sulfuric acid

H₂SO₄

Formula for ethanoic acid

CH₃COOH

Formula for phosphoric acid

H₃PO₄

What do reactions between a metal oxide (a base) and an acid produce?

It will produce a salt and water.

What do reactions between a metal hydroxide (an alkali) and an acid produce.

A salt and water. This is a neutralisation reaction.

How to carry out a titration practical

1. Measure out 25cm³ of acid using a pipette.

2. Transfer the acid to the conical flask

3. Fill the burette with an alkali solution. Take the initial reading on the burette and record it to 2dp.

4. Add a few drops of indicator (either phenolphthalein or methyl orange) into the conical flask

5. Add the alkali until the indicator changes color

6. Take the final reading on the burette

Why can't you use full range universal indicator in titration?

Full range universal indicator cannot be used in titration because it contains a mixture of indicators and has a range of colours, meaning the neutralisation point won’t be clear.