Acids, alkalis & titrations

What is the colour change of phenophthalein for acidic, neutral & alkaline solutions?

Acidic → colourless

Neutral → colourless

Alkaline → pink

What is the colour change of methyl orange for acidic, neutral & alkaline solutions?

Acidic → red

Neutral → yellow

Alkaline → yellow

What is the colour change of litmus solution / paper for acidic, neutral & alkaline solutions?

Acidic → red

Neutral → purple

Alkaline → blue

What is the pH scale?

A scale (0-14) that measures the acidity or alkalinity of a solution

How can the universal indicator be used to measure the approximate pH value of an aqueous solution?

Add a few drops of aqueous solution to a piece of universal indicator paper

Observe the colour change of the paper & compare it to the pH scale to find its pH value

What do acids produce in aqueous solutions?

Hydrogen ions (H⁺)

What do alkalis produce in aqueous solutions?

Hydroxide ions (OH^-)

What is a neutralisation reaction?

When an acid and alkali react together to neutralise each other, producing a salt & water (acid + alkali → salt + water)

The name of the salt can be worked out from the name of the acid & the alkali

What is a titre?

A small measured volume

What is an acid-alkali titration?

An experiment where titres of acid / alkali are added to a measured volume of acid / alkali containing a pH indicator until the solution becomes neutral

Describe the method of an acid-alkali titration

1. Wash a burette using acid & then water

2. Fill burette to 100cm³ with acid

3. Use a pipette filler to put 25cm³ of alkali into a pipette & then add it into a conical flask

4. Add a few drops of phenophthalein indicator to the conical flask

5. Add titres of the acid from the burette into the alkali until the solution turns from pink to colourless

6. Calculate the volume of acid needed to neutralise the alkali by subtracting the new reading of the burette from 100cm³

7. Repeat the experiment & calculate an average