A5 Universal Indicator

Describe the use of universal indicators to measure pH of a solution

Universal indicator is perhaps the main way in which the pH of a solution is tested, this is because not only does it tell you whether the solution is an acid, alkali or neutral it also tells you how acidic or alkaline the solution is.

As you know each number on the pH scale is given a colour, ranging from deep red to deep purple. These colours represent the colours universal indicator goes when presented with such a chemical. For example putting universal indicator into an acid solution we may find that the indicator turns an orangey red and comparing with our pH scale we can easily see that this chemical has a pH of roughly 2. And obviously this works for alkalis too.

You must remember that Universal indicator solution is green on its own so when presented with a neutral solution it will stay green. But Universal indicator paper is often a yellow colour, so when presented with a neutral colour will change to be green.

A4 Solutions

Describe solutions which have a pH value less than 7 as acidic, those with a pH value of more than 7 as alkaline and those with a pH of 7 as neutral.

Any solution with a pH value that is less than 7 is classed as acidic, the further away from 7 it is, the more acidic it is, i.e. pH 2 is more acidic than pH 5.

Any solution with a pH that is greater than 7 is called an alkali, and as with acids, the further away from the number 7 it is the more alkaline it is i.e. pH 14 is much more alkaline than pH 9

A solution with a pH of 7 is neutral, there aren’t many neutral substances, but pure water is a good example, it is neither acidic nor alkaline.

A3 The pH Scale

Describe the pH scale, running from 0-14, as a scale of strength acidity and alkalinity.

The pH scale is a scale running from 0 to 14 measuring the acidity or alkalinity of a substance; 0 being the most acidic and 14 being the most alkaline. The pH scale also comes in a scale of colours, acidic being red through brown (0-6) and alkaline being the blues and purples, a neutral substance has a pH of seven and is represented by a green colour. Below is a common pH scale showing the full range from 0 to 14 and their corresponding colours.

A2 Testing for Acids and Alkalis

Recall the colours produced by the following indicators in acidic solution and alkaline solution: litmus, phenolphthalein, methyl orange and universal indicator.

Blue Litmus Paper: Blue

•          In an Alkali: No change
•          In an Acid: Turns Red

Red Litmus Paper: Red

•          In an Alkali: Blue
•          In an Acid: No change

Universal indicator: Green solution (Yellow Paper)

•          In an Alkali: Green to Purple
•          In an Acid: Red to Green
•          In a Neutral solution: Green

Phenolphthalein: Colourless

•          In an Alkali: Violet
•          In an Acid: Colourless

Methyl Orange: Orange

•          In an Alkali: Orange
•          In an Acid: Pink/Red

A1 Testing for acidity and alkalinity

Topic 5, Physical Chemistry

Recall how to test for acidity and alkalinity, using suitable indicators

There are several indicators that can be used to test the acidity of a substance. The ones that are needed in the IGCSE course are: Litmus paper (Blue and Red), Universal indicator, Phenolphthalein and Methyl Orange. Each of these colour tests are explained in the next objective (A2)

UPDATE

From now on I will be posting every single topic objective for all the Topics that are covered this year: Topic 5, Physical Chemistry and Topic 6 Electricity and Chemistry.
If I have time over the course of the year I may also write out explanations for all the previous topics and post them here. I hope this helps everyone that needs it.

A6. Cracking

6. Describe how long-chain hydrocarbons are cracked to give more short-chain hydrocarbons using silica or alumina as a catalyst at a temperature in the range of 600-700°C


Because fractional distillation forms more long-chain molecules than the more in-demand short-chain molecules, a process called Cracking is used to break down the longer molecules.


The longer molecules are heated to a temperature in the range of 600-700°C and passed over a catalyst like alumina or silica, this breaks down the molecules into smaller ones.