What is an indicator-?

Answer 1

A substance that reveals, through characteristic color changes, the degree of acidity or basicity of solutions. Could also be a light to notify of a components operation.

Answer 2

A formal charge or accusation of a serious crime

Answer 3

The definition of the word indicator is "a thing, especially a trend or fact, that indicates the state or level of something."

Answer 4

In chemistry, usually: an indicator is a substance which changes color depending on the pH or temperature.

Answer 5

As a KS3 science student, you will be familiar with using indicators to tell whether a substance is acidic, alkaline or neutral. Indicators are usually made from plants, for example, you can use beetroot juice as a simple indicator. You have probably used the two types of litmus paper

When an acid is dissolved in water we get an acidic solution. When a base dissolves in water it is an alkali and makes an alkaline solution. If a solution is neither acidic nor alkaline it is neutral. Pure water is neutral, and so is paraffin.

Indicators are substances that change colour when they are added to acidic or alkaline solutions. Litmus, phenolphthalein, and methyl orange are all indicators that are commonly used in the laboratory.

Litmus

Litmus indicator solution turns red in acidic solutions, blue in alkaline solutions, and purple in neutral solutions.

Litmus paper comes as red litmus paper and blue litmus paper.

The pH scale

The chemical properties of many solutions enable them to be divided into three categories - acidic, alkaline and neutral solutions.

The pH scale is used to measure acidity and alkalinity:

solutions with a pH less than 7 are acidic

solutions with a pH of 7 are neutral

solutions with a pH greater than 7 are alkaline

If universal indicator is added to a solution it changes to a colour that shows the pH of the solution. Universal indicator is a mixture of a variety of other indicators and can be used to measure the approximate pH of a solution. A more accurate value can be obtained using a pH probe

When atoms or groups of atoms lose or gain electrons, charged particles called ions are formed. Ions can be either positively or negatively charged.

Acids

When acids dissolve in water they produce hydrogen ions, H+. These are sometimes called protons, because hydrogen ions are the same as a hydrogen nucleus (which is a proton).

For example, take a look at the equation for hydrochloric acid:

HCl(aq) → H+(aq) + Cl-(aq)

Note that (aq) means the substance is in solution.

Acids are often produced from non-metal oxides. For example, sulfur oxides make sulfuric acid.

Alkalis

When alkalis dissolve in water they produce hydroxide ions, OH-.

For example, take a look at the equation for sodium hydroxide:

NaOH(aq) → Na+(aq) + OH-(aq)

Ammonia is slightly different. This is the equation for ammonia in solution:

NH3(aq) + H2O(l) → NH4+ (aq) + OH-(aq)

Bases

A base is chemically opposite to an acid. Some bases dissolve in water and are called alkalis. But other bases, including many metal oxides, do not dissolve in water.

Neutralisation reaction

When the H+ ions from an acid react with the OH- ions from an alkali, a neutralisation reaction occurs to form water. This is the equation for the reaction:

H+(aq) + OH-(aq) → H2O(l)

For example, hydrochloric acid and sodium hydroxide solution react together to form water and sodium chloride solution. The acid contains H+ ions and Cl– ions, and the alkali contains Na+ ions and OH– ions. The H+ ions and OH– ions produce the water, and the Na+ ions and Cl– ions produce the sodium chloride, NaCl(aq).

Because neutralisation reactions involve the loss and gain of hydrogen ions, this process is sometimes referred to as ‘proton transfer’.