Purity, Formulations and Chromatography

This section explains purity, formulations and chromatography also covering the applications of chromatography. 

Purity

Purity refers to how much of a substance is present without contamination by other substances. A pure substance contains only one type of particle. In the context of chemistry, the purity of a substance is essential because it influences the physical and chemical properties of the substance.

• Pure substances: These are made up of only one element or compound. For example, pure water consists only of H₂O molecules, and pure gold consists only of gold atoms.
• Impure substances: Most substances in the real world are impure. For instance, crude oil is a mixture of hydrocarbons, and even a simple substance like salt can contain impurities.

The purity of a substance can be measured by determining its melting point and boiling point. Impurities lower the melting point and raise the boiling point of a substance.

Example:

• Water: Pure water has a freezing point of 0°C and a boiling point of 100°C. If the water is impure, the boiling and freezing points will change.

Formulations

A formulation is a mixture that has been carefully designed and manufactured to have a specific set of properties. These are made by mixing different substances in exact proportions to achieve the desired outcome.

Formulations are used in a wide range of everyday products. Here are some examples:

• Fuels: Petrol and diesel are formulated to burn efficiently in engines, with the right balance of chemicals to maximise energy output.
• Cleaning Products: Household cleaning products like washing-up liquid, bleach, and detergents are formulated to clean effectively. Each product has a specific formulation designed for its intended purpose.
• Paints: Paints are formulated to provide good coverage, durability, and resistance to environmental factors. The formulation of paint includes pigments, solvents, binders, and other chemicals.
• Medicines: Medications are formulated to have the right chemical composition to treat specific conditions. For example, tablets might contain active ingredients and additional substances to make them easier to consume, more stable, or better absorbed by the body.
• Foods: Food products are formulated to ensure consistency, taste, and nutrition. A packaged food item may contain preservatives, flavourings, and nutrients all carefully measured to provide the desired properties.
• Fertilisers: Fertiliser formulations contain chemicals like nitrogen, phosphorus, and potassium to provide essential nutrients for plants. The correct formulation ensures that crops grow healthily.

Chromatography

Chromatography is a technique used to separate and analyse the components of a mixture. It works by allowing the mixture to move through a medium (like paper or a column), where different substances will travel at different rates depending on their solubility and interactions with the medium.

There are various types of chromatography, but one of the most common in GCSE Chemistry is paper chromatography.

What is Chromatography?

Chromatography is a method used to separate different substances that are dissolved in a solution. The substances are separated based on their different rates of movement through a stationary medium, which is usually a piece of chromatography paper or a column filled with a solid material.

• The process:
  1. A small sample of the mixture is placed on a chromatography paper.
  2. The bottom of the paper is dipped in a solvent (often water or alcohol).
  3. As the solvent moves up the paper, the substances in the mixture move with it at different rates.
  4. The components of the mixture form separate spots on the paper.
• How it works: Each substance in the mixture has different solubility in the solvent, and each interacts with the stationary phase (the paper) to a different degree. This causes them to separate as they move up the paper at different rates.

Applications of Chromatography

• Identifying substances: Chromatography can be used to identify substances in a mixture by comparing the positions of the spots on the chromatogram with those of known substances.
• Purity testing: It can help check the purity of a substance, as pure substances will form only one spot, while impurities will result in multiple spots.
• Forensic analysis: Chromatography is used to analyse substances found at crime scenes, such as drugs or poisons.

Example:

• Food Colouring: If you have a mixture of food colourings, you can use paper chromatography to separate the different pigments. After applying the mixture to the paper, the various dyes will move up the paper and form distinct coloured bands. This is useful for identifying the different components in the colouring mixture.

Summary

• Purity: A pure substance contains only one type of particle. Impurities can be detected by changes in physical properties like melting and boiling points.
• Formulations: Carefully designed mixtures used in everyday products, such as fuels, medicines, and foods, to ensure they work as intended.
• Chromatography: A technique for separating mixtures into their individual components based on their different rates of movement through a medium. It is used for both analysis and purity testing.

By understanding these concepts, you can analyse substances and mixtures effectively, whether it's ensuring the purity of water or identifying the components of a complex formulation.