Elements, compounds and mixtures Revision Notes

Principles of Chromatography: Stationary and Mobile Phases

Chromatography works by separating components of a mixture based on their differential interaction with a stationary phase and a mobile phase. The stationary phase remains fixed, while the mobile phase moves through it. Substances that are more soluble in the mobile phase and less adsorbed by the stationary phase will travel further and faster. This differential movement leads to the separation of the mixture's components. Examples of stationary phases include chromatography paper or a thin layer of silica gel, while mobile phases can be various solvents.

Paper Chromatography Techniques

In paper chromatography, the stationary phase is a strip of chromatography paper. The mobile phase is a suitable solvent. A small spot of the mixture is applied near one end of the paper, called the baseline. The end of the paper is then dipped into the solvent, ensuring the solvent level is below the applied spot. As the solvent moves up the paper by capillary action, it carries the components of the mixture with it at different rates, depending on their solubility in the solvent and their adsorption to the paper. This results in the separation of components into distinct spots.

Thin Layer Chromatography (TLC) Techniques

Thin Layer Chromatography (TLC) is similar to paper chromatography but uses a thin layer of an adsorbent material (e.g., silica gel or alumina) spread onto a glass or plastic plate as its stationary phase. Like paper chromatography, a spot of the mixture is applied to a baseline, and the plate is placed in a solvent (mobile phase). The solvent moves up the plate by capillary action, separating the components. TLC is generally faster, provides better separation, and is more sensitive than paper chromatography. Visualisation agents may be needed if components are colourless.

Variables and Rf Values in Chromatography

For both paper and thin layer chromatography, the choice of solvent (mobile phase) and stationary phase are crucial variables that affect separation. The distance a substance travels can be quantified using an Rf (retardation factor) value. The Rf value is calculated by dividing the distance moved by the substance by the distance moved by the solvent front. Rf values are always between 0 and 1, are unique for a given substance under specific conditions, and can be used to identify components by comparing them to known standards.

Covalent Bonding and Dot and Cross Diagrams

Covalent bonds form between non-metal atoms. Atoms share pairs of electrons to achieve a stable outer electron shell, typically a full shell of eight electrons (octet rule), or two for hydrogen. In a dot and cross diagram, valence electrons from one atom are shown as dots, and valence electrons from the other atom are shown as crosses. Shared electron pairs are placed in the overlapping region between the atomic symbols. For example, in a water molecule (H2O), oxygen (Group 16, 6 valence electrons) forms two single covalent bonds with two hydrogen atoms (Group 1, 1 valence electron each). Each hydrogen shares one electron with oxygen, and oxygen shares one electron with each hydrogen. Oxygen completes its octet, and each hydrogen completes its duet.

Ionic Bonding and Dot and Cross Diagrams

Ionic bonds typically form between a metal atom and a non-metal atom. This type of bonding involves the complete transfer of valence electrons from the metal atom to the non-metal atom. The metal atom loses electrons to form a positive ion (cation), and the non-metal atom gains electrons to form a negative ion (anion). Both ions achieve a stable, full outer electron shell. In the dot and cross diagram, the electron transfer is indicated. The resulting ions are shown separately, enclosed in square brackets, with their new electron configurations (using either dots or crosses, but consistently tracking the origin of electrons) and their overall charges written outside the brackets. For instance, in sodium chloride (NaCl), sodium (Group 1, 1 valence electron) transfers its electron to chlorine (Group 17, 7 valence electrons). Sodium becomes Na+ with an empty outer shell that reveals a full inner shell, and chlorine becomes Cl- with a full outer shell of 8 electrons.

Steps for Drawing Dot and Cross Diagrams

1. Determine the number of valence electrons for each atom involved. This is usually the group number for main group elements (e.g., Group 1 has 1 valence electron, Group 17 has 7 valence electrons). 2. Identify whether the bonding is covalent (non-metal + non-metal) or ionic (metal + non-metal). 3. For covalent bonds: Draw the atoms with their valence electrons using dots for one atom and crosses for the other. Overlap the outer shells and place shared pairs of electrons in the overlapping region. Ensure each atom achieves a stable outer shell. 4. For ionic bonds: Draw the metal atom transferring its valence electrons to the non-metal atom. Show the resultant positive and negative ions with their full outer shells in square brackets and appropriate charges external to the brackets. 5. Make sure the total number of electrons in the diagram accurately reflects all valence electrons initially present and any electron transfers.

What Bacteria Spreader Is Used For

Bacteria Spreader is practical apparatus for handling cultures during microbiology practical work. In GCSE or introductory practical science, the annotated diagram helps connect the apparatus to the method, measurement, or observation being tested.

How To Use It In An Exam Answer

Use the marker labels to name the apparatus first, then say why it is suitable. A strong answer links bacteria spreader to handling cultures during microbiology practical work, rather than only saying that it is used in a lab.

Syllabus Link

Primary private mapping: OCR GCSE chemistry-a-gateway J248, Elements, compounds and mixtures.

Common Trap

Do not describe apparatus by appearance only. Marks usually come from explaining its function in the practical method.

What Bacteria Spreader Is Used For

Bacteria Spreader is practical apparatus for handling cultures during microbiology practical work. In GCSE or introductory practical science, the annotated diagram helps connect the apparatus to the method, measurement, or observation being tested.

How To Use It In An Exam Answer

Use the marker labels to name the apparatus first, then say why it is suitable. A strong answer links bacteria spreader to handling cultures during microbiology practical work, rather than only saying that it is used in a lab.

Syllabus Link

Primary private mapping: OCR GCSE chemistry-a-gateway J248, Elements, compounds and mixtures.

Common Trap

Do not describe apparatus by appearance only. Marks usually come from explaining its function in the practical method.