GCSE Chemistry B (Combined) OCR

This subject is broken down into 59 topics in 6 modules:

  1. Air and Water 13 topics
  2. Chemical Patterns 11 topics
  3. Natural Environments 10 topics
  4. Material Choices 8 topics
  5. Chemical Analysis 10 topics
  6. Making Useful Chemicals 7 topics
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  • 6
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  • 59
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  • 20,624
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  • 2+
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This page was last modified on 28 September 2024.

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Chemistry B (Combined)

Air and Water

States of Matter

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States of Matter

States of Matter

  • Matter is anything that has mass and takes up space.
  • Matter exists in four states: solid, liquid, gas, and plasma.
  • The state of matter is determined by the arrangement of particles and the energy of particles.

Solids

  • In a solid, particles are tightly packed in a fixed, regular arrangement.
  • The particles vibrate in fixed positions but do not move around.
  • Solids have a definite volume and shape.

Liquids

  • In a liquid, particles are close together but can move around each other in a random manner.
  • Liquids take the shape of their container and have a definite volume.
  • The particles in a liquid are constantly colliding and moving past each other.

Gases

  • In a gas, particles are far apart and move in all directions in a random manner.
  • Gases completely fill their container and can be easily compressed.
  • The particles in a gas move fast and collide with each other and the walls of the container.

Plasma

  • Plasma is an ionised gas, meaning it is a gas that has been energised until atomic electrons are dislodged.
  • As a result, plasma contains a mixture of free-moving ions, electrons, positive ions, and neutral particles.
  • Plasma is only found in certain conditions such as in large concentrations of energy (like the sun) or in fluorescent lights.

Changing States of Matter

  • Melting is the transition from solid to liquid, and occurs when the temperature reaches the melting point of the substance.
  • When liquid turns into gas, this is called evaporation.
  • If a gas transforms into a liquid, this process is known as condensation.
  • The transition from solid to gas (or vice versa) without passing through the liquid state is known as sublimation.
  • The energy required to change a substance from a solid to a liquid (or from a liquid to a gas) is called latent heat.

Remember, in an exam situation, you should aim to use correct terminology and make sure to cover all the key points in your answers. Address the question directly, and do not include irrelevant information. Good luck with your revision!

Course material for Chemistry B (Combined), module Air and Water, topic States of Matter

Chemistry B (Combined)

Natural Environments

Covalent Bonding

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Covalent Bonding

Covalent Bonding

  • Covalent bonds are strong bonds that form when two atoms share a pair of electrons.
  • These bonds occur between non-metal atoms, leading to the formation of molecules and covalent compounds.

Formation of Covalent Bonds

  • Covalent bonds form when two adjacent atoms have unshared pairs of electrons or free electron slots in their outermost energy levels, also called valence shells.
  • The strong bonding is a result of both atoms being attracted to the same shared pair of electrons.
  • Each shared pair of electrons constitutes one covalent bond.

Properties of Covalent Compounds

  • Covalent compounds have low boiling and melting points because they consist of simple molecules and the intermolecular forces between these molecules are relatively weak.
  • Most covalent compounds are gases, liquids or soft solids under normal environmental conditions.
  • Covalent compounds do not conduct electricity as no free electrons or ions are available to carry the charge.

Diagrammatic Representation of Covalent Bonds

  • Dot-and-cross diagrams are useful tools to represent covalent bonds. Each dot or cross represents an electron; shared electrons are represented by a pair of electrons (dot and cross) lying between the two atomic symbols.
  • In displaying the formation of covalent bonds, it is important to only fill in the outermost energy levels of the atoms involved.

Polar and Nonpolar Covalent Bonds

  • A polar covalent bond forms when there is a significant difference in electronegativity between the two atoms and the shared electrons spend more time around the more electronegative atom.
  • In a nonpolar covalent bond, the electrons are shared more or less equally between the atoms because the atoms have similar levels of electronegativity.

Multiple Covalent Bonds

  • Some atoms can form double or triple covalent bonds if they share two or three pairs of electrons respectively. For example, oxygen forms a double bond in an oxygen molecule, and nitrogen forms a triple bond in a nitrogen molecule.

Course material for Chemistry B (Combined), module Natural Environments, topic Covalent Bonding

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