GCSE Chemistry (Combined) Eduqas

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This page was last modified on 28 September 2024.

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Chemistry (Combined)

Core

Purity of Substance

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Purity of Substance

Purity of Substance

Definition and Importance

  • Purity refers to how free a substance is from other substances.
  • In chemical terms, a pure substance contains only one type of element or compound.
  • The purity of substances is important in a variety of fields such as medicine, manufacturing, and research.

Identifying Pure Substances

  • A pure substance will always have definite physical and chemical properties such as boiling point, melting point, density, and solubility.
  • Pure substances always melt and boil at specific temperatures because their microscopic structure is uniform.
  • A pure substance's boiling point or melting point can be used to identify the substance, as these properties are unique to each substance.

Impure Substances

  • Mixtures are considered impure substances as they contain two or more different substances not chemically combined together.
  • For instance, air is a mixture of mainly nitrogen and oxygen gases.
  • Impure substances have varying boiling points and melting points because their microscopic structure differs.
  • As a result, they often melt or boil over a range of temperatures instead of a specific temperature.

Methods of Purity Check

  • Melting Point Testing: The melting point of a substance is compared against its known standard value. The closer the test value is to the standard value, the purer the substance is likely to be.
  • Boiling Point Testing: Similar to the melting point test, the boiling point of a substance can also be used to check its purity by comparing it against known values.
  • Chromatography, in its various forms (paper, thin layer, gas etc.), is used to separate and identify different substances in a mixture. The distances moved by different substances up a chromatography paper or the time taken for substances to appear in a gas chromatograph can help identify and determine the purity of substances.

Purity in Industry

  • In pharmaceuticals, it is vital that each drug contains the exact amount of pure substance stated to ensure it's safe and effective.
  • In food production, the purity of substances such as additives and preservatives is significant for health and safety.

Course material for Chemistry (Combined), module Core, topic Purity of Substance

Chemistry (Combined)

Core

Reactivity Series

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Reactivity Series

Reactivity Series

Definition

  • The reactivity series is a list of metals, arranged in order of their reactivity from highest to lowest.
  • It is also known as the activity series.

Order of Metals in the Reactivity Series

  • The reactivity series from highest to lowest reactivity is: Potassium, Sodium, Lithium, Calcium, Magnesium, Aluminium, Zinc, Iron, Lead, Hydrogen, Copper, Silver, Gold, Platinum.

Reactions to Observe Reactivity

  • Reactivity can be observed through displacement reactions where a more reactive metal will displace a less reactive metal from its oxide, hydroxide or sulfate.
  • Highly reactive metals can react with cold water to form a metal hydroxide and hydrogen.
  • Medium reactive metals react with steam to produce metal oxide and hydrogen.
  • Less reactive metals only react with oxygen and heat, and do not react with water.

Implications of the Reactivity Series

  • Metals higher up in the series, like potassium and sodium, are very reactive. They will readily participate in chemical reactions.
  • Elements near the bottom of the series, like gold and platinum, are unreactive and resist corrosion and tarnishing.
  • The reactivity series helps to predict the outcomes of chemical reactions. For example, in a redox reaction, a higher reactive metal will reduce a lower reactive metal ion.

Using the Reactivity Series in Extraction of Metals

  • Metals less reactive than carbon can be extracted from their ores by reduction using carbon. For example, iron oxide is reduced to iron in a blast furnace using carbon.
  • Very reactive metals, such as aluminium and sodium, cannot be extracted using carbon and are instead obtained using electrolysis.

Hydrogen and the Reactivity Series

  • Hydrogen is included in the reactive series because it also shows displacement reactions with metals and helps in determining the reactivity of metals.
  • Metals above Hydrogen in the activity series can displace it from acids while metals below cannot.

Course material for Chemistry (Combined), module Core, topic Reactivity Series

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