Elements, Compounds & Mixtures (Edexcel IGCSE Chemistry)

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  • State what is meant by the term element.

    An element is a substance made of atoms that all contain the same number of protons and cannot be split into anything simpler.

  • True or False?

    Nitrogen is an element.

    True.

    Nitrogen is an element.

  • True or False?

    Water is an element.

    False.

    Water is a compound composed of hydrogen and oxygen atoms chemically combined.

  • State what is meant by the term compound.

    A compound is a pure substance made up of two or more different elements chemically combined together.

  • True or False?

    Compounds can be separated into their elements by physical means.

    False.

    Compounds cannot be separated into their elements by physical means.

  • What is a mixture?

    A mixture is a combination of two or more substances (elements and/or compounds) that are not chemically combined.

  • What is the difference between a compound and a mixture?

    A compound consists of two or more elements that are chemically combined, while a mixture consists of two or more substances that are not chemically combined.

  • True or False?

    All substances can be classified as elements, compounds, or mixtures.

    True.

    All substances can be classified into one of these three types.

  • Classify the following as an element, compound or mixture:

    • Air

    • Ammonia, NH3

    • Chlorine, Cl2

    • Copper sulfate, CuSO4

    The classifications of the following are:

    • Air = mixture

    • Ammonia, NH3 = compound

    • Chlorine, Cl2 = element

    • Copper sulfate, CuSO4 = compound

  • What is the difference between an element and a compound?

    An element consists of only one element, while a compound consists of two or more elements that are chemically combined.

  • Is a copper wire an element or compound?

    Copper wire is an element.

  • Why is copper sulfate not on the Periodic Table?

    Copper sulfate is not in the Periodic Table because it is a compound / not an element.

  • What is the difference between a pure substance and a mixture?

    A pure substance consists of only one substance (this can be an element or a compound), while a mixture contains two or more different substances that are not chemically combined.

  • True or False?

    Salt dissolved in water is a pure substance.

    False.

    A solution of salt dissolved in water is a mixture.

  • State what is meant by the term melting point.

    The melting point is the temperature at which a solid turns into a liquid.

  • What is a boiling point?

    The boiling point is the temperature at which a liquid turns into a gas.

  • What property can be used to distinguish a pure substance from a mixture?

    Melting point and boiling point can be used to distinguish a pure substance from a mixture.

  • True or False?

    Pure substances have a range of melting and boiling points.

    False.

    Pure substances have sharp, specific melting and boiling points, while mixtures have a range of melting and boiling points.

  • What is a cooling curve?

    A cooling curve is a graph that shows how a substance changes from a gas to a liquid to a solid as temperature decreases.

  • What does a horizontal line on a cooling curve indicate?

    A horizontal line on a cooling curve indicates where a substance is changing from a gas to a liquid or from a liquid to a solid.

  • What is a pure substance?

    A pure substance is a single element or compound that contains no other substances.

  • How can melting and boiling points distinguish between pure substances and mixtures?

    Pure substances have specific and sharp melting and boiling points, while mixtures have a range of melting and boiling points.

  • State the use of melting point analysis.

    Melting point analysis is used to assess the purity of a substance.

  • What does simple distillation separate?

    Simple distillation is used to separate:

    • a liquid and soluble solid from a solution (e.g., water from a solution of salt water)

    • a pure liquid from a mixture of liquids

  • State what is meant by the term simple distillation.

    Simple distillation is a technique used to separate a liquid from a solution (e.g., water from a salt solution) by boiling the solution and condensing the vapour.

  • What is fractional distillation?

    Fractional distillation is a technique used to separate two or more liquids that have different boiling points from a mixture.

  • What does filtration separate?

    Filtration separates an undissolved solid from a mixture of solid and liquid.

  • What is crystallisation used for?

    Crystallisation is used to separate a dissolved solid from a solution.

  • In terms of the solid (solute), what is important about the solvent?

    It is important that the solid (solute) is more soluble in hot solvent than in cold.

  • Which property does paper chromatography depend upon?

    Paper chromatography depends upon the solubility of different substances.

  • What is paper chromatography used for?

    Paper chromatography is used to separate substances that have different solubilities in a given solvent, such as identifying the components of a mixture.

  • What is the mobile phase in paper chromatography?

    The mobile phase in chromatography is the solvent / liquid that travels up the paper.

  • Why should the baseline (starting line) in chromatography be drawn in pencil?

    The baseline (starting line) should be drawn in pencil because it will not contaminate the results / will not run.

  • True or False?

    In paper chromatography, a pure substance produces more than one spot.

    False.

    In paper chromatography, a pure substance only produces one spot.

  • Which technique separates solutions with more than one type of dissolved solid (solute)?

    The technique that separates solutions with more than one type of dissolved solid is chromatography.

  • What does a single spot on a chromatogram indicate?

    A single spot on a chromatogram indicates that the substance is pure.

  • True or False?

    Multiple spots on a chromatogram indicate a pure substance.

    False.

    Multiple spots on a chromatogram indicate that the substance is a mixture.

  • State the equation to calculate the retention factor (Rf value).

    The equation for the retention factor (Rf value) is:

    Rf = fraction numerator Distance space travelled space by space sample over denominator Distance space travelled space by space solvent space front end fraction

  • What does an Rf value closer to 1 indicate?

    An Rf value close to 1 indicates that the component is very / highly soluble in the solvent used.

  • True or False?

    The Rf value of a compound is always the same, regardless of the solvent used.

    False.

    The Rf value of a compound depends on the solvent used.

    If the solvent is changed, the Rf value will change.

  • What is the purpose of calculating Rf values in paper chromatography?

    The purpose of calculating Rf values is to allow chemists to identify unknown substances.

  • What does the Rf value range between?

    The Rf values ranges between 0 and 1.

  • What should the initial line on a chromatogram be drawn with?

    The initial line on a chromatogram should be drawn with a pencil.

  • How many substances are in sample D?

    Chromatogram

    There are 3 substances in D.

    Chromatogram
  • True or False?

    The solvent level should start above the pencil line on a chromatogram.

    False.

    The solvent level should not start above the pencil line, as this would ruin the chromatogram.

  • Identify the mobile and stationary phase in the chromatography experiment below.

    Chromatogram 2

    The mobile phase is the water, and the stationary phase is the chromatography paper.

    Chromatogram 2
  • True or False?

    If two substances are the same, they will produce identical chromatograms.

    True.

    If two substances are the same, they will produce identical chromatograms.

  • Why does an Rf value have no units?

    An Rf value does not have units because it is a ratio.

  • Which colours from A-E does black food colouring contain?

    Chromatogram for black ink

    The black food colouring contains A, E and an unknown.

    Chromatogram for black ink