(1.8-1.13) Elements, compounds and mixtures

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11 Terms

1
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What is an element?

A substance in which all atoms have the same atomic number.

2
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What is a compound?

The combination of two or more elements through a chemical reaction so the atoms are chemically bonded to one another.

3
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What is a mixture?

A substance in which atoms from 2 or more different elements are present but they are not chemically bonded.

4
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What distinguishes a pure substance from a mixture?

A pure substance has a fixed melting/boiling point but a mixture can melt or boil over a range of temperatures.

5
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Describe simple distillation to separate mixtures.

A process that uses evaporation and condensation to separate solution:

  • Water from the salt solution boils and becomes steam.

  • The steam travels from the flask into the condenser and cools down.

  • It condenses and becomes liquid water which drips into the beaker.

<p>A process that uses evaporation and condensation to separate solution:</p><ul><li><p>Water from the salt solution boils and becomes steam.</p></li><li><p>The steam travels from the flask into the condenser and cools down.</p></li><li><p>It condenses and becomes liquid water which drips into the beaker.</p></li></ul>
6
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Describe fractional distillation to separate mixtures.

  • The mixture is heated to a temperature at which part of it will vaporise.

  • The fractionating column ensures that the separate parts are collected separately (different substances have different boiling points.)

<ul><li><p>The mixture is heated to a temperature at which part of it will vaporise.</p></li><li><p>The fractionating column ensures that the separate parts are collected separately (different substances have different boiling points.)</p></li></ul>
7
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Describe filtration to separate mixtures.

  • Filter paper is put into a funnel which is held over a beaker.

  • The mixture is poured into the funnel and the filter paper. due to its tiny holes, collects the residue and lets the filtrate into the beaker.

<ul><li><p>Filter paper is put into a funnel which is held over a beaker.</p></li><li><p>The mixture is poured into the funnel and the filter paper. due to its tiny holes, collects the residue and lets the filtrate into the beaker.</p></li></ul>
8
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Describe crystallisation to separate mixtures.

  • Heat the mixture in an evaporating dish over a Bunsen burner.

  • The majority of the water will evaporate away.

  • Leave the dish, now containing the salt and some water, in a warm place and leave the water to slowly evaporate, leaving crystals.

<ul><li><p>Heat the mixture in an evaporating dish over a Bunsen burner.</p></li><li><p>The majority of the water will evaporate away.</p></li><li><p>Leave the dish, now containing the salt and some water, in a warm place and leave the water to slowly evaporate, leaving crystals.</p></li></ul>
9
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Describe chromatography to separate mixtures.

  • Draw a line on chromatography paper in pencil and mark an x.

  • Put mixture on the x and put the paper into the solvent, dot shouldn’t touch water.

  • As the solvent travels up the paper, it takes the mixture with it to different distances.

  • Measure how far each substance has reached.

10
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How does a chromatogram provide information about a mixture’s composition?

By measuring the solvent front, we can find out how many substances are present in the mixture.

11
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State the Rf value equation. How is it helpful?

Rf value =

distance travelled by substance
——————————————
distance travelled by solvent front

If dots have the same Rf value, they are the same substance.