C2.1

• What is relative atomic mass (Ar)?

  It compares the average mass of one atom of an element using carbon-12 as having 12 unit mass.
• How does the relative atomic mass (Ar) of an element with one isotope compare to its mass number?

  Its Ar will be the same as its mass number.
• How is the relative atomic mass (Ar) calculated for an element with more than one isotope?

  It is the average of the mass numbers of all the different isotopes, taking into account how much there is of each one.
• What does a chemical formula indicate?

  It tells you how many atoms of each element there are in a unit of a substance.
• What is the chemical formula of water?


  H₂O
• What does the chemical formula H₂O tell you about water?

  Each molecule of water contains two hydrogen atoms and one oxygen atom joined together.
• What is an empirical formula?

  It shows the simplest whole-number ratio of the atoms of each element in a compound.
• How can some chemical formulae be simplified?

  Using their highest common factor (HCF).
• What is relative formula mass (Mr)?

  It is the total of all the Ar of all the atoms in a molecule.
• What are the definitions of the following keywords: Element, Compound, and Molecule?

  • Element: Made up of 1 type of atom only.
  • Compound: 2 or more different types of atoms bonded together.
  • Molecule: 2 or more of the same or different atoms bonded together.
• What happens to atoms in a chemical reaction according to symbolic equations?

  Atoms rearrange, and no atoms are created or destroyed, maintaining the same number of atoms on each side of the equation.
• What does a balanced chemical equation show?

  It shows the formulae and number of units for all the substances in a reaction.
• How can you check that you have correctly calculated the Mr values for all substances in a balanced equation?

  The total on the left of the arrow must equal the total on the right.
• What defines a pure substance?

  A pure substance consists of just one element or compound.
• Why are mixtures considered impure substances?

  Because they contain more than one element or compound.
• Can mixtures be useful?

  Yes, many useful materials are mixtures of different substances, often deliberately chosen to produce desired properties.
• What are formulations?

  Formulations are useful mixtures with a precise purpose made by following a 'formula'.
• What is an alloy?

  An alloy is a mixture of a metal with one or more other elements.
• Why is pure gold often mixed with copper for jewelry?

  Because pure gold is very soft, so a harder mixture is often used.
• What is the melting point of a pure substance?

  The melting point is a single temperature at which it changes from solid to liquid.
• How does the melting point of an impure substance compare to that of a pure substance?

  The melting point of an impure substance is less than that of the pure substance and often melts over a range of temperatures.
• What does a greater difference between the measured melting point and the accepted melting point indicate?

  The lower the purity of the substance is likely to be.
• How can you determine the melting point of a substance?

  By heating it and measuring the temperature at which it melts or plotting a graph of temperature over time.
• Why is it important to heat a substance slowly when determining its melting point?

  Heating slowly allows the temperature of the whole sample to increase evenly.
• What is the purpose of stirring a substance as it melts?

  Stirring ensures that the entire sample is at the same temperature.
• What is a solution?

  A solution is when one substance dissolves in another.
• What is the solute in a solution?

  The solute is the substance that dissolves.
• What is the solvent in a solution?

  The solvent is the substance it dissolves in.
• What happens to particles when a substance dissolves?

  The particles separate and become completely mixed with the particles of the solvent.
• What does it mean if a substance is soluble in a solvent?

  It means the substance can dissolve in that solvent.
• What does it mean if a substance is insoluble in a solvent?

  It means the substance cannot dissolve in that solvent.
• Can a substance be soluble in one solvent but insoluble in another?

  Yes, a substance can be soluble in one solvent but insoluble in another.
• What is filtration?

  Filtration separates an insoluble substance in the solid state from substances in the liquid state.
• How does filter paper work in filtration?

  Filter paper has tiny microscopic holes that allow small particles to pass through while retaining larger grains.
• What happens when you filter a mixture of sand and water?

  The water passes through as the filtrate, while the sand stays behind in the filter paper as the residue.
• Can particles of a dissolved solute pass through filter paper?

  Yes, particles of a dissolved solute are small enough to pass through filter paper.
• What is crystallisation?

  Crystallisation is the process where heating a solution causes the solvent to evaporate, leaving the solute behind.
• What happens if you heat a solution too strongly during crystallisation?

  You get a powder instead of regularly shaped crystals.
• What is a saturated solution?

  A solution is saturated when no more solute can be dissolved at that temperature.
• What happens as a saturated solution cools?

  As the solution cools, the solubility of the solute decreases, so more crystals form.