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CHEMISTRY FORM 1


i Common Chemistry Laboratory Chemicals
ii Common Chemistry Laboratory Apparatus
iii Safety in the Chemistry Laboratory
iv Why we should learn Chemistry
1. INTRODUCTION TO CHEMISTRY
2. SIMPLE CLASSIFICATION OF SUBSTANCES AND SEPERATION OF MIXTURES
3. ACIDS, BASES AND INDICATORS
4. AIR AND COMBUSTION
5. WATER AND HYDROGEN
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Simple Classification of Substances and Separation of Mixtures: Effect of impurities on melting and boiling points

2.0 Simple Classification of Substances and Separation of Mixtures


2.14 Effect of impurities on melting and boiling points


To study the effects of impurities, we should first know the melting and boiling points of a pure substance.


What are the melting and boiling points of pure water?


Materials and substances required
  1. Ice cubes or blocks frozen from pure water
  2. Boiling tube
  3. Thermometer
  4. Retort stand, clamp, clock
  5. Bunsen burner

Set-up melting point and boiling point, high school chemistry

Alternative A: Small quantities of ice and is faster. The flame shouldn't be so close


Set-up melting point and boiling point high school chemistry

Alternative B: Slower but no spurting (jumping out) of hot liquid


melting point and boiling point  high school chemistry

Temperature is read and recorded every 1 or 2 minutes during melting and all the way until the liquid boils for 3 to 5 minutes. The table can be made longer. A graph is plotted of Temperature (y-axis) against temperature.


NB: The heating should be steady, with the flame kept the same distance below the boiling tube or beaker. Use card shields to block direct wind if necessary. Observe the demonstrations on melting and boiling points of a pure substance.


Questions 2.14(a)

  1. What happens to the temperature of pure ice during melting?
  2. What happens to the temperature of pure water (obtained from pure ice) during boiling?
  3. What would be the shape of a temperature-versus-time heating graph (curve) for a pure substance heated from below its melting point till it boils for about 3-5 minutes?
  4. What would be the shape of a temperature-versus-time cooling graph (curve) for a pure substance cooled from its boiling point till it freezes?

Answers to Questions 2.14(a)


When temperature remains steady during melting or boiling, the substance is said to have a sharp melting point and a sharp boiling point respectively. These confirm that the substance is pure.


What is the effect of impurities on melting and boiling points?


Materials and substances required
  1. Ice cubes (blocks) and hammer to break them
  2. Sodium chloride
  3. Thermometer
  4. Beaker or boiling tube

Set-up effect of impurities on melting and boiling points high school chemistry

Open the link, effect of impurities on melting point,


in which sodium chloride is added as an impurity to ice cubes. Note what happens to ice at the point of application of salt and the change in temperature.


Questions 2.14(b)

  1. What happens to the ice and temperature at the point where sodium chloride is added (before heating)?
  2. What does this tell us about the effect of impurities on melting point?

Answers to Questions 2.14(b)


Continuation of investigation


Set-up effect of impurities on melting and boiling points high school chemistry

effect of impurities on melting and boiling points high school chemistry

NB: When all the ice has melted, the mixture is heated steadily till it boils for 3-5 minutes.
Observe the demonstration on the effects of impurities on boiling points.


Questions 2.14(c)

  1. Does temperature remain steady or increase during boiling of impure water?
  2. How do impurities affect boiling point?
  3. Pure naphthalene melts at 80 °C and boils at 218 °C. Pure camphor melts at 175 °C and boils at 209 °C. A small amount of camphor is mixed with naphthalene and the mixture is ground into a fine powder. Complete the following statements about the mixture.

    The mixture will start melting (a) --- (at, above, below) (b) ----°C.

    The mixture will begin boiling (c) --- (at, above, below) (d) -----°C.

    Select the appropriate words given in the brackets to fill in (a) and (c), and two of the values given to complete (b) and (d).
  4. What is the shape of a heating curve for impure substance?
  5. What is the shape of a cooling curve for an impure substance?

Answers to Questions 2.14(c)


Summary of differences between pure and impure substances


difference between pure and impure substances high school chemistry

At home and on roads

Sodium chloride salt is sometimes added to food at the beginning of cooking. Among other reasons, the salt raises boiling point of water, enabling the food to cook faster. Also the salt is added to ice to lower (depress) its melting point; so it cools things more effectively.


In the cold temperate and polar regions, calcium chloride is used as an impurity to melt away snow cover on roads. Take a look.