Salts and water

Salts and water
How do salts dissolve in water?
Which salts do/ do not dissolve in water?
How do metal oxides react with water?
Precipitation reactions
What are salt hydrates?
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Slide 1: Slide
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This lesson contains 13 slides, with text slides.

time-iconLesson duration is: 45 min

Items in this lesson

Salts and water
How do salts dissolve in water?
Which salts do/ do not dissolve in water?
How do metal oxides react with water?
Precipitation reactions
What are salt hydrates?

Slide 1 - Slide

Condensed formula of a salt
  • Fe2+ and OH-
  • Fe(OH)2
  • Do not forget the brackets!

Slide 2 - Slide

When a salt goes into water...
  • Ions are "pinched off" the ionic grid by water molecules, this process is known as hydration
  • Once an ion is in solution, it is surrounded by a small mantle of water molecules (usually 4-8) and we say it has been dissolved in water
  • The bond between the water molecules and the ion is called the ion-dipole bond
  • Whether hydration even occurs - i.e. whether the salt dissolves in water - depends on the relative  strength of the ionic bonds compared to the ion-dipole bonds
  • this determines a salt's solubility (Binas tabel 45A)

Slide 3 - Slide

When a salt goes into water...
  • Binas tabel 45A:
  • g: Soluble (e.g.: NaNO3)
  • m: slightly soluble (e.g.: MgSO3)
  • s: insoluble (e.g.: CaCO3)
  • r: reacts with water (e.g.: CaO)
  • i: unstable (e.g. NH4OH)

Slide 4 - Slide

Dissolution equations for salts
  • Do not include water in the reaction!

  • General form: Solid salt (s) --> dissolved ions (aq)
  • Multiple ions in the condensed formula means coefficient higher than 1

NaCl(s)Na+(aq)+Cl(aq)
CuCl2(s)Cu2+(aq)+2Cl(aq)

Slide 5 - Slide

Evaporation equations for salts
  • Simply the reverse of dissolution
  • General form: dissolved ions (aq) --> solid salt (s)

Na+(aq)+Cl(aq)NaCl(s)

Slide 6 - Slide

Metal oxides
  • Some metal oxides (Na2O, K2O, CaO en BaO) react with water
  • These have an "r" in Binas 45A
  • For these you DO have to include water in the reaction equation!

Slide 7 - Slide

Precipitation reactions
What can happen when you mix 2 salt solutions:
  • The mixture remains a clear solution; all ions are dissolved
  • The mixture becomes turbid: 2 of the ionic species react, creating solid particles; a precipitate.

Slide 8 - Slide

Binas 45A

Slide 9 - Slide

Note: this resembles an evaporation equation!
The reaction then becomes

Slide 10 - Slide

Salt hydrates
Water molecules are inserted in the crystal lattices of some special salts. These salts (BEFORE adding water!) are called hygroscopic.
The absorbed (technically: "intercalated") water inside the crystal structure is called "water of hydration"

Hydration is always exothermic


Slide 11 - Slide

Zouthydraten
Examples:
  • Gypsum
  • Cement

Slide 12 - Slide

Zouthydraten
Nomenclature:
  • (name of the salt) [centre dot] (no. of watermolecules) 
  • e.g.: 
  • Name: Copper sulfate pentahydrate
  • Trivial names in Binas 66A en 66B
CuSO45H2O
(H2O)

Slide 13 - Slide