Salts and water

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• Leerjaar 4

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?

Condensed formula of a salt

• Fe²⁺ and OH^-
• Fe(OH)₂
• Do not forget the brackets!

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)

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)

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)

CuCl​2​​(s)→Cu​2+​​(aq)+2Cl​−​​(aq)

Evaporation equations for salts

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

Na​+​​(aq)+Cl​−​​(aq)→NaCl(s)

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!

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.

Binas 45A

Note: this resembles an evaporation equation!

The reaction then becomes

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

Zouthydraten

Examples:

• Gypsum
• Cement

Zouthydraten

Nomenclature:

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

CuSO​4​​⋅5H​2​​O

(H​2​​O)