Does a reaction occur when aqueous solutions of calcium chloride and potassium hydroxide are combined?

Calcium chloride, #"CaCl"_2#, and sodium hydroxide, #"NaOH"#, are soluble in water, which implies that they exist as ions in aqueous solution.

#"CaCl"_ (2(aq)) -> "Ca"_ ((aq))^(2+) + 2"Cl"_ ((aq))^(-)#

#"NaOH"_ ((aq)) -> "Na"_ ((aq))^(+) + "OH"_ ((aq))^(-)#

When you mix aqueous solutions of calcium chloride and sodium hydroxide, the calcium cations will combine with the hydroxide anions to form calcium hydroxide, #"Ca"("OH")_2#, an insoluble solid that precipitates out of solution. The other product is aqueous sodium chloride, itself a soluble ionic compound that exists as ions in solution.

So the two cations are exchanging partners, which is why this reaction is a double replacement reaction.

The calcium cations start combined with the chloride anions in solid calcium chloride and end up combined with the hydroxide anions.

Similarly, the sodium cations start combined with the hydroxide anions in solid sodium hydroxide and end up combined with the chloride anions--keep in mind that if we were to evaporate all the water, we would get solid sodium chloride as the second product of the reaction.

So, put all this together to get

#"CaCl"_ (2(aq)) + 2"NaOH"_ ((aq)) -> "Ca"("OH")_ (2(s)) darr + 2"NaCl"_ ((aq))#

The complete ionic equation looks like this

#"Ca"_ ((aq))^(2+) + 2"Cl"_ ((aq))^(-) + 2"Na"_ ((aq))^(+) + 2"OH"_ ((aq))^(-) -> "Ca"("OH")_ (2(s)) darr + 2"Na"_ ((aq))^(+) + 2"Cl"_ ((aq))^(-)#

The net ionic equation, which you get by eliminating the spectator ions

#"Ca"_ ((aq))^(2+) + color(red)(cancel(color(black)(2"Cl"_ ((aq))^(-)))) + color(red)(cancel(color(black)(2"Na"_ ((aq))^(+)))) + 2"OH"_ ((aq))^(-) -> "Ca"("OH")_ (2(s)) darr + color(red)(cancel(color(black)(2"Na"_ ((aq))^(+)))) + color(red)(cancel(color(black)(2"Cl"_ ((aq))^(-))))#

looks like this

#"Ca"_ ((aq))^(2+) + 2"OH"_ ((aq))^(-) -> "Ca"("OH") _(2(s)) darr#

This is a precipitation reaction: Ca(OH)2 is the formed precipitate.

Reactants:

CaCl2 – Calcium chloride
- Other names: Calcium(II) chloride
  , Calcium dichloride
  , E509
  Calcium chloride (anhydrous)
  , Calcium chloride anhydrous
  , CaCl2
  , Calcium chloride, anhydrous
  , Calcium Chloride
  , Calcium(2+) chloride
- Appearance: White powder, hygroscopic
  ; Odourless hygroscopic colourless or white crystals
KOH – Potassium hydroxide
- Other names: Caustic potash
  , Lye
  , Potash lye
  Potassia
  , Potassium hydrate
  , KOH
  , E525
  , Potassium lye
  , Lye [Potassium hydroxide]
- Appearance: White solid, deliquescent
  ; White hygroscopic solid in various forms
  ; Odorless, white or slightly yellow lumps, rods, flakes, sticks, or pellets. [Note: May be used as an aqueous solution.]

Products:

KCl
- Names: Potassium chloride
  , Sylvite
  , Muriate of potash
  Klor-Con M15
  , K-Lease
  , Klor-Con 8
  , K+10
  , K+8
  , Klor-Con M10
  , Micro-K 10
  , Kato
  , Kaon Cl-10
  , Kaon-Cl
  , Kaon Cl
  , Micro-K
  , Klor-Con 10Meq
  , Klor-Con 10
  , Micro-K Ls
  , K-dur
  , Klor-Con
  , Klor-Con M20
  , Slow-K
  , Ten-K
  , Klotrix
  , [KCl]
  , Kaliumchlorid
  , Chloride of potash
  , Kaon-CL 10
  , Klor-CON
  , KCl
  , Monopotassium chloride
  , K-Tab
  , Chlorid draselny
  , Potassium Chloride
- Appearance: White crystalline solid
  ; Colourless hygroscopic crystals
Ca(OH)2 – Calcium hydroxide
- Other names: {{ubl
  , Slaked lime
  , Calcium hydrate
  Caustic lime
  , Hydrated lime
  , Ca(OH)2
  , Hydralime
  , Calcium dihydroxide
- Appearance: White powder
  ; Colourless crystals or white powder
  ; White, odorless powder. [Note: Readily absorbs CO2 from the air to form calcium carbonate.]