Sodium hydroxide and reactions with it

Chemical properties of caustic soda

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Gen­er­al char­ac­ter­is­tics

Sodi­um hy­drox­ide is a sub­stance that is clas­si­fied as an al­ka­li. Oth­er names for sodi­um hy­drox­ide are caus­tic soda, caus­tic, lye and caus­tic al­ka­li. It is a hard white sub­stance, which can ab­sorb wa­ter va­por and car­bon diox­ide from the air. For ex­am­ple, if you leave sodi­um hy­drox­ide in an open jar, the sub­stance swift­ly ab­sorbs wa­ter va­por from the air, and af­ter a while will turn into a form­less mass. So it is sold in her­met­ic vac­u­um con­tain­ers.

It is also ad­vis­able not to store crys­tals in glass con­tain­ers, as sodi­um hy­drox­ide can en­ter into a re­ac­tion with glass and cor­rode it. When sodi­um hy­drox­ide is dis­solved in wa­ter, a large amount of warmth is re­leased, and the so­lu­tion heats up.

Sodi­um hy­drox­ide is a dan­ger­ous and caus­tic sub­stance, which can cause se­vere burns to the skin. So when work­ing with caus­tic soda, one must ob­serve safe­ty reg­u­la­tions.

At present, NaOH is a pop­u­lar sub­stance in in­dus­tri­al pro­duc­tion, for ex­am­ple around 57 mil­lion tons of it are pro­duced and used world­wide each year.

Phys­i­cal prop­er­ties

[Wikimedia]

It is a hard white sub­stance, hy­dro­scop­ic, and dis­solves well in wa­ter. It has a melt­ing and boil­ing tem­per­a­ture of 323 and 1403 de­grees Cel­sius re­spec­tive­ly.

Ob­tain­ing NaOH

Sodi­um hy­drox­ide is ob­tained by elec­tro­chem­i­cal meth­ods. One of these meth­ods is the elec­trol­y­sis of a so­lu­tion of halite, a nat­u­ral min­er­al which main­ly con­sists of sodi­um chlo­ride (ta­ble salt). Hy­dro­gen and chlo­rine also form when this method is used.

2Na­Cl + 2H₂O → H₂ + Cl₂ + 2NaOH

Halite [Flickr]

Ap­pli­ca­tion of sodi­um hy­drox­ide

Al­though the sub­stance is caus­tic and dan­ger­ous, it has found an ap­pli­ca­tion in many spheres of in­dus­try and oth­er fields as well, and on a very large scale:

  • in the pulp and pa­per in­dus­try;
  • in soap man­u­fac­ture;
  • in chem­i­cal branch­es of in­dus­try;
  • in the man­u­fac­ture of bio diesel fuel;
  • in the house­hold for clean­ing pipes;
  • in cos­met­ics;
  • and many oth­er spheres.

The chem­i­cal prop­er­ties of sodi­um hy­drox­ide

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Sodi­um hy­drox­ide dis­solved in wa­ter has a strong al­ka­line medi­um. Al­ka­lis in so­lu­tions are de­ter­mined by spe­cial in­di­ca­tors, for ex­am­ple lit­mus pa­per will turn bright blue. Here you’ll find an im­pres­sive ex­per­i­ment with sodi­um hy­drox­ide.

In the in­ter­ac­tion of sodi­um hy­drox­ide with acids, a neu­tral­iza­tion re­ac­tion takes place. For ex­am­ple, in the re­ac­tion with hy­drochlo­ric acid, sodi­um chlo­ride (ta­ble salt) and wa­ter form:

NaOH + HCl → NaCl + H₂O

The neu­tral­iza­tion process is checked with phe­nolph­thalein, which is added to the al­ka­line so­lu­tion, turn­ing it a crim­son col­or. As acids are added to the NaOH so­lu­tion, the crim­son col­or will grad­u­al­ly dis­ap­pear.

Sodi­um hy­drox­ide re­acts well both with met­als (zinc, alu­minum, ti­ta­ni­um) and non-met­als: halo­gens, sul­fur and phos­pho­rus. In the in­ter­ac­tion of sodi­um hy­drox­ide with alu­minum, sodi­um tetrahy­drox­oa­lu­mi­nate and hy­dro­gen form. This re­ac­tion was used to pro­duce hy­dro­gen to fill air-ships and air bal­loons.

2Al + 2NaOH + 6H₂O → 2Na[Al(OH)₄] + 3H₂↑

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In the in­ter­ac­tion of caus­tic soda with phos­pho­rus, sodi­um hy­pophos­phate and phos­phine (phos­pho­rus hy­dride) are formed:

4P + 3NaOH + 3H₂O → PH₃↑ + 3NaH₂PO₂

In the in­ter­ac­tion of sodi­um hy­drox­ide with sul­fur and halo­gens, a dis­pro­por­tion­ing re­ac­tion takes place, for ex­am­ple with chlo­rine and sul­fur, the re­ac­tions will take place as fol­lows:

3S + 6NaOH → Na₂­SO3 + 2Na₂S + 3H₂O

3Cl₂ + 6NaOH → Na­ClO3 + 5Na­Cl + 3H₂O (on heat­ing)

Cl₂ + 2NaOH → Na­ClO + NaCl + H₂O (room tem­per­a­ture)

When caus­tic soda makes con­tact with fats, an ir­re­versible saponi­fi­ca­tion re­ac­tion takes place, a process used in the man­u­fac­ture of sham­poo, soap and oth­er prod­ucts.

On in­ter­ac­tion with mul­tiatom­ic al­co­hols, white crys­talline sub­stances are formed that dis­solve well in wa­ter, and are known as al­co­ho­lates:

HOCH₂CH₂OH + 2NаOH → NaOCH₂CH₂ONa + 2H₂O

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