What Does Sodium Hydroxide React With?

A purple cloud of powder

Sodium hydroxide (NaOH) is a strongly basic organic compound that is also very hygroscopic. Also known as caustic soda or lye, it reacts with a number of compounds to produce very different results.

Dissolving in Water

Sodium hydroxide ionises when it is dissolved in water. While this may not necessarily be a chemical reaction, it is an important characteristic of how this basic compound behaves in an aqueous solution.

In water, sodium hydroxide is able to completely dissociate by breaking apart into sodium ions (Na+) and hydroxide ions (OH-). When dissolved in this way, the solution takes on a very strong alkaline medium represented by a high pH.

The aqueous solution will also feel very slippery, but it is important to avoid contact with skin since sodium hydroxide is most corrosive when dissolved in water. This is because of the excess OH- ions which cause the solution to be extremely basic and caustic.

Reactions with Acids

Sodium hydroxide undergoes a neutralisation reaction when it is combined with acid. This makes it well-suited as a pH regulator in many industries, like water treatment, because of its ability to neutralise or control acidity.

NaOH reacts with acid to produce a water and an ionic compound. For example, when sodium hydroxide reacts with hydrochloric acid (HCl), the hydroxide ion from NaOH reacts with the hydrogen ion in HCl to form water (H2O).

The rest of the acid then combines with the remaining sodium ions to form the ionic compound sodium chloride (NaCl). The neutralisation reaction that happens here can then be checked by phenolphthalein, a common indicator used in acid-base titrations. The reaction between sodium hydroxide and hydrochloric acid can be shown in the following equation:

NaOH + HCl → NaCl + H2O

In a similar way, sulphuric acid (H2SO4) is neutralised by sodium hydroxide. As before, this neutralisation reaction produces water and an ionic compound, this time sodium sulphate (Na2SO4).

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Reactions with Dissolved Metals

When sodium hydroxide reacts with certain dissolved metals, it forms a solid. This reaction is commonly used to remove dissolved metals from a solution, particularly if they are toxic.

When a reaction involves transforming soluble ions into an insoluble solid it is known as a precipitation reaction. Sodium hydroxide can be used to facilitate this reaction in many transition metals, like copper sulphate (CuSO4).

Transition metals are very soluble in water and form coloured solutions when dissolved. Copper sulphate, for example, turns the solution a characteristic light blue. Zinc, on the other hand, produces a white colour.

If sodium hydroxide is introduced into an aqueous solution containing a soluble transition metal, the transition metal is displaced from its compound. This process is called a displacement reaction, and it happens because sodium is a much more reactive metal.

Once the transition metal has been displaced, an insoluble transition metal hydroxide is formed. In the case of copper sulphate, sodium hydroxide displaces the copper and copper hydroxide (Cu(OH)2) is formed. Instead of dissolving, this compound precipitates and appears as a light blue solid in the liquid. At this point, the solution turns colourless and solid copper hydroxide sits at the bottom.

This reaction can be used to identify what transition metal ion exists in a solution, identified by the colour of the final precipitate. A light blue solid shows that there was a presence of Cu2+ ions. A green solid shows that there was a presence of Fe2+ ions whereas a red/brown solid suggests Fe3+ ions. Sodium hydroxide can be used with all of these compounds to bring about precipitation.

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Reactions with Aluminium

Sodium hydroxide reacts with aluminium (Al) powder to form sodium aluminate and hydrogen (H). This was a common method used to fill airships and air balloons because of the rapid evolution of hydrogen gas it produces. It has also been proposed as a source of fuel for hydrogen-powered cars.

The reaction can be carried out in the lab by mixing aluminium powder with a concentrated solution of NaOH in a flask. The gas produced can then be captured by placing a balloon tightly over the flask. The general equation for this reaction is:

2Al (s) +6 NaOH (aq) → 2Na3AlO3 (aq) + 3H2 (g)

If attempting this experiment, it is important to take the necessary safety precautions. The reaction between aluminium and sodium hydroxide is extremely rapid, exothermic and caustic. Hydrogen gas is also explosive, so immense care should be taken.

The by-product that this reaction forms, sodium aluminate, is an important compound on its own and is commonly used as a source for aluminium hydroxide. It is also utilised in water treatment, construction technology and the paper industries.

What Else Does Sodium Hydroxide React With?

Reactions involving sodium hydroxide do not stop here. Some other examples of what sodium hydroxide reacts with include, but are not limited to:

  • Irreversible saponification reaction with fats, which is used in the manufacture of soap, shampoos and other cosmetic products
  • Interaction with multi-atomic alcohols to form alcoholates, white crystalline substances that dissolve well in water
  • Disproportionation reaction with non-metals, such as sulphur or chlorine, which involves a single substance getting oxidised and reduced

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