Hardness in Water (Leaving Cert Chemistry): Revision Notes

Hardness in Water

What is Hard Water?

Hard water is water that contains high levels of dissolved minerals, primarily calcium ($\text{Ca}^{2+}$) and magnesium ($\text{Mg}^{2+}$) ions.

These ions react with soap to form scum, a greyish, insoluble precipitate, making it difficult to form a lather with soap.

Types of Hardness in Water

There are two types of hardness in water: temporary hardness and permanent hardness.

Temporary Hardness

Cause:

Temporary hardness is caused by the presence of calcium hydrogen carbonate ($\text{Ca(HCO}_3)_2$) and magnesium hydrogen carbonate in the water.

Removal:

It can be removed by boiling. When boiled, calcium hydrogen carbonate decomposes:

$$\text{Ca(HCO}_3)_2 \xrightarrow{\text{heat}} \text{CaCO}_3 \downarrow + \text{H}_2\text{O} + \text{CO}_2$$

The calcium carbonate ($\text{CaCO}_3$) forms a solid precipitate, which is not soluble in water, leading to softer water.

Permanent Hardness

Cause:

Permanent hardness is caused by the presence of calcium sulfate ($\text{CaSO}_4$) or magnesium sulfate ($\text{MgSO}_4$). Unlike temporary hardness, this type cannot be removed by boiling.

Removal:

Permanent hardness requires more advanced methods like ion exchange or chemical treatment to soften the water.

Problems Caused by Hard Water

• Wastes Soap: Hard water reacts with soap to form scum, leading to the need for more soap to produce a lather.
• Scale Formation: Hard water can form scale, a hard deposit, on heating elements in appliances like kettles and boilers. This scale is mainly calcium carbonate, which reduces efficiency and can eventually block pipes.
• Aesthetic Issues: The scum formed by hard water looks dirty and can stain clothing or surfaces.

Tests on Scale Deposits in a Kettle

Scale deposits form when water is heated, especially if the water contains temporary hardness. To test these deposits:

Observation: Look for white or greyish deposits inside the kettle or heating appliances.

Reaction with Acid: If you add an acid, such as vinegar or hydrochloric acid, to the scale, it will fizz as carbon dioxide ($\text{CO}_2$) is released, confirming the presence of calcium carbonate:

$$\text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2$$

Methods of Removing Hardness

Boiling (for temporary hardness):

• Boiling decomposes the hydrogen carbonates into insoluble carbonates, which can be filtered out.
• This method is not effective for permanent hardness.

Distillation:

• Water is boiled, and the vapour is condensed back into liquid.
• This removes all dissolved substances, including the ions responsible for hardness.
• However, distillation is an expensive method and not practical for everyday use.

Washing Soda (Sodium Carbonate):

• Adding washing soda ($\text{Na}_2\text{CO}_3$) can remove hardness by precipitating calcium as calcium carbonate:

$$\text{Ca}^{2+} + \text{CO}_3^{2-} \rightarrow \text{CaCO}_3 \downarrow$$

• This method is less common now due to modern detergents.

Ion Exchange Resin:

• This is the most effective method for removing both temporary and permanent hardness.
• In an ion exchange system, calcium and magnesium ions are exchanged for sodium ions ($\text{Na}^+$) or hydrogen ions ($\text{H}^+$), softening the water.
• Ion exchange systems are widely used in households (e.g., water softeners) and laboratories.

Deionisation and Distillation

Deionisation

• Deionisation involves removing all ions from water, including both cations like $\text{Ca}^{2+}$and anions like $\text{SO}_4^{2-}$
• This is done using a deionising resin, which exchanges ions in the water for $\text{H}^+$ and $\text{OH}^-$, leaving only pure water.
• Use: Deionised water is used in laboratories and industrial processes where pure water is required.

Distilled Water

• Distillation involves boiling water and then condensing the steam back into liquid form, which removes all impurities, including dissolved ions, gases, and organic compounds.
• Use: Distilled water is even purer than deionised water and is often used in medical applications, such as in autoclaves, and for preparing certain chemicals.

Comparison of Deionised and Distilled Water

• Both deionised and distilled water are extremely pure, but distilled water is generally purer because it removes organic impurities and non-ionic substances as well as ions.