Physical & Chemical Properties of Acids

Physical Properties of Acids

acid turns blue litmust paper red

Acids have the following properties:

  • Acids have a sour taste
  • Acid solutions have pH values less than 7. (More about pH values in the next few sub-topics)
  • Acid solutions turn blue litmus paper (an indicator) red.

All acids have a sour taste in dilute solution.

  • The sour taste found in lemon juice is due to citric acid.
  • Vinegar is sour because it contains ethanoic acid.
  • Sour milk contains lactic acid.

Chemical Properties of Acids

There are three common reactions of acids:

  • $\text{Acid} + \text{metal} \rightarrow \text{salt} + \text{hydrogen gas}$
  • $\text{Acid} + \text{base} \rightarrow \text{salt} + \text{water}$
  • $\text{Acid} + \text{carbonate} \rightarrow \text{salt} + \text{water} + \text{carbon dioxide gas}$

We will look into these reactions in detail below:

Acid Reacting With Metals

MAGNESIUM IN ACID Magnesium reacts in strong and weak acids. Magnesium reacts at different rates in a strong acid solution (left) and weak acid solution (right).
Magnesium reacts in strong and weak acids.
Magnesium reacts at different rates in a strong acid solution (right) and weak acid solution (left).

Many acids react with reactive metals to liberate hydrogen gas.

Metals above copper in the electrochemical series react with dilute acid to produce hydrogen gas. (Electrochemical series will be introduced in the “Metals” topic)

One example of an acid reacting with a metal:

  • When a piece of magnesium metal is placed into dilute hydrochloric acid, it dissolves rapidly and effervescence of colourless, odourless gas is produced which gives a ‘pop’ sound when it is lit up. The gas produced in hydrogen.
  • $\text{Mg} (\text{s}) + 2 \text{HCl} (\text{aq}) \rightarrow \text{MgCl}_{2} (\text{aq}) + \text{H}_{2} (\text{g})$
  • $\text{MgCl}$ is the salt produced from the chemical reaction between dilute hydrochloric acid and magnesium.

However, there are some acid and metal reactions that do not give off hydrogen gas.

  • When unreactive metals (such as copper or silver) are added to dilute acids, there is no reaction.
  • Concentrated nitric acid reacts with metals such as copper but it does not give off hydrogen gas. Instead, a nitrate (salt), water and nitrogen dioxide gas are formed.
  • Lead appears not to react (or react very slowly) with dilute hydrochloric acid and dilute sulphuric acid. This is because a layer of insoluble salt, lead(II) chloride or lead(II) sulphate, is formed from the initial reaction between lead and the dilute acid. This layer is insoluble in water and quickly forms a coating around the metal. The coating protects the metal from further reaction with the acid.

Very reactive metals such as sodium, potassium and calcium react explosively with acids.

Acid Reacting With Bases

Note: The introduction to bases is in the next sub-topic.

Acids react with bases to form a salt and water only. No other products are formed.

  • E.g. Copper(II) oxide reacting with nitric acid. $\text{CuO} (\text{s}) + 2 \text{HNO}_{3} (\text{aq}) \rightarrow \text{Cu}(\text{NO}_{3})_{2} (\text{aq}) + \text{H}_{2}\text{O} (\text{l})$

Lead(II) oxide and lead(II) hydroxide react very slowly with hydrochloric acid due to the formation of of insoluble lead salts.

Acid Reacting With Carbonates

Acids react with carbonate or hydrogen carbonate to form salt, water and carbon dioxide gas.

  • E.g. Calcium carbonate (limestone), $\text{CaCO}_{3}$, and dilute hydrochloric acid react to give off carbon dioxide gas.
  • $\text{CaCO}_{3} (\text{s}) + 2 \text{HCl} (\text{aq}) \rightarrow \text{CaCl}_{2} (\text{aq}) + \text{H}_{2}\text{O} (\text{l}) + \text{CO}_{2} (\text{g})$

calcium carbonate in acid

The carbon dioxide given off produces an effervescence. Carbon dioxide turns the limewater chalky.

This reaction is often used as a test for acid.

Preparation of Acids

There are two general ways to prepare acids:

  • dissolving a non-metallic oxide in water
  • heating the salt of a volatile acid with concentrated sulphuric acid

Dissolving Non-metallic Oxide In Water

Non-metallic oxides that are soluble in water will dissolve to form an acidic solution. Several examples are listed below.

Carbon dioxide (oxide of carbon) dissolves in water to form carbonic acid.

$$\text{CO}_{2} (\text{g}) + \text{H}_{2}\text{O} (\text{l}) \rightleftharpoons \text{H}_{2}\text{CO}_{3} (\text{aq})$$

Sulphur dioxide (oxide of sulphur) dissolves in water to form sulphurous acid.

$$\text{SO}_{2} (\text{g}) + \text{H}_{2}\text{O} (\text{l}) \rightleftharpoons \text{H}_{2}\text{SO}_{3} (\text{aq})$$

Sulphur trioxide (oxide of sulphur dissolves in water to form sulphuric acid.

$$\text{SO}_{3} (\text{g}) + \text{H}_{2}\text{O} (\text{l}) \rightarrow \text{H}_{2}\text{SO}_{4} (\text{aq})$$

Nitrogen dioxide (oxide of nitrogen) dissolves in water to form nitrous and nitric acid.

$$2\text{NO}_{2} (\text{g}) + \text{H}_{2}\text{O} (\text{l}) \rightarrow \text{HNO}_{2} (\text{aq}) + \text{HNO}_{3} (\text{aq})$$

Phosphorus (V) oxide (oxide of phosphorus) dissolves in water to form phosphoric acid.

$$\text{P}_{2}\text{O}_{5} (\text{s}) + 3\text{H}_{2}\text{O} (\text{l}) \rightarrow 2\text{H}_{3}\text{PO}_{4} (\text{aq})$$

Heating The Salt of a Volatile Acid

When the sodium or potassium salt of a volatile acid is heated with concentrated sulphuric acid, the volatile acid is driven off. The acid can then be condensed or dissolved in water.

Volatile acids such as hydrochloric acid is prepared by heating sodium chloride with concentrated sulphuric acid.

$$\text{NaCl} (\text{s}) + \text{H}_{2}\text{SO}_{4} (\text{aq}) \rightarrow \text{NaHSO}_{4} (\text{aq}) + \text{HCl} (\text{g})$$

Nitric acid is prepared by heating sodium nitrate with concentrated sulphuric acid.

$$\text{NaNO}_{3} (\text{s}) + \text{H}_{2}\text{SO}_{4} (\text{aq}) \rightarrow \text{NaHSO}_{4} (\text{aq}) + \text{HNO}_{3} (\text{g})$$

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