Acids and Alkalis

Definition and Properties

Acids are substances that release hydrogen ions (H⁺) when dissolved in water. Alkalis are a special type of base that dissolve in water to release hydroxide ions (OH⁻).

The strength of an acid or alkali is often measured using the pH scale, which ranges from 0 to 14. Acids have pH values less than 7, alkalis have pH values greater than 7, and a pH of 7 is neutral (pure water).

Common examples of acids include:

• Hydrochloric acid (HCl) – found in stomach acid and used in industry
• Sulfuric acid (H₂SO₄) – used in car batteries and fertilisers
• Ethanoic acid (CH₃COOH) – vinegar

Common alkalis include:

• Sodium hydroxide (NaOH) – used in drain cleaners
• Potassium hydroxide (KOH) – used in soaps
• Ammonia solution (NH₃ in water) – used as a cleaning agent

Neutralisation Reactions

Neutralisation is the reaction between an acid and an alkali to form a salt and water. This happens because the hydrogen ions (H⁺) from the acid react with the hydroxide ions (OH⁻) from the alkali to produce water (H₂O).

The general equation for neutralisation is:

Acid + Alkali 12 Salt + Water

In ionic form:

H⁺ (aq) + OH⁻ (aq) 12 H₂O (l)

Indicators such as litmus, phenolphthalein, or methyl orange can be used to show when neutralisation has occurred by changing colour at different pH levels.

For example, when hydrochloric acid (HCl) neutralises sodium hydroxide (NaOH), the salt formed is sodium chloride (NaCl):

HCl + NaOH 12 NaCl + H₂O

For instance, if you mix equal amounts of hydrochloric acid and sodium hydroxide, the solution becomes neutral, and the indicator changes colour accordingly.

Reactions with Metals

Acids react with certain metals to produce a salt and hydrogen gas. This is a common reaction for metals like magnesium, zinc, and iron.

The general word equation is:

Acid + Metal 12 Salt + Hydrogen

For example, hydrochloric acid reacts with magnesium to produce magnesium chloride and hydrogen gas:

Mg + 2HCl 12 MgCl₂ + H₂

The rate of reaction depends on factors such as:

• Surface area of the metal (more surface area = faster reaction)
• Concentration of the acid (higher concentration = faster reaction)
• Temperature (higher temperature = faster reaction)

For instance, if magnesium ribbon is added to dilute hydrochloric acid, bubbles of hydrogen gas are seen, and the magnesium gradually disappears as it forms magnesium chloride.

Reactions with Carbonates

Acids react with carbonates to produce a salt, water, and carbon dioxide gas. Carbonates are compounds containing the carbonate ion (CO₃^22).

The general word equation is:

Acid + Carbonate 12 Salt + Water + Carbon Dioxide

For example, hydrochloric acid reacts with calcium carbonate (marble chips) to produce calcium chloride, water, and carbon dioxide:

2HCl + CaCO₃ 12 CaCl₂ + H₂O + CO₂

The carbon dioxide gas produced can be tested by bubbling it through limewater (a solution of calcium hydroxide). If carbon dioxide is present, the limewater turns cloudy (milky).

For instance, when vinegar (ethanoic acid) is added to baking soda (sodium bicarbonate), bubbles of carbon dioxide gas are produced, causing fizzing.