Physical properties:

• Sour taste (found in vinegar, sour milk, lemons)
• Most are soluble in water
• Conduct electricity when dissolved
• Corrosive to metals and skin

Chemical properties:

• pH less than 7
• Turn blue litmus paper red
• React with metals to produce hydrogen gas
• React with carbonates to produce carbon dioxide gas

Physical properties:

• Bitter taste
• Slippery or soapy feel
• Most are insoluble in water (soluble bases are called alkalis)
• pH greater than 7

Chemical properties:

• Turn red litmus paper blue

Acids react with different substances in specific ways:

1. Reaction with Metals

Acids react with metals above hydrogen in the reactivity series (such as zinc, magnesium, iron) to produce a salt and hydrogen gas.

Example: Zinc + Hydrochloric acid $Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)$

The hydrogen gas produced can be tested with a lighted splint, which produces a "pop" sound.

2. Reaction with Metal Oxides and Hydroxides

Acids react with metal oxides and hydroxides to produce salt and water only (neutralisation).

Example: Copper(II) oxide + Nitric acid $CuO(s) + 2HNO_3(aq) \rightarrow Cu(NO_3)_2(aq) + H_2O(l)$

3. Reaction with Carbonates and Hydrogencarbonates

Acids react with carbonates and hydrogencarbonates to produce salt, water, and carbon dioxide gas.

Example: Calcium carbonate + Hydrochloric acid $2HCl(aq) + CaCO_3(s) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$

The carbon dioxide gas turns limewater cloudy or milky.

When an acid and a base are mixed in correct proportions, they neutralise each other to produce a salt and water. This is called neutralisation.

General equation: $Acid + Base \rightarrow Salt + Water$

Worked example: Nitric acid reacts with potassium hydroxide: $HNO_3(aq) + KOH(aq) \rightarrow KNO_3(aq) + H_2O(l)$

When an acid reacts with a carbonate, carbon dioxide is also produced: $2HCl(aq) + Na_2CO_3(aq) \rightarrow 2NaCl(aq) + H_2O(l) + CO_2(g)$

Indicators are substances that change colour to show whether a solution is acidic or basic.

• Litmus: Blue litmus turns red in acid; red litmus turns blue in base
• Phenolphthalein: Colourless in acid, pink in base
• Methyl orange: Red in acid, yellow in base
• Universal indicator: Shows different colours for different pH values (red = strong acid, violet = strong base)

pH scale: A scale from 0 to 14 measuring acidity and alkalinity. pH 7 is neutral; below 7 is acidic, above 7 is basic.

A salt is formed from the positive ion of a base and the negative ion of an acid.

• Neutral salts: Formed from strong acid + strong base (e.g., NaCl, K₂SO₄)
• Acidic salts: Formed from strong acid + weak base (e.g., NH₄Cl, FeCl₃)
• Basic salts: Formed from weak acid + strong base (e.g., Na₂CO₃, CH₃COONa)

Treating insect stings: Bee stings are acidic (contain methanoic acid) and can be neutralised by rubbing baking soda (a base) on the affected area. Wasp stings are alkaline and can be neutralised with vinegar.

Relieving indigestion: Excess stomach acid causes indigestion. Antacids like magnesium hydroxide (milk of magnesia) or sodium hydrogencarbonate (baking soda) neutralise the acid.

Soil treatment: Acidic soil harms plant growth. Farmers add calcium oxide (quicklime) or calcium hydroxide to neutralise acidic soil.

Cleaning: Hydrochloric acid cleans metals before electroplating. Sodium hydroxide (caustic soda) is used in soap and drain cleaners.

In Tanzania, small-scale farmers in areas like Mbeya or Kilimanjaro often test their soil pH using simple indicators or by observing crop performance. If the soil is too acidic (common after heavy rains or use of ammonium-based fertilisers), they add agricultural lime (calcium oxide or calcium carbonate) to neutralise it and help crops like maize, beans, and coffee grow better. This direct use of acid-base chemistry improves harvests and household food security.