Acids and acidic solutions

• Acids are very common in our food. Citrus fruits such as lemons and oranges contain citric acid and ascorbic acid, which is more commonly known as vitamin C. Carbonated drinks, such as cola, contain phosphoric acid and carbonic acid. Vinegar contains ethanoic acid (also known as Acetic acid). Stale wine also contains ethanoic acid. Our own stomachs contains hydrochloric acid which aids the digestion of food and kills microbes we might have eaten or drunk. Oxalic acid is a poisonous acid found in the leaves of rhubarb plants and other plants. The sting of a bee contains an acid, and the medicine aspirin contains an acid found in the bark of the willow tree.

• Acids can be found as solids, liquids or as gases.

• Acids are substances that form acidic solutions in water. Acidic solutions contain water, and other ions, including hydrogen ions, H⁺ . When HCl (hydrogen chloride) gas is dissolved in water, hydrogen ions, H⁺ , and chloride ions, Cl^- , form because HCl is an acidic substance:

H₂O

HCl_(g) → H⁺ _(aq) + Cl^- _(aq)

• Common lab acids include:

hydrochloric acid (HCl),

nitric acid (HNO₃),

sulfuric acid (H₂SO₄)

• Acids have a sour taste. Lemons, vinegar and sour lollies and sweets contain acids.

• Acids change the colour of acid-base indicators. Different indicators turn different colours in acidic solution. The indicators you need to know about are Universal Indicator and Litmus. See the page on the pH scale and indicators.

• Acids can be strong or weak. Strong acids make more hydrogen ions, H⁺, than weak acids when they dissociate or form ions in water. Strong acids include the common laboratory acids (hydrochloric acid, nitric acid and sulfuric acid). Many weak acids are organic acids, such as vinegar, citric acid etc.

• Acid take part in many reactions. The reaction you need to know about is when acids react with bases to make a salt compound and water. See the page on neutralisation form more about this.

Bases and Basic solutions

• Bases are substances that are the 'chemical opposites' of acids. They can neutralise acids. Just like acids, bases can be found as solid, liquid (solution), or gases.

• Bases are found in many useful every substances e.g. in oven and drain cleaner, cleaning products, indigestion tablets (antacids), washing soda crystals, and soaps etc. We also use bases in cooking e.g. sodium carbonate (baking powder), and sodium hydrogen carbonate (bicarbonate of soda or baking soda).

• Bases are substances that form alkaline solutions in water. Alkaline solutions contain water, and other ions, including hydroxide ions, OH^- . All alkalis are bases, but only bases that dissolve in water to form solutions are alkalis. When NaOH (sodium hydroxide) solid is dissolved in water, hydroxide ions, OH^- , and sodium ions, Na⁺, form because NaOH is an basic substance:

H₂O

NaOH_(s) → Na⁺ _(aq) + OH^- _(aq)

• Common lab bases include:

sodium hydroxide, NaOH)

Calcium hydroxide, Ca(OH)₂, or as it more commonly known, limewater

ammonia solution, NH₃ (also known as ammonium hydroxide, NH₄OH

• Many metal oxides, metal hydroxides, metal carbonates and metal hydrogen carbonates are bases.

• Bases have a bitter taste and many bases, like soap, are slippery to the touch.

• Bases also change the colour of acid-base indicators. Different indicators turn different colours in basic solutions. The indicators you need to know about are Universal Indicator and Litmus. See the page on the pH scale and indicators.

• Bases can also be strong or weak. Strong bases make more hydroxide ions, OH^-, than weak bases when they dissociate or form ions in water.

• Bases also take part in many reactions. The reaction you need to know about is when bases react with acids to make a salt compound and water. See the page on neutralisation form more about this.

Neutral and Amphoteric substances

• Neutral substances are substances that are neither acidic or basic. Sodium chloride (table salt) dissolves in water to form a neutral solution. This is because pure water is neutral and only sodium and chloride ions dissociate (or are released) as sodium chlorides dissolves in water.

• Amphoteric substances are substances that can react with both acids and bases. A few metal oxides and hydroxides, such as aluminium oxide, (AlO₃) and aluminium hydroxide, Al(OH)₃ are amphoteric.

Links to pages on pH Scale & Neutralisation