Acids & Alkalis (Bases) Chemistry
What Acids & Alkalis are like? • Acids:
Acids: • Liquids. • Sharp, sour taste. • Weak acids are found in food, e.g.: Limes, vinegar. • Strong acids are ‘Corrosive’, e.g.: Hydrochloric acid.
Some Acids are dangerous: Some acids like Hydrochloric acid, Sulfuric acid, and Nitric acid are dangerous. They are considered strong acids, so they are ‘Corrosive’. When we mix an acid we water, we say we ‘dilute’ them. e.g. Mixing Hydrochloric acid with water gives us ‘Dilute Hydrochloric acid’
Alkalis (Bases): • Alkalis are the dissolved forms of Bases, alkalis are bases that were dissolved in water. Hence, Alkalis are liquids (solutions), Bases are solids! • Bitter taste. • Some weak alkalis are safe to use, e.g.: Soap, toothpaste. • Strong alkalis are Corrosive, e.g.: Sodium hydroxide.
Some Alkalis are dangerous! When you get a strong alkali on your skin, it dissolves your Skin away! So, skin feels Soapy! This is called ‘Chemical burn’. Many kitchen cleaners contain alkalis that attack grease.
Telling Acids & Alkalis apart: • Sodium hydroxide, hydrochloric acids, lemonade, and water are all colorless liquids. They look the same. • How could we separate them?
We can also use the following ‘Indicators’: • Natural Indicators: Vegetable extracts: e.g. Red cabbage, beetroot juice. Fruit extracts: e.g. Blackcurrant juice. Lichen: e.g. Litmus.
Litmus. • Litmus is a dye. • It is a very common indicator. • In Acids: Litmus turns RED. • In Alkalis: Litmus turns BLUE. • In Neutral Solutions (like water) : Litmus is PURPLE. Lichens
Universal Indicator & the pH scale: • Universal indicator: is a special type of indicator that tells us how strong or weak an acid or an alkali is.
The pH scale: • From the pH scale, we can conclude that: • pH of Acids: 0 – 6.9 • pH of Alkalis: 7.1 – 14 • pH of Neutral solutions, e.g. Water: 7
pH scale on Universal indicator: Universal indicators are very precise in indicating strong & weak acids and alkalis:
Knowledge checkpoint: • This chart shows what happens when you test different substances with Universal indicator:
Examples of Acids: • Hydrochloric acid (HCl). • Sulfuric acid (H2SO4). • Nitric acid (HNO3). • Examples of Alkalis: • Sodium hydroxide (NaOH) • Potassium hydroxide (KOH) • Sodium bicarbonate (NaHCO3) • Magnesium Oxide (MgO) • Ammonia solution (NH4OH) • P.S.: Sodium, Potassium, and Magnesium present in alkalis are all Metals.
Neutralisation: • An acid reacts with an alkali to cancel it out. This reaction is called Neutralization. • The pH of the new substances (Salt, and Water) will be 7. Acid + Alkali Salt + Water
How Salt is formed & named? • When an acid & an alkali neutralise each other, they produce a salt. The name of the salt depends on which acid and alkali were used: E.g.: Hydrochloric acid + Sodium hydroxide Sodium chloride + Water The salt name here is SodiumChloride. Its first name is the same as the first name of the alkali, usually a metal. Its second name comes from the name of the acid. • Sodium hydroxide gives Sodium. • Hydrochloric acid gives Chloride. This gives the salt the name Sodium chloride.
Names of Alkalis are composed of a Metal + a Base: - Examples of Metal hydroxides: Sodium hydroxide, Calcium hydroxide, etc. - Examples of Metal Carbonates: Sodium carbonate, Potassium carbonate, etc. - Examples of Metal Oxides: Magnesium oxide, Zinc oxide, etc. - Ammonia Solution DOESN’T attach to a metal.
Knowledge checkpoint: • Give examples, and name the salt that results from the following word equations: Acid + Metal Hydroxide = Metal Salt + Water Acid + Metal Oxide = Metal Salt + Water Acid + Metal Carbonate = Metal Salt + Water + Carbon Dioxide Acid + Metal Hydrogen Carbonate = Metal Salt + Water + Carbon Dioxide Acid + Ammonia Solution = Ammonium Salt + Water
Name the following Salts: • Sodium hydroxide+ hydrochloric acid • Ammonium hydroxide+ sulphuric acid • Calcium oxide+ nitric acid • Magnesium+ hydrochloric acid
Where Neutralisation is important: 1- Soil Treatment - Farming The majority of plants grow best at pH 7. If the soil is acidic or alkaline the plant may grow badly. Therefore, chemicals can be added to the soil to change its pH. • If the soil is too acidic - the most common complaint - it is treated with a base (chemicals opposite to an acid) in order to neutralize it. Common treatments use quicklime (calcium oxide) or chalk (calcium carbonate).
2- Indigestion: We all have hydrochloric acid in our stomach - it helps breakdown food! However, too much acid leads to indigestion. Therefore, to cure this we need to neutralize the acid with a base such as, sodium hydrogen carbonate (baking soda), or an indigestion tablet (antacid). 3- Insect Stings: A bee sting contains acid. In order to relieve the painful symptoms of the sting we need to neutralize the acid. By rubbing on calamine lotion (zinc carbonate) or baking soda the acid can be neutralized.