ACIDS AND ALKALIS

1.Acids, Alkalis and Neutral Substances

Properties of acids

1. They are liquids.

2. They are solutions of compounds in water.

3. If concentrated they can be corrosive.

4. Acids taste sour (for example, vinegar).

5. Turn blue litmus paper red - this is an easy test for an acid!

6. Usually react with metals to form salts.

7. Acids contain hydrogen ions.

8. Turn Universal Indicator from green to red, and have a pH less than 7.

Examples of acids: are vinegar (ethanoic acid) and lemon juice (citric acid)

magnesium + hydrochloric acid -> magnesium chloride + hydrogen gas

Some common acids used in your laboratories at school will be:

1. Hydrochloric acid, HCl_(aq)

2. Nitric acid, HNO_3(aq)

3. Sulphuric acid, H₂SO_4(aq)

Properties of alkalis

1. They feel soapy to touch.

2. They are soluble bases.

3. Like acids, they can burn the skin.

4. They turn red litmus blue - this is how you test for an alkali!

5. Alkalis contain hydroxide ions (OH^-).

6. They taste bitter.

7. Turns Universal Indicator from green to blue or purple.

Some common alkalis used in your laboratories at school will be:

1. Sodium hydroxide, NaOH_(aq)

2. Ammonia, NH₃NH₄OH_(aq)

3. Calcium hydroxide, Ca(OH)_2(aq)

Properties of neutral substances

1. Litmus paper is not affected by neutral paper.

2. Tend to be harmless

3. Universal Indicator stays green.

Common examples of neutral substances:

1. Water

2. Sodium chloride solution, NaCl_(aq)(common salt)

3. Sugar solution C₆H₁₂O_6(aq)

The pH scale

The Strength of an Acid

Acids and alkalis can be strong or weak!

So how can we measure their strength?

The strength of an acid or alkali is shown using a scale of numbers called the pH scale. The numbers go from 0-14.

On the scale it follows that:

An acidic solution has a pH number less than 7

An alkaline solution has a pH number greater than 7

A neutral solution has a pH number of exactly 7.

You can find the pH of any solution using universal indicator. Universal indicator is a mixture of dyes. It comes as a solution or in paper.

Universal indicator will change from green to a different colour depending on the pH of the solution you place it in.

Note:

In a strong acid, nearly all the acid molecules form ions.

In a weak acid, only some of the acid molecules form ions.

The more OH^- ions (hydroxide ions), the more alkaline an alkali will be.

In other words, the more OH- ions there are the higher the pH number.

2. Neutralisation

Reactions between acids and alkalis

The Making of a Salt

When an acid reacts with an alkali it produces a salt and water.

This reaction is called neutralisation. The alkali has neutralised the acid by removing its H+ ions, and turning them into water.

Neutralisation always produces a Salt

Uses of Neutralisation

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 neutralise it. Common treatments use quicklime (calcium oxide) or chalk (calcium carbonate).

Indigestion

We all have hydrochloric acid in our stomach - it helps breakdown food! However, too much acid leads to indigestion. Therefore, to cure this ailment we need to neutralise the acid with a base such as, sodium hydrogen carbonate (baking soda), or an indigestion tablet.

Insect Stings

A bee sting contains acid. In order to relieve the painful symptoms of the sting we need to neutralise the acid. By rubbing on calamine lotion (zinc carbonate) or baking soda the acid can be neutralised.

Wasp stings are alkaline, hence acid is needed to neutralise and remove the painful sting. Vinegar (ethanoic acid) is used.

Waste from Factories

Waste from many factories are often acidic. If this acidic solution is not treated and enters rivers it can kill fish. Slaked lime (calcium hydroxide) is often used to neutralise the acid.