LESSON NOTES

ACIDS are corrosive (they eat away metals and burn your skin) and have a sour taste.

They are found in many common substances around the house.

Eg/ Citrus fruits (lemons) contain citric acid

Yoghurt contains lactic acid

Vinegar contains acetic acid

Soft drinks contain carbonic acid

Your stomach contains a very strong acid to help your body digest the food you eat called hydrochloric acid.

A DILUTE ACID is one which contains a large amount of water. The opposite of this is a CONCENTRATED ACID which contains only a little bit of water (these must be handled with care!)

BASES can be used to neutralise acid (they are opposite of acids) and to clean.

The main way that we use bases around the house are in toothpastes and in cleaning products to help us remove dirt and grease. Bases have a soapy feel.

ALKALIS are bases that are soluble in water.

INDICATORS: Not all substances are acids or bases. Some substances like water are neutral. A quick way to tell if a substance is acidic or basic is to use an indicator. Indicators change colour in the presence of an acid or a base.

Eg/ Litmus is an indicator which is blue in an acidic solution and red in a basic solution.

Acids and Teeth: Some food gets stuck between your teeth. Bacteria in your mouth feed on this food and produce weak acids as a waste product. This acid in turn starts to attack your teeth causing tooth decay. This is increased by plaque which is a mixture of food, bacteria, acids and saliva that sticks to your teeth like concrete speeding up tooth decay.

To overcome this you should floss and brush your teeth. Flossing helps to remove any food between your teeth and any plaque before it hardens. Toothpaste is made from a slightly basic substance which neutralises the acid made by the bacteria and helps to protect the enamel on your teeth.

Stomach Acid: Your teeth help to break the food you eat into smaller pieces but it needs to be broken down further in your stomach. There is a very strong acid in your stomach as well as digestive enzymes such as pepsin and rennin. The acid does not affect your stomach because there is a lining of sticky mucus which protects your body from the acid. If this layer is lost then your stomach acid can attack your body forming a stomach ulcer from the damage it does.

pH is simply a scale from 0 to 14 which tells you how acidic or basic a solution is. Acidic solutions are between 0 and 6, neutral substances are 7, and basic solutions are from 8 to 14. The best way to measure the pH of a substance is to use an Indicator.

Materials: as per handout sheet 2

Method: as per handout sheet 2

Questions and Conclusions: Answer Questions 1 to 3 from handout 2

Explaining Acids and Bases:

Eg/ All acids break down into ions in water.

Hydrochloric Acid HCl ® H⁺ + Cl^-

Sulfuric acid H₂SO_{4 ®} 2H⁺ + SO₄^2-

All basic solutions contain OH^- ions. These are called hydroxide ions and are a combination of one Hydrogen and one Oxygen atom.

Eg/ Sodium Hydroxide NaOH ® Na⁺ + OH^-

Not all acids are alike. Acids which react quickly with substances are called STRONG acids. The opposite are called WEAK acids (most acids around the house are weak)

Strong acids break up into H⁺ ions very easily, therefore there will be a large number of ions in the solution rather than of the whole molecule (which hasn’t broken up to be active).

It is very important not to get strong/weak confused with concentrated/dilute solutions (which is how much water has been added).

When acids and metals react they form a salt plus hydrogen gas

2HCl + Mg ® Mg Cl₂ + H₂

2HCl + CaCO3 ® CaCl2 + H2O + CO2

When an acid and a base are put together they cancel one another out by forming a salt plus water.

HCl + NaOH ® NaCl + H2O