 What makes something acidic? In this video you will find out what acids are and what the strange pH scale is all about. Acids are substances that taste sour like the citric acid in lemons and vinegar which is sour wine or van agre in French. There are many other natural acids such as lactic acid in sour milk and the poisonous oxalic acid in rhubarb. 250 years ago early chemists found that non-metals burnt to form acids so they named the gas in the air oxygen meaning acid maker. Salpha burns to form sulphur dioxide which dissolves in water to form sulphurous acid. Carbon dioxide forms carbonic acid making rain always slightly acidic and nitrogen oxides dissolve in water to form nitric acid. However oxygen also reacts with reactive metals to form alkalis such as sodium hydroxide and calcium oxide so oxygen is not only an acid maker but also an alkali maker. In water acids give up a hydrogen ion or proton to the water molecule forming a hydroxonium ion H3O+. For example hydrochloric acid produces the chloride ion and hydroxonium ion. Acetic acid the acetate ion. Sulfuric acid the sulphate ion. The amount of acidity can be measured on the pH scale a measure of the concentration of hydroxonium or hydrogen ions little p for power or strength and capital H for the hydrogen ion. Acids can change the color of indicators try making your own by extracting the color from red cabbage leaves or other colored plants add lemon juice then garden lime and watch the red color gradually change through green to blue as the acid is neutralized. Perhaps the most famous indicator is litmus which is extracted from lichens red in acid blue in alkali. A full range indicator contains a mixture of dyes allowing you to measure the pH from nought strong acid getting less acidic through neutral water at pH 7 getting more alkaline to strong alkali at pH 14. The pH scale is logarithmic meaning that for each change of one unit of pH there is a 10 fold change in acidity. To understand its logarithmic nature consider this you have collected a bucket full of acid rain with a pH of four and you want to make it less acidic for watering your garden. How much neutral water must you add to make the pH change from four to five? Pause the video and think. Well the answer is that you need to add nine buckets of neutral water just to change the pH by one unit since each change in pH is a 10 fold in acidity. To summarize most acids are naturally occurring substances which dissolve in water and are sour to the taste warning only taste food substances. They're also formed when oxides of non-metallic elements like sulfur dioxide dissolve in water. There are several different indicators which can help us determine the acidity of a substance and the degree of acidity is measured on the pH scale which is logarithmic.