14 Acids And Alkalis

ACIDS AND ALKALIS Introduction The concepts of acids and alkalis are of the utmost importance in chemistry. Acidity, neutrality and alkalinity play a fundamental role in life chemical reaction, in research work in the labs and industrial processes. From an experimental point of view ACIDS are those solutions that will turn bluelitmus red and ALKALIS those which turn red-litmus blue. Usually the substances that will dissolve in water to give acid solutions are called acids too. Some features about acids and alkalis are shown in the following chart. ACIDS

• • • • •

ALKALIS

Turn blue-litmus RED Have a sour taste Make carbonate solutions bubble (set carbon dioxide free) Have low pH (below 7) Are corrosive (attack many metals and skin)

• • • • •

Turn red-litmus BLUE Have a soapy touch Make ammonium salts solutions smell (set ammonia free) Have high pH (above 7) Are corrosive (attack a few metals and skin

Chemists compare acidity and alkalinity (also called basicity) of aqueous solutions using a scale called the pH scale. The pH scale ranges from zero for strongly acid solutions to fourteen for strongly alkaline (basic) solutions. Lemon juice and vinegar are acids. Soap and oven cleaners are alkaline. Solutions that show a pH of seven (that is, midway between 0 and 14) are called neutral: they are neither acid nor alkaline so they will not change any litmus, or decompose carbonates etc. Common salt solutions or alcohol solutions are neutral. NEUTRAL SOLUTIONS     

Do not change the colour of either litmus Do not have a sour taste or a soapy touch (generally) Do not make carbonate solutions bubble or ammonium salts solutions smell Are not corrosive (although some metals are quite sensitive to water) Have a pH value of 7

Many substances change their colours or their shades according to the pH number. Litmus is one of the first substances known that show this behaviour. These substances are called indicators. For example adding a few drops of phenolphthalein indicator to an aqueous solution will not change it if pH is below 8. But if it is just above (say 8.5) the solution will turn magenta (dark pink). Brilliant yellow will be yellow if pH is below 12 and red if pH is above 12. To determine pH values for different materials and so assess its acidity or alkalinity, we can use Universal Indicator solution or Universal Indicator paper strip usually called pH paper. It is a strip of paper coloured with a mixture of indicators that will show different colours according to pH. The approximate colour – pH relation for the paper strip is shown in the chart below. Red

Orange

Yellow

Strongly acid 0 to 2

Medium acid 3-4

Weakly acid 5-6

Greenish yellow Neutral 7

Yellowgreen Weakly basic 8-9

Bluegreen Medium basic 10 - 11

Navy blue Strongly basic 12 to 14

A more accurate procedure is to use a pH-meter. pH-meters are electronic devises that have a pH sensitive glass tip called glass electrode. If we dip it into a solution, a p.d. (potential difference or voltage) will be established between the inner and outer sides of the glass electrode. This p.d. depends on the pH of the solution that is being tested and its value in pH units will be shown on the display. Aims: To study acidic, alkaline and neutral solutions Lab ware Test tubes and racks, pipettes, watch glasses, glass rods, spatulas, Litmus and Universal Indicator Papers (and/or solutions), pH-meter Testing Acidity 1- You are given three samples of aqueous solutions labelled 1, 2 and 3, and some strips of blue and red litmus paper. Cut a red and a blue litmus paper in three parts. 2- Dip a different red and blue strip on each of the three solutions. 3- After 2 – 3 seconds check the colour of the strips and put them on a watch glass.

4- Complete the following chart: Colour litmus

of

red Colour litmus

of

blue Conclusion

Sample1 Sample2 Sample3 Estimating pH with Universal Indicator Paper 1- Cut the strip of Universal Indicator Paper (pH paper) in three parts. 2- With a glass rod (clean it after every test) put a drop of each of the solutions on a strip of pH paper. 3- Observe the colours after 2 – 3 seconds and deduce a possible ph for the three samples. 4- Complete the chart below. Colour paper

of

pH Estimated pH

Sample 1 Sample 2 Sample 3 Observing Some Characteristics of Acids and Alkalis Put three test tubes in a rack. Fill up to 2 cm from the bottom with the solution you found to be an acid. 1- Add the first tube with a tip of a spatula measure of sodium carbonate. Observe and record your results in the chart. 2- Drop a small lump of zinc into the second one. Observe and record your observations in the chart. Teacher will test the fizzing gas for you. This is a typical test for hydrogen gas. If there are no changes heat gently (teacher). 3- Add a tip of a spatula measure of ammonium chloride to the third one. Observe. In case you don’t see any changes, smell carefully. Record your results in the chart. Carbonate test Zinc test Ammonium test Acid solution Alkaline solution Neutral solution 4- Pour the contents of the test tubes into the sink and let water run. Be careful not to throw away the remainders of the zinc lump.

5- Rinse the test tubes with tap water two or three times and put them back in the rack. 6- Repeat all 1 to 6 (now fill the ALKALI chart below) but using the alkaline solution instead of the acid. 7- Repeat all 1 to 7 using the neutral solution. Testing pH on Different Materials a- Using pH paper (universal indicator) To test pH for different substances just dissolve or suspend a drop or the tip of a spatula measure of the substance to be tested in 2-3 cm3 distilled water in a test tube. Then shake thoroughly and using a glass rod put a drop of the solution (suspension) on a 1 cm strip of Universal Indicator paper (pH paper). Compare the colour with the scale provided. b- Using a pH-meter Instead of putting a drop on the pH paper, dip the pH-meter’s electrode into the solution. If the tip doesn’t reach the solution, pour it into a small beaker and test in it. Write down your results on a table writing the name of the substance tested and the pH value found at the right of it. Pure substances or materials to be tested will be either brought by the pupils or given by the teacher.