Acids and Bases

Neutral Substances are not proton donors or proton acceptors. Neutral substances, when liquid or if soluble in water, give a solution of pH 7. Neutral substances have no effect on litmus eg distilled water, petrol, salt & sugar

An Indicator is a substance that shows by a change in colour that a particular substance is present, the concentration of a particular substance an when a reaction has been completed. Litmus is an acid-base indicator as it has a different colour in an acid to its colour in a base: litmus is red in an acid and blue in a base. Universal Indicator is a mixture of indicators that gives a gradual but clearly distinguishable series of colour changes over the range from very acidic to very basic. Litmus and Universal Indicator are available in liquid or in test papers.

The pH Scale is a graded number system from 0 to 14 to give a measure of the level of acidity or basicity of a solution with 7 representing neutrality. Less than 7 is acidic and the lower the number the greater the acidity. Greater than 7 is basic and the higher the number the greater the basicity.

MandPractivity 13 Investigate the pH of a variety of materials using the pH scale.

1. Obtain a variety of everyday solutions – rainwater, milk, bleach, vinegar, fizzy drinks, fruit juices, tea, coffee, nettles, bee/wasp stinger (from already dead specimen)

2. Make up solutions of everyday soluble solids – glucose, table salt, baking powder, bath salts.

3. Put each solution into its own clearly labelled small dropping bottle or jar.

4. On a plastic sheet or white tile place small squares of blue and red litmus paper in pairs – 1 blue & 1 red.

5. Place a drop of each solution on a blue and red square of litmus paper and record the colour or colours.

Both red: the solution is acidic. Both blue: the solution is basic. Red & blue – the solution is neutral.

6. Dip the end of a strip of Universal Indicator paper into the solution.

7. Match the new or unchanged colour of the tip to a colour on the pH colour chart.

8. The number on the matching colour on the chart is the pH of the solution.

The pH of a Variety of Common Substances

Gastric Juice 2.0 Lemon Juice 2.5 Vinegar 3.3

Orange Juice 4.4 Rainwater 5.5 Milk 6.5

Pure Water 7.0 Blood 7.4 Saliva 7.8

Seawater 8.3 Toothpaste 9.0 Milk of Magnesia 10.5

Acids and Bases (ii)

Acids Hydrochloric Acid: (HCl) Sulphuric Acid (H2SO4)

Bases Sodium Hydroxide (NaOH) Calcium Hydroxide (Ca(OH)2) Alkalis are Soluble Bases.

Neutralisation If the exact amounts of equal strength acid and base are used then only salt and water remain after the reaction and the pH should be neutral at 7. Acid + Base » Salt + Water

Everyday Neutralisation Reactions

  • Indigestion, reflux of acid material from the stomach, is treated with a base.
  • Acid soils treated with lime.
  • Toothpaste counteracts the acid plaque on our teeth.

Acid + Base » Salt + Water

Hydrochloric Acid + Sodium Hydroxide » Sodium Chloride + Water

HCl + NaOH » NaCl + H2O

MandPractivity 18 Titrate HCl Against NaOH and Prepare a Sample of Salt

Titration is the use of a burette, to determine the volume of a solution needed to completely react with a particular volume of another solution.

1. Set up a burette with hydrochloric acid.

2. Using a pipette place a particular volume e.g. 25 cm3 , of sodium hydroxide solution in a conical flask.

3. Add a few drops of universal indicator to the sodium hydroxide solution.

4. The sodium hydroxide solution is now blue.

5. Slowly run the acid into NaOH solution while swirling the flask.

6. When the indicator turns the colour of pH 7 STOP ADDING the ACID.

7. Note the volume of acid added to neutralise the base.

8. Repeat exactly without the indicator.

9. A pure sodium chloride solution is in the beaker.

10. Pure sodium chloride salt remains when the water is evaporated.

Salts A salt is an ionic compound composed of a metal ionically bonded to a non-metal.

A salt is formed by the replacement of the hydrogen of an acid by a metal.

Different salts are produced from different acids and different metals.

MandPractivity 20 Investigate the Reaction between Zinc and HCl, and Test for Hydrogen.

Word Equation: Zinc + Hydrochloric Acid » Zinc Chloride + Hydrogen

Chemical Equation: Zn + 2HCl » ZnCl2 + H2

1. Place a piece of clean pure zinc into a test tube.

2. Half fill the test tube with dilute hydrochloric acid.

3. Note the bubbles of colourless gas forming on the zinc and rising to the surface.

4. Allow the gas to escape for a minute and then cover the top of the test with your thumb to trap the gas building up its concentration.

5. Bring a lighted taper to the test tube to ignite the gas.

6. The gas burns with a ‘pop’, a small explosion burning with a pale blue flame.

7. The ‘pop’ is the classic test for hydrogen as it reacts with oxygen of the air forming water.

8. Keep adding zinc until all the bubbling stops; then filter to remove the bits of excess zinc and then allow the filtrate to evaporate. A white salt, zinc chloride, remains.

Hydrocarbons and Acid Rain

A fuel is a material that is used as a source of energy. Burning is a process by which energy is released from many fuels. Nuclear fuels release energy by nuclear fission.

Fossil fuels are energy rich substances produced by the partial decay of dead organisms. The major fossil fuels are coal, oil, natural gas and peat. Coal and peat are composed mostly of carbon. Oil and gas are composed of hydrocarbons – compounds of hydrogen and carbon e.g.CH4 is methane the main component of natural gas.

Fossil fuels are non-renewable energy sources as their formation is extremely slow, much slower that the rate at which they are being used. Energy is released from fossil fuels by combustion i.e. by burning them with oxygen.