Acids and Alkalis

By the end of this unit you should be able to

  • give some examples of properties of acids , such as sour tast, ability to react with limestone and some meals

  • state that alkalis are ‘acid opposites’ that dissolve in water(can mention

  • recall some examples of common acids, including lab acids (hydrochloric, sulfiric) and food acids (vinegar, citric acid)

  • give examples of substances which are commonly alkaline, such as cleaners

  • state that neutralisation occurs when an acid reacts with an alkali

  • relate pH number to acidity, neutral or alkalinity

  • use litmus paper to find out whether a substance is acid, alkali or neutral

  • have some experience of universal indicator in relation to pH

  • have some experience of making an indicator and testing it e.g. from red cabbage or turmeic

  • describe the causes and effects of acid rain

  • have used a lab acid to prepare for and test for hydrogen gas


Many people think of acids as something dangerous.

Acids are a type of chemical. They have certain properties in common (properties are things like colour, state, smell etc.). For example acids –

  • are always sour (though many are too poisonous to taste)

  • will make baking soda fizz

  • turn a chemical called litmus red when it is blue (they make it change colour)

  • corrode many metals

However, you may have some preconceived ideas and find the following facts surprising:

  • not all acids will burn you

  • some acids occur in food (to make it sour)

  • many acids occur naturally

  • you have hydrochloric acid in your stomach

Acids have many different uses. The one thing all acids have in common is that they are a particular type of hydrogen compound.

Examples of acids:

Lab acids include hydrochloric acid, nitric acid and sulfuric acid. These acids are strong acids, which means that at high concentration they can become very reactive.

Food acids include vinegar (acetic acid), lemon juice (citric acid), grape juice (tartaric acid), apple juice (malic acid), vitamin C (ascorbic acid) and coca-cola (phosphoric acid). Many of these acids have complex chemical names e.g. acetic acid is properly called ethanoic acid.



Bases and Alkalis

An "acid opposites" is called a base.

Bases are chemicals which react with acids to take away their acidity and neutralise their acid properties. Bases which dissolve in water are called alkalis. In this unit we will only look at alkalis and ignore most other bases.

"Drano" is the powerful alkali sodium hydroxide.

acids worksheet 1.pdf

Examples

Lab alkalis include sodium hydroxide, ammonium hydroxide (also called ammonia).

Everyday alkalis include baking soda (sodium bicarbonate), washing soda (sodium carbonate), soap (sodium stearate)


Limestone (calcium carbonate) is a base but not an alkali because it reacts with acids to neutralise them but it can't dissolve in water. All alkalis are bases but not all bases are alkalis. Limestone is crushed to a powder to add to soil to make it less acid.

The pH scale

A scale of 'how acid' or 'how base' uses a number called the pH.

The most acid pH is 1 and the strongest alkalis have a pH of 14.

A pH of 7 is halfway between these extremes and is neither acid or base. We term it neutral.


To find out the pH we can use a special coloured paper or solution called Universal Indicator. This goes different colours at different pH values:

Indicators

Indicators are chemicals which change colour to ‘indicate’ different chemical conditions.
The commonest indicators are acid-base indicators, which change colour depending on pH.

Examples of these: litmus (red for acid, blue for base), phenolphthalein (colourless for acid and pink for base).

Universal indicator is a mixture of chemicals which change at a different pH for each one. As a result, the mixture is a rainbow of different colours between the lowest pH of 1 and the highest of 14.

Universal indicator is available as paper, to dip into solution, or as a liquid to add to a solution. The liquid form is particularly useful if you are doing a chemical reaction and want to see the pH change as it goes.

Other common indicators:

Litmus: Litmus is red for acid and blue for alkali.

You need to use BOTH red and blue litmus to be sure whether something is an acid, alkali or neutral.

Phenophthalein

Phenolphthalein is an indicator mostly used as a liquid. In neutral or acid pH it is colourless, but it goes a bright startling pink in the presence of an alkali. In the photo on the left the alkali hasn't completely mixed with water containing phenolphthalein, so only the part where the alkali is present is pink.

Neutralisation

Wnen you mix the correct amount of acid and alkali the pH goes to 7. This means the amount of acid and alkali are balanced and the solution is now neutral.

This is called a neutralisation reaction and the chemical produced is called a salt.

Many plant dyes change colour with different pH. Red cabbage, beetroot juice, turmeric and many flower colours are examples.

Some plants actually flower in different colours depending on the pH of the soil. The best known example of this is a common border plant found in NZ called hydrangea. These are pink in acid soil and blue in alkali soil. Gardeners can produce a colourful display by using an acid fertiliser, such as ammonium sulfate, to make the soil acid for pink flowers. They can add garden lime (a chemical called calcium carbonate) to produce blue flowers.

Red cabbage contains more than one pH sensitive chemical, so it can go more than just two colours. It has almost as many colours as Universal Indicator, but different colours for acid, alkali and neutral.

How to test if a substance is an indicator

  1. You need to extract and purify the colour. This will depend on the substance to be tested e.g. it may need to be ground up in a mortar and pestle, or dissolved in a solvent such as alcohol.

  2. You then need to divide your colour into at least 3 samples to be tested.

  3. You need to add an acid to one sample Isuch as hydrochloric acid), water to a second and an alkali (such as sodium hydroxide) to the third sample.

  4. You can work out from any resulting colour changes whether your material is an indicator.

Acids and food preservation

Acids stop many harmful bacteria, which spoil food, from growing.

In the days before refrigerators, they were an important way of making food last.

Pickling is putting the food into acetic acid (vinegar) or a similar acid e.g. onions, eggs etc.

Fermenting: some foods, such as milk or meat, produce lactic acid when they are fermented with bacteria. This is how yoghurt and salami are made.

Salting: adding salt makes it hard for bacteria to grow; salt can be used together with acid or by itself. Sauerkraut is cabbage which is fermented to produce lactic acid and also salted. In the days before fridges and supermarkets, sauerkraut saved many people in countries where the ground freezes in winter and nothing grows from getting scurvy. Scurvy is caused by a deficiency in another food acid - Vitamin C or ascorbic acid (normally found in fresh fruit and veges).

Acid Rain

Normal rainwater is very slightly acidic because of the presence of carbon dioxide in the atmosphere. This very dilute solution contains small amounts of the weak acid carbonic acid, and the pH of normal rainwater is no less than 5.7. Rainwater with a pH of lower than 5.7 is termed acid rain.

There are two main gases that can cause acid rain by dissolving in rainwater

  • sulfur dioxide: this can come from volcanoes or from pollution in 'dirty' coal and 'dirty' oil

  • nitrogen oxides: this is formed by lightning and in petrol and diesel engines

A certain amount of acid rain is natural, particularly after volcanic eruptions. However, human activity (partcularly coal-fired power stations and marine diesel engines) has greatly increased it.

The effects of acid rain can include:

  • eroding away limestone or marble in buildings

  • maing streamwater too acid for fish

  • removing nutrients from soil and interfering with plant growth

  • effects on human health (particularly from smog

  • corrosion of metals such as copper or bronze, or even galvanised steel

  • effects on birds eggs and organisms such as snails which make their shells from calcium carbonate



Ocean acidification

The increasing amount of carbon dioxide is lowering the pH of seawater, which is normally very slightly alkaline.

Many things that live in the sea, such as coral and shellfish, make their shells from a base called calcium carbonate (limestone is made from the fossil skeletons of these marine organisms). If the sea gets more acid, these creatures will find it harder to make their shells. Their numbers may decrease, causing further effects on the food chain and possible drops in the numbers of fish. Very little is known about this particular effect of human alteration of the atmosphere as yet.