Acids, Alkalis, pH Flashcards
acid…
A substance with a pH below 7 is acidic.
alkali…
An alkali is a soluble base – so it will neutralise an acid and also dissolves in water.
A substance with a pH above 7 is alkaline. Most alkalis are metal hydroxides, e.g. sodium hydroxide.
pH…
a measure of how acidic/basic a substance is. The range goes from 0 - 14, with 7 being neutral.
what is the pH scale…
The pH scale is a numerical scale from 0-14 that tells us how acidic or alkaline a substance is.
The pH scale below shows the colour of universal indicator at each pH.
pH 7 is neutral.
Any pH below 7 is acidic – the lower the number the stronger the acid.
Any pH above 7 is alkaline – the higher the number the stronger the alkali.
define pH 1-3 and an example…
strong acid
e.g.
battery acid, lemon juice, vinegar
define pH 4 - 6 and an example…
weak acid
e.g.
urine, saliva, acid rain, black coffee
define pH 7 and an example…
neutral
e.g.
“pure” water,
define pH 8 - 10 and an example…
weak alkali
e.g.
sea water, baking soda
define pH 11 - 13/14 and an example…
strong alkali
e.g.
liquid drain cleaner, bleach, oven cleaner, soapy water
what is an indicator?
A substance that changes colour when added to solutions with different pH numbers.
There are lots of different indicators, each one with its own colour changes. Some only have two or three colour changes. We often use universal indicator because it has a full range of colour changes that can tell us how strongly or weakly acidic or alkaline a substance is, not just whether it is acidic, alkaline or neutral
How does a universal indicator change colour depending on what substance it is?
When universal indicator is added to a solution, the colour change can indicate the approximate pH of the solution in which it depends. Acids cause universal indicator solution to change from green toward red. Bases cause universal indicator to change from green toward purple.
what is a neutralisation reaction?
Neutralisation is a chemical reaction between an acid and an alkali. If just the right amount of acid and alkali are added together they will react to form a neutral solution of a salt and water.
acid + alkali –> a salt + water
We need to use an indicator to show us when neutralisation is complete because there is no visible change to the solutions (although there will be a temperature rise). Neutralisation reactions can be useful, for example we can neutralise an acidic bee sting using a weak alkali.
what is a carbonate reaction?
A carbonate is a compound made of three elements a metal, carbon and oxygen, e.g. magnesium carbonate, MgCO3 (CO3 is always the formula of the carbonate part of the compound).
Carbonates react with acids to produce a salt, water and carbon dioxide gas.
acid + carbonate —-> a salt + water + carbon dioxide
e.g.
sulfuric acid + copper carbonate —-> copper sulfate + water + carbon dioxide
There will be fizzing and bubbling because a gas is produced. There may also be a colour change.
what is a metal + acid reaction?
Acids react with some metals to produce a salt and hydrogen gas.
Metal + acid → salt + hydrogen
The abbreviation M.A.S.H. can be used to remember this general reaction.
When a metal is put in acid, it gets smaller and smaller as it gets used up in the chemical reaction.
At the same time, bubbles of gas can be seen. The bubbles produced in the reaction are hydrogen gas.
This can be proven using a burning splint because hydrogen is flammable. When the burning splint is put into the test tube containing hydrogen gas, a small explosion occurs, making a squeaky pop sound. This shows that hydrogen is present.
e.g.
magnesium + nitric acid —-> magnesium nitrate + hydrogen
squeaky pop…
The ‘squeaky pop’ test is used to test for hydrogen gas. Use a lit splint to test the gas, if it is hydrogen it will burn with a squeaky pop.
limewater…
Limewater (calcium hydroxide solution) is used to test for carbon dioxide. Limewater is a colourless solution that turns cloudy or milky when carbon dioxide is bubbled through it.
