Acids are substances that often taste sour.
Examples:
Lemon juice
Vinegar
Curd
Orange juice
Hydrochloric acid
Never taste laboratory chemicals.
Acids turn blue litmus paper red.
Bases are substances that often taste bitter and feel slippery.
Examples:
Soap
Baking soda
Sodium hydroxide
Lime water
Never taste laboratory bases.
Bases turn red litmus paper blue.
Indicators are substances that tell us whether something is acidic or basic by changing colour.
Examples:
Litmus
Turmeric
Methyl orange
Phenolphthalein
Red cabbage juice
They work like colour-changing detectives.
Litmus is obtained from lichens.
Acid turns blue litmus red.
Base turns red litmus blue.
A neutral substance does not change litmus colour.
Turmeric is yellow.
It turns reddish-brown in a basic substance.
It stays yellow in an acidic or neutral substance.
Methyl orange changes colour like this:
Red in acid
Yellow in base
Phenolphthalein changes colour like this:
Colourless in acid
Pink in base
Some substances change their smell in acids or bases.
These are called olfactory indicators.
Examples:
Onion
Vanilla
Clove oil
Their smell may change or disappear in a basic solution.
Acids can react with some metals.
They produce:
A salt
Hydrogen gas
Example:
[ \text{Acid}+\text{Metal}\rightarrow\text{Salt}+\text{Hydrogen} ]
For example:
[ Zn+2HCl\rightarrow ZnCl_2+H_2 ]
Hydrogen gas makes a popping sound when tested with a flame.
Acids react with metal carbonates.
They produce:
Salt
Carbon dioxide
Water
Example:
[ \text{Metal carbonate}+\text{Acid}\rightarrow\text{Salt}+\text{Carbon dioxide}+\text{Water} ]
The bubbles are carbon dioxide.
Metal hydrogencarbonates also react with acids.
They produce:
Salt
Carbon dioxide
Water
Example:
[ NaHCO_3+HCl\rightarrow NaCl+H_2O+CO_2 ]
When carbon dioxide is passed through lime water, the lime water turns milky.
This happens because calcium carbonate is formed.
[ Ca(OH)_2+CO_2\rightarrow CaCO_3+H_2O ]
If too much carbon dioxide is passed through, the milkiness can disappear.
An acid and a base can cancel each other’s effects.
They produce:
Salt
Water
Example:
[ HCl+NaOH\rightarrow NaCl+H_2O ]
This is called a neutralisation reaction.
Neutralisation is like an acid and a base calming each other down.
General reaction:
[ \text{Acid}+\text{Base}\rightarrow\text{Salt}+\text{Water} ]
Examples:
Antacid neutralises excess acid in the stomach.
Lime neutralises acidic soil.
Toothpaste neutralises acids in the mouth.
Baking soda can help neutralise an acid sting.
Metal oxides usually behave like bases.
They react with acids to form salt and water.
Example:
[ \text{Metal oxide}+\text{Acid}\rightarrow\text{Salt}+\text{Water} ]
Copper oxide reacts with hydrochloric acid to form copper chloride and water.
Some non-metal oxides behave like acids.
Carbon dioxide reacts with calcium hydroxide to form a salt and water.
This shows that non-metal oxides are often acidic.
Acids produce hydrogen ions when dissolved in water.
Hydrogen ions are written as:
[ H^+ ]
These ions are responsible for many acidic properties.
Bases produce hydroxide ions in water.
Hydroxide ions are written as:
[ OH^- ]
These ions are responsible for many basic properties.
Acid solutions contain ions that can move.
Moving ions carry electric current.
That is why acid solutions can conduct electricity.
Glucose and alcohol may contain hydrogen, but they do not produce hydrogen ions in water.
They do not make enough ions to conduct electricity well.
So simply having hydrogen in a compound does not make it an acid.
A strong acid produces many hydrogen ions in water.
Examples:
Hydrochloric acid
Sulphuric acid
Nitric acid
A weak acid produces fewer hydrogen ions.
Examples:
Acetic acid
Carbonic acid
A strong base produces many hydroxide ions.
Examples:
Sodium hydroxide
Potassium hydroxide
A weak base produces fewer hydroxide ions.
Example:
Ammonium hydroxide
A concentrated solution has a lot of dissolved substance.
A dilute solution has less dissolved substance.
A strong acid can be dilute, and a weak acid can be concentrated. Strength and concentration mean different things.
Always add acid to water slowly.
Do not pour water into concentrated acid.
Adding water to acid suddenly can create a lot of heat and cause splashing.
Hydrogen ions do not stay alone in water.
They join water molecules to make hydronium ions:
[ H^+ + H_2O\rightarrow H_3O^+ ]
Hydronium ions are responsible for acidic behaviour in water.
The pH scale tells us how acidic or basic a solution is.
The scale usually goes from 0 to 14.
pH less than 7: acidic
pH equal to 7: neutral
pH greater than 7: basic
A lower pH means stronger acidity.
A higher pH means stronger basicity.
A universal indicator is a mixture of indicators.
It shows different colours for different pH values.
It gives us a better idea of how strongly acidic or basic something is.
pH is important in many daily activities.
The stomach contains hydrochloric acid to help digest food.
Too much acid can cause indigestion.
Antacids help neutralise the extra acid.
Bacteria in the mouth can produce acids.
If the pH in the mouth becomes too low, tooth enamel can be damaged.
Toothpaste is usually basic and helps neutralise acids.
Plants grow best in soil with a suitable pH.
Farmers may add lime to acidic soil.
A bee sting is acidic.
A mild base such as baking soda can help neutralise it.
A wasp sting is often basic.
A mild acid such as vinegar may help neutralise it.
A salt is often formed when an acid reacts with a base.
Example:
[ HCl+NaOH\rightarrow NaCl+H_2O ]
Sodium chloride, or common salt, is one example.
Different acids and bases make different salts.
Salts made from the same acid belong to one family.
For example, hydrochloric acid makes chloride salts:
Sodium chloride
Copper chloride
Zinc chloride
Salts made from the same base can also form a family.
Common salt is sodium chloride:
[ NaCl ]
It comes mainly from seawater or underground salt deposits.
It is used in food and to make many useful chemicals.
Electricity can be passed through salty water, or brine.
This produces:
Sodium hydroxide
Chlorine gas
Hydrogen gas
This process is called the chlor-alkali process.
[ 2NaCl+2H_2O\rightarrow2NaOH+Cl_2+H_2 ]
Used to make:
Soap
Paper
Artificial fibres
Used to:
Disinfect drinking water
Make bleaching powder
Make plastics
Used as a fuel and to make other chemicals.
Bleaching powder is made by passing chlorine over dry slaked lime.
It is used to:
Bleach clothes and paper
Kill germs in drinking water
Disinfect places
Baking soda is sodium hydrogencarbonate:
[ NaHCO_3 ]
It is a mild base.
It is used in:
Baking
Antacids
Fire extinguishers
Baking powder contains:
Baking soda
A mild edible acid
When heated, it produces carbon dioxide.
The gas makes cakes and bread rise.
Baking soda reacts with acids and releases carbon dioxide.
This is why it can help an acid sting and why it is useful in fire extinguishers.
Washing soda is sodium carbonate:
[ Na_2CO_3\cdot10H_2O ]
It is used for:
Washing clothes
Removing permanent hardness of water
Making glass
Making soap
Making paper
Plaster of Paris is made by heating gypsum.
Its formula is:
[ CaSO_4\cdot\frac{1}{2}H_2O ]
When mixed with water, it becomes hard gypsum again.
It is used for:
Plaster casts
Making statues
Making moulds
Making decorative items
Some crystals contain a fixed number of water molecules inside them.
This water is called water of crystallisation.
For example, copper sulphate crystals contain water and look blue.
When heated, they lose this water and become white.
Adding water makes them blue again.
Acids are usually sour and turn blue litmus red.
Bases are usually bitter or slippery and turn red litmus blue.
Indicators change colour to show acids and bases.
Acids react with metals to make hydrogen.
Acids react with carbonates to make carbon dioxide.
Acids and bases neutralise each other.
The pH scale measures acidity and basicity.
Salts are made in many reactions and are useful in everyday life.