Testing the pH of a solution means using an indicator.

The pH scale is a measure of how acidic or alkaline a solution is.

• The strongest acid has a pH 0.
• The strongest alkali has a pH 14. 
• A neutral substance has a pH 7 (E.g pure water)

Acids and bases neutralise each other

• An acid has a pH of less than 7. Acids form hydrogen ions(H+) in water.
• A base has a pH of greater than 7
• An alkali is  a base that dissolves in water. Alkalis form hydroxide ions (OH-) in water .
• So H+ ions make solutions acidic. OH- ions make them alkaline.

The reaction between acids and bases is called neutralisation.  Acid + Base --> Salt + Water

When an acid neutralises a base the products are neutral. An indicator can be used to show that (indicator will go green)

(s): solid (l): Liquid  (g): Gas  (aq): Dissolved in water

Acids Reacting With Metals 

• Metals react with acids to give salts
• Acid  + metal --> Salt + Hydrogen
• The more reactive the metal, the faster the reacion will go
• Very reactive metals ( E.g sodium) react explosively
• Copper does not react wiht dilute acids at all
• The rate of reaction is shown by how fast the bubbles of hydrogen are give off
• You can twst for hydrogen using the burning splint test. It will give a spueaky pop
• The salt made depends on which metal is used, and which acid is used.

• Metal oxides and metal hydrooxides are bases
• The ones that dissolve in water are alkalis
• All metal oxides and metal hydroxides react with acids to form a salt and water

The type of metal and acid decides the salt

• hydrochloric acid + copper oxide --> copper chloride + water
• hydrochloric acid + sodium hydroxide--> sodium chloride +water
• sulfuric acid + zinc oxide --> zinc sulfate + water
• sulfuric acid + calcium hydroxide --> calcium sulfate + water
• nitric acid + magnesium oxide  -->magnesium nitrate + water
• nitric acid + potassium hydroxide -->potassium nitrate + water

Ammonia can be neutralsied with nitric acid to make a fertiliser

• Ammonia dissolves in water to make an alkaline solution
• This can be reated with acid to prodcue an ammonium salt.
• Ammonium salts are good fertilisers ( plant food )

Acids and bases

When acids react with bases, a salt and water are made:

• acid + metal oxide → salt + water
• acid + metal hydroxide → salt + water

Remember that most bases do not dissolve in water. But if a base can dissolve in water, it is also called an alkali.

Reactive metals

Acids will react with reactive metals, such as magnesium and zinc, to make a salt and hydrogen:

• acid + metal → salt + hydrogen

The hydrogen causes bubbling during the reaction, and can be detected using a lighted splint.

The name of the salt produced in a neutralisation reaction can be predicted. The first part of the name is 'ammonium' if the base used is ammonia. Otherwise, it is the name of the metal in the base. The second part of the name comes from the acid used:

• Chloride (if hydrochloric acid is used)
• Nitrate (if nitric acid is used)
• Sulfate (if sulfuric acid is used)

Acid+Base→Salt + Water Hydrochloric acid  + Copper oxide → Copper chloride + water Sulfuric acid + Sodium hydroxide → Sodium sulfate + water Nitric acid + Calcium hydroxide → Calcium nitrate + water

Many artificial fertilisers are ammonium salts, made by the reaction of an acid with ammonia solution. For example:

AcidAlkaliFertiliser Nitric acid Ammonia solution Ammonium nitrate Phosphoric acid Ammonia solution Ammonium phosphate Sulfuric acid Ammonia solution Ammonium sulfate

• If the base dissolves in water, you need to add just enough acid to make a neutral solution.
• Check a small sample with universal indicator paper.
• If ammonia solution is used, you can add a little more than needed to get a neutral solution.
• Warm the salt solution to evaporate the water.
• You get larger crystals if you evaporate the water slowly.
• Copper oxide, and other transition metal oxides or hydroxides, do not dissolve in water.
• If the base does not dissolve in water, you need an extra step.
• You add the base to the acid until no more will dissolve and you have some base left over (called an excess).
• You filter the mixture to remove the excess base, then evaporate the water in the filtrate to leave the salt behind.

Insoluble salts do not dissolve in water. They can be made by mixing appropriate solutions of ions together.

SolubleInsoluble All nitrates None Most sulfates Lead sulfate, barium sulfate Most chlorides, bromides and iodides Silver chloride, silver bromide, silver iodide, lead chloride, lead bromide, lead iodide Sodium carbonate, potassium carbonate Most other carbonates Sodium hydroxide, potassium hydroxide Most other hydroxides

You need a soluble silver salt and a soluble chloride salt to make it. Silver nitrate and sodium chloride are both soluble. When you mix their solutions together, you make soluble sodium nitrate and insoluble silver chloride:

• silver nitrate + sodium chloride → sodium nitrate + silver chloride
• AgNO₃(aq) + NaCl(aq) → NaNO₃(aq) +AgCl(s)

The silver chloride appears as tiny particles suspended in the reaction mixture - it forms a precipitate. The precipitate can be filtered, washed with water on the filter paper, and then dried in an oven.

Using precipitation reactions

Precipitation reactions can be used to remove unwanted ions in solution. This is useful for treating drinking water and waste water.

Electrolysis is the process by which ionic substances are broken down into simpler substances using electricity. During electrolysis, metals and gases may form at the electrodes.

What is electrolysis?

Ionic substances contain charged particles called ions. For example, lead bromide contains positively charged lead ions and negatively charged bromide ions.

Electrolysis is the process by which ionic substances are decomposed (broken down) into simpler substances when an electric current is passed through them.

For electrolysis to work, the ions must be free to move. Ions are free to move when an ionic substance is dissolved in water or when melted. For example, if electricity is passed through molten lead bromide, the lead bromide is broken down to form lead and bromine.

Here is what happens during electrolysis:

• Positively charged ions move to the negative electrode during electrolysis. They receive electrons and are reduced.
• Negatively charged ions move to the positive electrode during electrolysis. They lose electrons and are oxidised.

The substance that is broken down is called the electrolyte.

Electroplating

Electrolysis is used to electroplate objects. This is useful for coating a cheaper metal with a more expensive one, such as copper or silver.

Electroplating with silver

The object to be plated, such as a metal spoon, is connected to the negative terminal of the power supply. A piece of silver is connected to the positive terminal. The electrolyte is silver nitrate solution.

Electroplating with copper

The object to be plated, such as a metal pan, is connected to the negative terminal of the power supply. A piece of copper is connected to the positive terminal. The electrolyte is coppersulfate solution.

This arrangement can also be used to purify copper during copper manufacture. In this case, both electrodes are made from copper. The negative electrode gradually gets coated with pure copper as the positive electrode gradually disappears.

Aluminium is the most abundant (found in large quantities) metal on Earth. But it is expensive, largely because of the amount of electricity used up in the extraction process.

Aluminium ore is called bauxite. The bauxite is purified to yield a white powder - aluminium oxide - from which aluminium can be extracted.

The extraction is done by electrolysis. But first the aluminium oxide must be melted so that electricity can pass through it. Aluminium oxide has a very high melting point (over 2000°C) so it would be expensive to melt it. Instead, it is dissolved in moltencryolite - an aluminium compound with a lower melting point than aluminium oxide. The use of cryolite reduces some of the energy costs involved in extracting aluminium.

Ionic substances in solution break down into elements during electrolysis. Different elements are released depending on the particular ionic substance.

At the negative electrode

At the negative electrode, positively charged ions gain electrons. This is reduction, and you say that the ions have been reduced.

Metal ions and hydrogen ions are positively charged. Whether you get the metal or hydrogen during electrolysis depends on the position of the metal in the reactivity series:

• The metal will be produced if it is less reactive than hydrogen
• Hydrogen will be produced if the metal is more reactive than hydrogen

If an electric current is passed through concentrated sodium chloride solution, hydrogen gas forms at the negative electrode and chlorine gas forms at the positive electrode. A solution of sodium hydroxide forms.

You might have expected sodium metal to be deposited at the negative electrode. But sodium is too reactive for this to happen, so hydrogen is given off instead.

During electrolysis:

• Hydrogen ions H⁺ (from the water) are discharged at the negative electrode as hydrogen gas, H₂
• Chloride ions Cl^– are discharged at the positive electrode as chlorine gas, Cl₂
• Sodium ions Na⁺ and hydroxide ions OH^– (from the water) stay behind - they form sodium hydroxide solution, NaOH

The three products of the electrolysis of concentrated sodium chloride solution have important uses in the chemical industry:

• Hydrogen is used as a fuel and for making ammonia
• Chlorine is used to kill bacteria in water, and to make bleach and plastics
• Sodium hydroxide is used to make soap and bleach