Chemical Reactions

chemical reaction is a change in which atoms are rearranged to make new substances. The atoms are joined together in one way before the reaction and in a different way after the reaction. Elements and compounds take part in chemical reactions. 

Chemical reactions are not easily reversible. At the end of the reaction, it is very difficult to get back to the substance you started with. 

As well as making new substances, chemical reactions also transfer energy to or from the surroundings. 

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Chemical reaction or not?

To know if there has been a chemical reaction, you can check for these things: 

  • see huge flames or tiny sparks
  • notice a sweet smell or foul stink
  • feel the substance getting hotter or colder
  • hear a loud bang or gentle fizzing
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Why are chemical reactions useful?

Chemical reactions are very useful. They make very useful substances. These include:

  • medicines, such as paracetamol
  • fabrics, such as polyester
  • building materials, such as cement

Chemical reactions also transfer energy. This transfer can be useful. Burning petrol makes vehicles go. Burning coal or gas heats water to produce steam to generate electricity. 

Some chemical reactions are not useful. Rusting may damage cars, bicycles, boats and bridges. Chemical reactions make food rot. 

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Are all changes chemical reactions>

Not all changes involve chemical reactions. If you warm chocolate, it melts. But you still have chocolate. Changes of state, and dissolving, are reversible. This means you can back what you started with. These are called physical changes.

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Acids and Alkalis introduction

Acids are sour chemicals that can sometimes be corrosive. An example of an acid is hydrochloric acid (stomach acid). It helps us digest food.

Alkalis are the chemical opposites if acids. Soap solution is an alkali, and so is toothpaste. most alkalis feel soapy. 

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Using acids and alkalis safely

It is safe to eat the acid in lemons and to use alkaline soaps, but there are hazards linked to some acids and alkalis. 

The hazard symbols on bottles show that the solution in the bottle is corrosive. It could burn your skin and eyes.

You can control risks from corrosive solutions by: 

  • wearing eye protection
  • keeping the solution off your skin

Some acidic and alkaline solutions are labelled with different hazard symbols.  

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Measuring Acids and Alkalis

The pH scale shows the strength of acids and alkalis. 

  • It goes form 0-14
  • Anything with a pH below 7 is an acid. The strongest acid has a pH 0
  • The lower the number, the more acidic and dangerous it is
  • Anything with a pH above 7 is an alkali. The strongest alkali has pH 14
  • A neutral substance has pH 7 (like water)

An indicator is something that changes colour depending on whether it's an acid or an alkali. 

  • Litmus paper is an indicator. Acids turn litmus paper red. Alkalis turn it blue.
  • Universal indicator solution is a liquid indicator. It gives the colours shown on the pH chart. 
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Neutralisation Reactions

Acids and alkalis neutralise each other.

  • Acids react with alkalis to form a salt and water
  • acid + alkali --> salt + water
  • This is a neutralisation reaction. The products have a neutral pH (a pH of 7). 

Making salts by neutralisation

  • Wearing eye protection, add acid to an alkali. Stop when the solution is neutral.
  • Boil-off some of the liquid so you're left with a really concentrated solution
  • Leave the solution overnight for the rest of the solution to evaporate. Nice big salt crystals will form.
  • The reaction between hydrochloric acid and sodium hydroxide makes the salt sodium chloride.
  • Hydrochloric acid always reacts to make a salt with chloride in its name. For example, sodium chloride.
  • Sulfuric acid always reacts to make a salt with sulfate in the name. For example, copper sulfate.  
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Reactivity Series and metal extraction

The reactivity series is how well a metal reacts. The reactivity series lists metals in order of how reactive they are. 

Reactions of Metals with Dilute Acid

  • Metal + acid --> salt + hydrogen
  • Metals above hydrogen in the reactivity series will react with acids to make a salt and hydrogen
  • The metals below hydrogen in the reactivity series don't react with acids
  • The reaction becomes less and less exciting as you go down the series.
  • More reactive metals react more violently. 


  • zinc + sulfuric acid--> zinc sulfate + hydrogen
  • Zn + H2SO4--> ZnSo4 + H2
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Reaction of Oxides with Acids

Oxides are chemicals with oxygen in them.

  • Metals react with oxygen to create metal oxides
  • Example: magnesium + oxygen--> magnesium oxide
  • Metal oxides in solution have a pH which is higher than 7. They're alkaline.
  • So metal oxides react with acids to make a salt and water
  • Acid + metal oxide--> salt + water
  • Non-metals also react with oxygen to make oxides
  • Example: sulfur + oxygen--> sulfur dioxide
  • Non-metal oxides are acidic
  • The oxides of non-metals have a pH of below 7. This means they're acidic
  • So non-metals will react with oxides to make a salt and water
  • alkali + non-metal--> salt + water
  • Example: sodium hydroxide + silicon dioxide--> sodium silicate + water
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Displacement reactions

Displacement means "taking the place of"

A more reactive metal will replace a less reactive metal from its compound.

  • A more reactive metal will will take the place of a less reactive metal in a compound
  • The less reactive metal gets kicked out of its compound
  • It then coats itself in on the reactive metal
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Word equations and symbol equations

Chemicals are formed in Chemical Reactions

  • In a chemical reaction, chemicals combine together or split apart to form new substances
  • The chemicals you start with are called REACTANTS
  • The chemicals you end up with are called PRODUCTS
  • Word equations show what's happening in a chemical reaction
  • A word equation has the names of all chemicals written out in full
  • Example: sodium + water--> sodium hydroxide + hydrogen
  • sodium and water are the reactants and sodium hydroxide and hydrogen are the products
  • Scientists usually use symbol equations from the periodic table 
  • The formula for a compound is made up of the symbols of the elements inside it
  • A symbol equation uses symbols and formulas to sho what's happening in a chemical reaction
  • The big numbers show you the amounts of reactants and products in the reaction
  • 2NA+ 2H2O--> 2NAOH+ H2
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Properties of Metals

Metals can be found in the periodic table

  • Most of the elements in the periodic table are metals. About 91 of the elements in the periodic table are metals. 

Metals conduct electricity

  • This means metals allow electrical current to pass through them easily
  • So metals can be used to make wires to allow electricity to move around a circuit

Metals conduct energy

  • Metals transfer energy from a hot place to a cold place quickly and easily
  • That's why saucepans are usually made out of metal

Metals are strong and tough

  • Metals have high tensile strength. This means they can be pulled hard without breaking
  • So they make good building materials
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Properties of Metals Part 2

Metals are shiny and polished

  • Polished or freshly cut metals have a smooth surface
  • This means they reflect light well, so they look shiny

Metals are sonorous

  • This means they make a dong sound when they are hit. Only metals do that

Metals are malleable

  • Metals are easily shaped (malleable). This means metals can be hammered into thin sheets or bent
  • Shaped metal is used in cars and planes

Metals are ductile

  • This means they can be drawn into wires and it means metals aren't brittle
  • Metals just bend and stretch 
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Properties of Metals Part 3

Metals have high melting points and boiling points

  • A lot of energy is needed to melt metals, this is because their atoms are joined together with strong bonds
  • Things that get really hot are usually made of metal so they don't melt

Metals have high densities

  • Density is all to do with how much stuff there is squeezed into a certain place
  • Metals feel heavy for their size, they're very dense. It's because they have lots of atoms in a small space

Some metals are Magnetic

  • Magnetic means attracted to magnets 
  • Most metals aren't magnetic
  • Iron, nickel and cobalt are magnetic
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Properties of Non-Metals

Non-metals can be found in the periodic table. There are fewer non-metals than metals.

Non-metals don't conduct electricity

  • Most non-metals are electrical insulators. This means electrical current can't flow through
  • This useful; non-metals are used to make things like plugs and electric cable coverings

Non-metals don't conduct energy by heating well

  • Non-metals don't transfer energy from a hot place to a cold place quickly or easily
  • This makes non-metals really good thermal insulators
  • Oven gloves, saucepan handles and loft insulation are usually made of non-metals

Non-metals are not strong or hard-wearing

  • The forces between the particles in non-metals are weak. This means non-metals break easy
  • Its also easy to scrub atoms or molecules off them, so it's easy to wear them.
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Properties of Non-Metals Part 2

Non-Metals are dull

  • Most non-metals don't reflect light well. Their surfaces are not usually as smooth as metals. 
  • This makes them look dull.

Non-Metals are brittle

  • Non-metal structures are held together by weak forces. This means they can shatter easily.

Non-metals have low melting points and boiling points

  • The forces which hold the particles in non-metals together are very weak. This means they melt and boil easily
  • At room temperature, most non-metals are gases or solids. Only one is liquid

Non-metals are not magnetic

  • Only a few metals (iron, cobalt and nickel) are magnetic.
  • All non-metals are definitely not magnetic. 
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