All substances are made up of atoms. Atoms contain 3 types of particles;
- neutrons (except hydrogen)
Protons are positively charged.
Electrons are negatively charged.
Neutrons have no charge.
Elements and Compounds
Elements are substances made up of only one type of atom.
Compounds are substances made up of two or more types of atom.They are held together by chemical bonds.
Each element is represented by different chemical symbols eg. Fe for iron, Na for sodium.
These symbols are all arranged in the periodic table.
Chemical symbols are used with numbers to write formulae that represent molecules in compounds e.g. H2O
Formulae are used to show;
- the different elements in a compound
- the number of atoms of each element in one molecule of a compound.
Food is cooked in order to ;
- improve flavour/ taste
- improve texture
- make it easier to digest
- kill microorganisms (make it safe to eat)
Cooking food causes chemical changes to take place;
- new substances are made
- there may be a change in mass
- there is a temperature change
- the change cannot be reversed
Cooking eggs and meat; potatoes; baking powder
Eggs and meat contain protein, cooking protein causes the shape of the molecule to change and this change is irreversible. This is called DENATURING.
Potatoes are a good source of carbohydrates . When potatoes are cooked they soften because the cell walls break down and the starch is released. The body can easily digest starch.
Baking powder contains sodium hydrogen carbonate. When this is heated it breaks down, thermal decomposition, to form water and carbon dioxide gas. Baking powder is added to cake mixture because the carbon dioxide gas causes the cake to rise.
Testing for gases
There are 3 main tests we use in school to test for gases;
Limewater turns milky (cloudy) in the presence of carbon dioxide gas.
A glowing splint will relight in the presence of oxygen gas.
A lit splint will give a squeaky pop in the presence of hydrogen gas.
Chemicals are added to food ;
- antioxidents - stop it reacting with oxygen
- food colours - improve the appearance
- flavour enhancers - bring out the flavour of food without adding extra flavour
- flavourings - add another flavour to a food
- emulsifiers - help mix ingredients which would normally separate eg mayonnaise.
- have two types of ends to their molecules
- one end 'likes' water molecules - called hydrophillic
- one end 'repels' water molecules - hydrophobic ( and it 'likes' the oil droplets)
- the emulsifier molecule joins the droplets of oil and water and keeps them mixed,
Two important types of active packaging are used by food manufacturers, one improves the quality of the food; eg the widget that causes Guiness cans to form the right type of froth, or the button on coffe cans that heat it up.
The other type is used to improve the safety of the food; eg adding a substance to the packaging that absorbs water, this stops mould or bacteria from growing.
Perfumes and solvents
Smells are molecules which travel up your nose and stimulate sense cells.
- evaporate easily - so it can travel to your nose
- be non-toxic - so it does not poison you
- not irritate - so it does not cause rashes
- insoluble in water -so it wont wash off
- not react with water - so it doesnt react with sweat
- come from natural sources such as plants
- examples are; lavender, rose, patchouli, pine and musk
- esters made from organic acids reacted with an alcohol
Fossil fuels; crude oil
Crude oil, natural gas and coal are all fossil fuels. They are finite resources. They are described as non-renewable resources. This means they will eventually run out.
Crude oil is a thick, black, sticky liquid that is a mixture of hydrocarbons. Oil spills and slicks
- float on the sea surface
- block light from plankton which is the start of ocean food chains
- gets onto fish, birds and other animals and can kill them
- get onto shore and affect even more wildlife
- Crude oil is a mixture of hydrocarbons.
- hydrocarbons are compounds containing carbon and hydrogen atoms ONLY.
- different hydrocarbons have different boiling points
- crude oil can be separated into fractions with different boiling points.
- The fractionating column is hot at the bottom and cooler at the top.
- Fractions with long chain hydrocarbons have high boiling points and are collected at the bottom of the column
- fractions with short chain hydrocarbons have lower boiling points and are collected higher up the column.
Alkanes, alkenes and cracking
- saturated hydrocarbons
- all C-C bonds are single
- large molecules of alkanes are in less demand
- unsaturated hydrocarbons
- have one or more C=C double bonds
- alkene molecules can be used to make polymers which have many uses
- Not enough petrol in crude oil to meet demand
- Cracking is used to break down larger molecular hydrocarbons into smaller more useful alkenes.
The alkenes made during cracking can be used as monomers, which can be reacted together to produce polymers. These are long chain molecules, some of which are used to make plastics.
Alkenes are very good at joining together, and when they do so without produing another product we call it polymerisation.
Polymerisation requires high temperatures and a catalyst.
Designer polymers / disposal of plastics
- Gore- Tex - nylon fibres coated with a membrane or poly (tetrafluoroethene) (PTFE). This makes the holes in the fabric much smaller. PTFE would be too weak to use on its own.
- Gore-Tex is breathable; allows tiny molecules of sweat (water vapour) to diffuse through the small holes, but stops water droplets passing through.
- Used to make clothing for outdoor pursuits and activities.
Disposal of plastics;
- non-biodegradable - will not rot, last for hundreds of years
- give off toxic fumes if burnt
- rapidly fill landfill sites
- recyle plastics
- develop biodegradable plastics
Using carbon fuels
Choosing a fuel;
- energy value?
- storage needs
- ease of use
Fuel + oxygen --> carbon dioxide + water
Fuel + oxygen --> carbon monoxide + water
Exothermic or endothermic?
Exothermic = chemical reaction that releases heat energy into it's surroundings.
Endothermic = chemical reaction that absorb energy from it's surroundings.
To compare fuels;
- burn the same mass of fuel for the same length of time
- same mass/volume of water
- Record start temperature of water and end temperature of water, then calculated the temperature rise.
Calculating energy changes
Bond between atoms in the reactants need to be broken - this takes energy in.
New bonds need to be made to form the products - this releases energy.
The amount of energy transferred by a fuel can be calculated using the following equation;
ENERGY = Mass (g) x Specific heat capacity (J/g0C) x Temperature change (0C)
NB - Specific Heat Capacity is a constant that is specific for a particular material. For water it is 4.2 J/g0C
To calculate the energy transferred per gram of fuel we use;
Energy per gram(J/g) = Energy supplied(J) / mass of fuel burnt (g)