GCSE OCR Gateway Science - C1 A-D

We cook our food to kill bacteria, the texture, taste and flavour are improved and it makes it easier to digest.

Cooking food is a chemical change, which is irreversible.

Proteins are large molecules that have a definite shape. When cooked, they change shape and by denaturing.

The starch grains burst when a potato is cooked, which makes it easier to digest.

Decomposition of baking powder

sodium hydrogencarbonate ^heat^ sodium carbonate + carbon dioxide + water

Testing for carbon dioxide - pass it through limewater. It turns the limewater from colourless to milky white.

Food Additives are used -

- To preserve food from reacting with oxygen, bacteria or mould

- To give a different sensory experience, such as to enhance the colour or flavour of food.

Tinned fruit and wine contain ascorbic acid, an antioxidant which stops food from reacting with oxygen.

Active packaging changes the condition of the food to extend its shelf life. It prevents the need for additives to be added to foods. It often involves the removal of water to make bacteria difficult to grow. It uses a polymer and a catalyst as a packaging film that scavenges for oxygen.

Intelligent packaging uses sensors to monitor the quality of the food and lets the customer know when the food is no longer fresh.

Emulsifiers -

• Some paints
• Milk, which is an emulsion of oil in water
• Mayonnaise is an emulsion of oil and vinegar and egg. Egg is the emulsifier.

The mayonnaise doesn't separate as the egg yolk has a molecule that has two parts :

- a water-loving part that attracts vinegar to it, called the hydrophillic head.

- a water-hating part that attracts oil to it, called the hydrophobic tail.

The hydrophobic tail is attracted into the lump of oil but the head isn't. The hydrophilic head is attracted to water and 'pulls' the oil on the tail into the water.

Making an Ester

Alcohol + Organic Acid ^ Ester + Water

Esterrs are used as perfumes as they can be recognised by their sweet smell.

A perfume :

• evaporates easily (volatile) - so that the smelly esters can easily reach the nose
• is non-toxic - so that it deos not poison you
• does not react with water - so that it does not react with perspiration (sweat)
• does not irritate the skin - so that it can be put onto the skin without causing harm
• is insoluble in water - so that it does not wash off easily

Volatility of Perfumes

Volatility - the ease with which a liquid can evaporate

During evaporation particles escape from the liquid into the air.

Only particles with lots of energy can escape the attraction of othe rparticles in the liquid to evaporate.

In a perfume the attraction between the particles is weak, so the particles do not need much energy to evaporate.

Crude oil, coal and natural gas are all fossil fuels. These have all formed naturally but the processes have taken millions of years.

The fossil fuels are finite resources. We also call fossil fuels non-renewable fuels because once they have been used up, we cannot make new supplies to replace them.

Crude oil is a mixture of hydrocarbons which are made up of hydrogen and carbon.

Fractional Distillation

Crude oil is a mixture of hydrocarbons. These hydrocarbons can be split up into fractions which are groups of hydrocarbons with similar boiling points.

Fractional Distillation

1. First of all, the crude oil is heated in a furnace to about 350 C when most of it evaporates into a gas.

2. It is then passed into the lower parts of the fractionating column where the fractions that have evaporated rise up the column.

3. The liquid parts are also caught in special plates further down the column.

4. As the rise, the condense back into liquids in different parts of the column, depending on their boiling points.

5. As they condense, they get trapped in special plates and separated.

6. The fractions with the lowest boiling points come off at the top of the column and the fractions with the highest boiling points come off at the bottom of the column.

Cracking - breaks some of the larger, less useful alkanes to make more of the smaller and more useful ones.

In the cracking reaction hydrocarbons belonging to another chemical family, the alkenes, are also formed which are useful to make plastic.

Cracking is used because there is a high demand for low chain fractions for example, petrol, so long chain fractions are broken up.