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C8 A) Purity
C8 A) Purity
A substance is pure if it is made up of a single element or a single compound. If there are more than one signal element or more than one compound, it means that the substance is a mixture and therefore not pure. Sometimes we will want to have a mixture of substances and other times we will want to have a pure substance. An example of a mixture of substances is air. Air is made up of many different gases; 78% of air is nitrogen, 21% is oxygen, 1% is argon and other substances make up the rest (these other substances are carbon dioxide, methane, hydrogen etc.).
We are able to test whether a substance is pure by melting or boiling the substance that we are testing. We then compare the melting or boiling point that we observe against the known melting or boiling point for the pure substance that we are testing for. If the melting point or boiling point for the substance that we are testing and the known meting or boiling point for the pure substance are the same, it means that the substance that we are testing is a pure substance. If there is a difference between the melting and boiling points, it means that the substance that we are testing is not pure.
The more impure our sample is, the greater the difference will be between the known melting or boiling points and the melting or boiling points that we observe.
For example, ice (solid water) melts at 0°C. If we have a block of ice and we want to test if the ice is pure water, we can slowly increase the temperature of the ice from -10°C and find the temperature where the ice starts melting. If the ice starts to melt at a temperature that is lower than 0°C, it means that the ice is not pure and instead is a mixture. For example, if the ice contains salt, the ice would start melting at a temperature below 0°C. Impurities making the melting or freezing point of water lower is why we spread salt on the roads in winter. When the salt is spread on the roads, it will dissolve in the water on the roads, which makes the solution a mixture of salt and water. As the solution on the roads is a mixture, it means it will lower the freezing point of the solution on the roads. This means that the solution on the roads wont freeze until a lower temperature, which means that there is less likely to be ice on the roads, thus making the roads safer. The more salt that is added to water, the further the freezing point will be away from 0°C (the more salt, the lower the melting point).
The boiling point of water with salt in is greater than the boiling point of pure water. For example, pure water boils at 100°C and sea water, which contains salt, boils at 100.6°C. The more salt that is added to the solution (the more impure the sample is), the further the boiling point will be above 100°C; the more salt, the higher the boiling point.
We are able to test whether a substance is pure by melting or boiling the substance that we are testing. We then compare the melting or boiling point that we observe against the known melting or boiling point for the pure substance that we are testing for. If the melting point or boiling point for the substance that we are testing and the known meting or boiling point for the pure substance are the same, it means that the substance that we are testing is a pure substance. If there is a difference between the melting and boiling points, it means that the substance that we are testing is not pure.
- The melting point for impure substances will be lower than the melting point of the pure substance. The impurities will also mean that the sample will melt over a range of temperatures.
- The boiling point of an impure substance will be higher than the boiling point for the pure substance. The impurities will also mean that the sample will boil over a range of temperatures.
The more impure our sample is, the greater the difference will be between the known melting or boiling points and the melting or boiling points that we observe.
For example, ice (solid water) melts at 0°C. If we have a block of ice and we want to test if the ice is pure water, we can slowly increase the temperature of the ice from -10°C and find the temperature where the ice starts melting. If the ice starts to melt at a temperature that is lower than 0°C, it means that the ice is not pure and instead is a mixture. For example, if the ice contains salt, the ice would start melting at a temperature below 0°C. Impurities making the melting or freezing point of water lower is why we spread salt on the roads in winter. When the salt is spread on the roads, it will dissolve in the water on the roads, which makes the solution a mixture of salt and water. As the solution on the roads is a mixture, it means it will lower the freezing point of the solution on the roads. This means that the solution on the roads wont freeze until a lower temperature, which means that there is less likely to be ice on the roads, thus making the roads safer. The more salt that is added to water, the further the freezing point will be away from 0°C (the more salt, the lower the melting point).
The boiling point of water with salt in is greater than the boiling point of pure water. For example, pure water boils at 100°C and sea water, which contains salt, boils at 100.6°C. The more salt that is added to the solution (the more impure the sample is), the further the boiling point will be above 100°C; the more salt, the higher the boiling point.
Formulations
Formulations are mixtures that have been designed as useful products. Many products are complex mixtures in which each of the chemicals has a particular purpose. We make formulations by mixing the different substances in carefully measured quantities to ensure that the products have the required properties.
Examples of formulations are fuels, cleaning agents, paints, medicines, alloys and food.
Let’s have a look at the drug example in more detail. The formulation of drugs needs to ensure that the drug gives the correct concentration and quantity of a certain substance to the correct part of the body whilst ensuring that the drug is able to be consumed and have a long enough shelf life.
Formulations are mixtures that have been designed as useful products. Many products are complex mixtures in which each of the chemicals has a particular purpose. We make formulations by mixing the different substances in carefully measured quantities to ensure that the products have the required properties.
Examples of formulations are fuels, cleaning agents, paints, medicines, alloys and food.
Let’s have a look at the drug example in more detail. The formulation of drugs needs to ensure that the drug gives the correct concentration and quantity of a certain substance to the correct part of the body whilst ensuring that the drug is able to be consumed and have a long enough shelf life.
Let’s now have a look at the paint example in more detail. Some of the different chemicals in paint are the pigments (chemicals that give the paint its colour), solvent (chemicals that dissolve all of the different chemicals and make the paint viscous [able to flow]), binders (creates a film that holds the paint onto the wall) and other additives to give the paint different properties (such as chemicals that result in the paint being water resistant so it can protect a wall or something that is metal).