Elements, Compounds, and Mixtures Flashcards
Define an atom and element, giving examples. (4)
An atom is the smallest particle of a chemical element that can exist. For example, a single atom of hydrogen.
An element is a pure substance that is listed in the periodic table and only has one type of atom in it. For example, carbon.
What is the structure of an atom?
Give a brief description of the sub-atomic particles; protons, neutrons, and electrons. (4)
An atom has a central nucleus. This is surrounded by electrons arranged in shells.
The nucleus is tiny compared to the atom as a whole:
the radius of an atom is about 0.1 nm (1 × 10-10 m)
the radius of a nucleus (1 × 10-14 m) is less than of the radius of an atom.
For comparison, the radius of a typical bacterium is 1 × 10-6 m and the radius of a human hair is about 1 × 10-4 m.
Protons carry a positive electrical change, while electrons are negatively charged, and neutrons are neutral.
What are the 5 most common periodic elements and what are their symbols? (5)
- Oxygen (O)
- Silicon (Si)
- Aluminium (Al)
- Iron (Fe)
- Calcium (Ca)
Define a mixture and give examples. (2)
An impure substance made from different elements or compounds mixed together that are not chemically joined.
Mixtures can usually be separated by physical techniques such as filtering and distillation.
Air is a mixture that contains the elements nitrogen, oxygen and argon, and also the compound carbon dioxide.
Distinguish between a solute, solvent and solution. (3)
A solution is made when a substance dissolves into a liquid. The liquid is called the solvent. The substance that has been dissolved is called the solute.
Describe how to separate mixtures using simple filtration, evaporation, distillation, fractional distillation, and chromatography. (12)
Filtration is a method for separating an insoluble solid from a liquid. When a mixture of sand and water is filtered:
the sand stays behind in the filter paper (it becomes the residue)
the water passes through the filter paper (it becomes the filtrate)
Evaporation is used to separate a soluble solid (i.e. a solid that dissolves) from a liquid. For example, copper sulfate is soluble in water – its crystals dissolve in water to form copper sulfate solution.
During evaporation, the water evaporates away leaving solid copper sulfate crystals behind.
Simple distillation is a method for separating the solvent from a solution. For example, water can be separated from salt solution by simple distillation. This method works because water has a much lower boiling point than salt. When the solution is heated, the water evaporates. It is then cooled and condensed into a separate container. The salt does not evaporate and so it stays behind.
Fractional distillation is a method for separating a liquid from a mixture of two or more liquids. For example, liquid ethanol can be separated from a mixture of ethanol and water by fractional distillation. This method works because the liquids in the mixture have different boiling points.
When the mixture is heated, the liquid with the lowest boiling point boils first. The vapour condenses in the condenser before the other liquid boils. The long fractionating column ensures that the second liquid does not get into the condenser until most of the first one has been removed.
Paper chromatography is a method for separating dissolved substances from one another. It is often used when the dissolved substances are coloured, such as inks, food colourings and plant dyes. For instance, ink is a solution of dyes dissolved in a solvent of water or oil.
It works because some of the coloured substances dissolve in the solvent better than others, so they travel further up the paper.
Define a compound and give an example of a common compound. (3)
A compound is a pure substance made from more than one type of element chemically bonded together.
Elements bond in fixed ratios and so can be represented by a chemical formula. For example, sodium chloride has the same number of sodium ions and chloride ions, so its formula is NaCl, whereas water is always made from twice the number of hydrogen atoms as oxygen atoms, so it is H2O.
What must occur for a compound to occur? (1)
A chemical reaction.
Define a chemical change and give an example of a chemical change. (3)
Chemical changes happen when chemical reactions occur. They involve the formation of new chemical elements or compounds.
For example, liquid water decomposing into hydrogen and oxygen, eg when an electric current is passed through water, is a chemical reaction.
List three ways you can observe a chemical reaction has taken place. (3)
Evidence for a chemical reaction can include any of the following:
Bubbles – Many chemical reactions you see in the science lab make a chemical which is a gas, so you see bubbles.
A colour change – If the new chemicals are a different colour from the original chemicals, there will be a colour change.
A large energy change – Many chemical reactions give off lots of energy, like burning, and a few absorb energy, so they feel cold.
How do you write a word equation? (2)
What are reactants and products? (2)
A word equation represents a chemical reaction using the names of the substances involved. Word equations do not show any chemical symbols or formulae.
In a chemical reaction, reactants are the substances that react together, and products are the substances formed.
Word equations always take the form, reactants → products. A + sign separates two or more reactants or products.
How do you write a word equation? (2)
What are reactants and products? (2)
A word equation represents a chemical reaction using the names of the substances involved. Word equations do not show any chemical symbols or formulae.
In a chemical reaction, reactants are the substances that react together, and products are the substances formed.
Word equations always take the form, reactants → products. A + sign separates two or more reactants or products.
How do you name compounds? (Three Rules) (6)
Rule one
The element that is furthest left in the periodic table comes first, eg Sodium Chloride/Carbon dioxide
Rule two
If there are only two elements in the compound then the compounds name ends in –ide, eg A compound of copper and sulfur is called copper sulfide.
Rule three
If the compound contains three elements one of which is oxygen then the compound name will end in –ate or –ite, eg Calcium carbonate contains calcium, carbon and oxygen.
Sometimes the name of the compound gives information about the formula of that compound. Names of these compounds have prefixes that give the number of atoms of certain elements in each molecule.
