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K-12Corpus

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These compressions and rarefactions arise because the source vibrates longitudinally and the longitudinal motion of air produces pressure fluctuations.

The most common example of electromagnetic (EM) radiation is visible light

Everyone is very familiar with light in everyday life, you can only see things because light bounces off them and enters your eyes

This alone makes it worthwhile to learn about it but there are also very many other applications of EM radiation

It is called electromagnetic because there are electric and magnetic fields making up the radiation

We will look at this in more detail a little later.In everyday experience, light doesn't seem to have many special properties but it does: A huge spectrum: The light we can see (visible EM radiation) is only a small part of all of the EM radiation (electromagnetic spectrum) that exists

Nature's speed limit: Nothing moves faster than the speed of light

Wave nature: All EM radiation has the ability to behave like a wave which we call wave-like behaviour

Particle nature: All EM radiation has the ability to behave like a particle which we call particle-like behaviour

No medium required: EM radiation can propagate without a medium through which to move even though they are waves

We will discuss this in the following sections and in even more detail in Grades 11 and 12.

Diagrams of compounds are very useful because they help us to picture how the atoms are arranged in the compound and they help us to see the shape of the compound

There are three types of diagrams that are commonly used: Wireframe or stick models In this model, the bonds between atoms are shown as “sticks”

Ball and stick models This is a 3-dimensional molecular model that uses “balls” to represent atoms and “sticks” to represent the bonds between them

The centres of the atoms (the balls) are connected by straight lines which represent the bonds between them

Table 12.1 shows examples of the different types of models for all the types of compounds.Covalent molecularCovalent networkIonic networkMetallic networkName of compoundglucosegraphitesilver chloridezincFormula or Stick modelBall-and-stick modelSpace-filling model Table 12.1: Different representations for compounds CanvasMol (www.alteredqualia.com/canvasmol) is a website that allows you to view several compounds

You do not need to know these compounds, this is simply to allow you to see one way of representing compounds.

The synthesis (forming) of water () from hydrogen gas () and oxygen gas () is another example of chemical change

A simplified diagram of this reaction is shown in Figure 13.3

The chemical bonds between in and between in are broken and new bonds between and (to form ) are formed

A chemical change has taken place.Video: VPbhyA mixture of hydrogen and oxygen gas is used as a fuel to get rockets into space.There are some important things to remember about chemical changes: Arrangement of particles During a chemical change, the particles themselves are changed in some way

In the example of hydrogen peroxide that was used earlier, the molecules were split up into their component atoms

The number of particles will change because each molecule breaks down into two water molecules () and one oxygen molecule ()

Energy changes The energy changes that take place during a chemical reaction are much greater than those that take place during a physical change in matter

During a chemical reaction, energy is used up in order to break bonds and then energy is released when the new product is formed

Reversibility Chemical changes are far more difficult to reverse than physical changes

When hydrogen peroxide decomposes into water and oxygen, it is almost impossible to get back to hydrogen peroxide

Mass conservation Mass is conserved during a chemical change, but the number of molecules may change

In the example of the decomposition of hydrogen peroxide, for every two molecules of hydrogen peroxide that decomposes, three molecules are formed (two water and one oxygen)

Table 13.1 highlights these concepts for the decomposition of hydrogen peroxide.Moleculestwo moleculesthree moleculesEnergy changesenergy taken in when bonds are brokenenergy given off when bonds are formedMass is conservedAtoms are conserved oxygen atoms, hydrogen atoms oxygen atoms, hydrogen atoms Table 13.1: Important concepts in chemical change Exercise 13.1See solutions For each of the following say whether a chemical or a physical change occurs

Melting candle wax.Mixing sodium chloride () and silver nitrate () to form silver chloride ().Mixing hydrochloric acid () and magnesium ribbon () to form magnesium chloride ().Dissolving salt in water.Tearing a piece of magnesium ribbon

As we have already mentioned, a number of changes can occur when elements are combined with one another

One way of representing chemical changes is through balanced chemical equations

A chemical equation describes a chemical reaction by using symbols for the elements involved

For example, if we look at the reaction between iron and sulfur to form iron sulfide , we could represent these changes in a sentence, in a word equation or using chemical symbols: Sentence: Iron reacts with sulfur to form iron sulfide

A chemical formula shows each element by its symbol and also shows how many atoms of each element are found in that compound

The number of atoms (if greater than one) is shown as a subscript.The following exercise serves as revision

Magnetism is an interaction that allows certain kinds of objects, which are called 'magnetic' objects, to exert forces on each other without physically touching

A magnetic object is surrounded by a magnetic 'field' that gets weaker as one moves further away from the object

A second object can feel a magnetic force from the first object because it feels the magnetic field of the first object

The further away the objects are the weaker the magnetic force will be.Video: VPfkkHumans have known about magnetism for many thousands of years

For example, lodestone is a magnetised form of the iron oxide mineral magnetite

It is referred to in old European and Asian historical records; from around BCE in Europe and around BCE in Asia.Magnetic objects stuck to a magnetThe root of the English word magnet is from the Greek word magnes, probably from Magnesia in Asia Minor, once an important source of lodestone.

Electrostatics is the study of electric charge which is at rest or static (not moving)

In this chapter we will look at some of the basic principles of electrostatics as well as the principle of conservation of charge.Video: VPfmg

When you measure the potential difference across (or between) the terminals of a battery that is not in a complete circuit you are measuring the emf of the battery

This is the maximum amount of work per coulomb of charge the battery can do to drive charge from one terminal, through the circuit, to the other terminal.The volt is named after the Italian physicist Alessandro Volta (1745–1827).Electrical potential difference is also called voltage.When you measure the potential difference across (or between) the terminals of a battery that is in a complete circuit you are measuring the terminal potential difference of the battery

Although this is measured in volts it is not identical to the emf

The difference will be the work done to drive charge through the battery.BatteriesOne lead of the voltmeter is connected to one end of the battery and the other lead is connected to the opposite end

The voltmeter may also be used to measure the voltage across a resistor or any other component of a circuit but must be connected in parallel.

Many reactions in chemistry and all biological reactions (reactions in living systems) take place in water

In this chapter we will look at some of these reactions in detail

Almost all the reactions that occur in aqueous solutions involve ions

We will look at three main types of reactions that occur in aqueous solutions, namely precipitation reactions, acid-base reactions and redox reactions

Before we can learn about the types of reactions, we need to first look at ions in aqueous solutions and electrical conductivity.Video: VPbls

Sometimes it is important to know exactly how many particles (eg atoms or molecules) are in a sample of a substance, or what quantity of a substance is needed for a chemical reaction to take place.The amount of substance is so important in chemistry that it is given its own name, which is the mole.Mole The mole (abbreviation “mol”) is the SI (Standard International) unit for “amount of substance”

The mole is a counting unit just like hours or days

We can easily count one second or one minute or one hour

If we want bigger units of time, we refer to days, months and years

We call this number Avogadro's number.Avogadro's number The number of particles in a mole, equal to

If we had this number of cold drink cans, then we could cover the surface of the earth to a depth of over

If you could count atoms at a rate of 10 million per second, then it would take you 2 billion years to count the atoms in one mole!The original hypothesis that was proposed by Amadeo Avogadro was that “equal volumes of gases, at the same temperature and pressure, contain the same number of molecules”

His ideas were not accepted by the scientific community and it was only four years after his death, that his original hypothesis was accepted and that it became known as “Avogadro's Law”

In honour of his contribution to science, the number of particles in one mole was named Avogadro's number.We use Avogadro's number and the mole in chemistry to help us quantify what happens in chemical reaction

If we measure of carbon we have one mole or carbon atoms

You will remember that when the mass, in grams, of an element is equal to its relative atomic mass, the sample contains one mole of that element

This mass is called the molar mass of that element.You may sometimes see the molar mass written as

We will use in this book, but you should be aware of the alternate notation.It is worth remembering the following: On the periodic table, the relative atomic mass that is shown can be interpreted in two ways

The mass (in grams) of a single, average atom of that element relative to the mass of an atom of carbon

The average atomic mass of all the isotopes of that element

ElementRelative atomic mass (u)Molar mass ()Mass of one mole of the element (g)MagnesiumLithiumOxygenNitrogenIron Table 19.1: The relationship between relative atomic mass, molar mass and the mass of one mole for a number of elements

Calculate the number of moles of iron (Fe) in an sample

If we look at the periodic table, we see that the molar mass of iron is

This means that 1 mole of iron will have a mass of .If 1 mole of iron has a mass of , then: the number of moles of iron in must be:There are 2 moles of iron in the sample

You need to imagine that the horizontal line is like a division sign and that the vertical line is like a multiplication sign

So, for example, if you want to calculate , then the remaining two letters in the triangle are and and is above with a division sign between them

Your calculation will then be Remember that when you use the equation , the mass is always in grams (g) and molar mass is in grams per mol ()

Always write the units next to any number you use in a formula or sum

Calculate the number of moles of copper there are in a sample that with a mass of

Calculate the number of atoms there are in a sample of aluminium that with a mass of

However, you need to remember that all your calculations will apply to the whole compound

So, when you calculate the molar mass of a covalent compound, you will need to add the molar mass of each atom in that compound

The number of moles will also apply to the whole molecule

For example, if you have one mole of nitric acid () the molar mass is and there are molecules of nitric acid

This is the mass of all the atoms in one formula unit of the compound

For example, one mole of sodium chloride () has a formula mass of and there are molecules of sodium chloride in one formula unit.In a balanced chemical equation, the number that is written in front of the element or compound, shows the mole ratio in which the reactants combine to form a product

If there are no numbers in front of the element symbol, this means the number is '1'.Video: VPezceg In this reaction, 1 mole of nitrogen molecules reacts with 3 moles of hydrogen molecules to produce 2 moles of ammonia molecules

Scalars are physical quantities which have only a number value or a size (magnitude)

A scalar tells you how much of something there is.Scalar A scalar is a physical quantity that has only a magnitude (size)

For example, a person buys a tub of margarine which is labelled with a mass of

The mass of the tub of margarine is a scalar quantity

It only needs one number to describe it, in this case, .Vectors are different because they are physical quantities which have a size and a direction

A vector tells you how much of something there is and which direction it is in.Vector A vector is a physical quantity that has both a magnitude and a direction

For example, a car is travelling east along a freeway at

The car is moving at (this is the magnitude) and we know where it is going – east (this is the direction)

Your weight is proportional to your mass (magnitude) and is always in the direction towards the centre of the earth

This chapter is about how things move along a straight line or, more scientifically, how things move in one dimension

This is useful for learning how to describe the movement (motion) of cars along a straight road or of trains along straight railway tracks

There are three features of motion that we use to describe exactly how an object moves

They are:The jerk is the name we give to how fast the acceleration is changing.Traffic often moves along a straight line