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The word energy comes from the Greek word energeia (ένέργεια), meaning activity or operation

Energy is closely linked to mass and cannot be created or destroyed

In this chapter we will consider gravitational potential and kinetic energy.Video: VPgjm

As far as we know, the Earth we live on is the only planet that is able to support life

Amongst other factors, the Earth is just the right distance from the sun to have temperatures that are suitable for life to exist

Also, the Earth's atmosphere has exactly the right type of gases in the right amounts for life to survive

Our planet also has water on its surface, which is something very unique

In fact, Earth is often called the “Blue Planet” because most of it is covered in water

This water is made up of freshwater in rivers and lakes, the saltwater of the oceans and estuaries, groundwater and water vapour

Together, all these water bodies are called the hydrosphere.The EarthVideo: VPbzp

In this chapter learners will explore Newtons three laws of motion and Newtons law of universal gravitation

Learners will also learn more about forces and the different kinds of forces

The following provides a summary of the topics covered in this chapter.Different kinds of forces

The types of forces covered are: normal force, frictional force, applied force and tension.Force diagrams and free body diagrams

In this section learners will see how to take a problem and draw diagrams to show all the forces acting on a body

They will learn what a force diagram is and what a free body diagram is

Each law is covered in detail and practical applications such as rockets, lifts and seat belts are covered

Learners are introduced to the ideas of weight and mass as well as the acceleration due to gravity

In this chapter we will learn how a net force is needed to modify the motion of an object

We will recall what a force is and learn about how force and motion are related

We are also introduced to Newton's three laws and we will learn more about the force of gravity.Ratio and proportion - Physical Sciences, Grade 10, Science skillsEquations - Mathematics, Grade 10, Equations and inequalitiesUnits and unit conversions - Physical Sciences, Grade 10, Science skills

Covalent bonding involves the sharing of electrons to form a chemical bond

The outermost orbitals of the atoms overlap so that unpaired electrons in each of the bonding atoms can be shared

By overlapping orbitals, the outer energy shells of all the bonding atoms are filled

As they move, there is an attraction between these negatively charged electrons and the positively charged nuclei

This attractive force holds the atoms together in a covalent bond

Covalent bond A form of chemical bond where pairs of electrons are shared between atoms

We will look at a few simple cases to deduce some rules about covalent bonds

Remember that it is only the valence electrons that are involved in bonding, and so when diagrams are drawn to show what is happening during bonding, it is only these electrons that are shown

For this case we will look at hydrogen chloride and methane

The electron configuration of hydrogen is and the electron configuration for chlorine is .The hydrogen atom has valence electron and the chlorine atom has valence electrons

The Lewis diagrams for hydrogen and chlorine are: Notice the single unpaired electron (highlighted in blue) on each atom

This does not mean this electron is different, we use highlighting here to help you see the unpaired electron

Notice how the two unpaired electrons (one from each atom) form the covalent bond

The dot and cross in between the two atoms, represent the pair of electrons that are shared in the covalent bond

We can also show this bond using a single line: Note how we still show the other electron pairs around chlorine

From this we can conclude that any electron on its own will try to pair up with another electron

So in practise atoms that have at least one unpaired electron can form bonds with any other atom that also has an unpaired electron

Remember that we said we can place unpaired electrons at any position (top, bottom, left, right) around the elements symbol

Water is made up of one oxygen and two hydrogen atoms

From what we learnt in the first examples we see that the unpaired electrons can pair up

Instead of writing the Lewis diagram for hydrogen twice, we simply write it once and use the in front of it to indicate that two hydrogens are needed for each oxygen

And now we can answer the questions that we asked before the worked example

We see that oxygen forms two bonds, one with each hydrogen atom

Oxygen however keeps its electron pairs and does not share them

If an atom has an electron pair it will normally not share that electron pair

A lone pair stays on the atom that it belongs to

A lone pair can be used to form a dative covalent bond

In the example above the lone pairs on oxygen are highlighted in red

When we draw the bonding pairs using lines it is much easier to see the lone pairs on oxygen

Notice the two electron pairs between the two oxygen atoms (highlighted in blue)

Because these two covalent bonds are between the same two atoms, this is a double bond

This forms a double covalent bond (which is shown by a double line between the two oxygen atoms)

Hydrogen has valence electron, carbon has valence electrons and nitrogen has valence electrons

Notice the three electron pairs (highlighted in red) between the nitrogen and carbon atom

Because these three covalent bonds are between the same two atoms, this is a triple bond

Nitrogen keeps its electron pair and shares its three unpaired electrons with carbon

A dative covalent bond is also known as a coordinate covalent bond

Earlier we said that atoms with a pair of electrons will normally not share that pair to form a bond

But now we will see how an electron pair can be used by atoms to form a covalent bond

One example of a molecule that contains a dative covalent bond is the ammonium ion () shown in the figure below

The hydrogen ion does not contain any electrons, and therefore the electrons that are in the bond that forms between this ion and the nitrogen atom, come only from the nitrogen

Notice that the hydrogen ion is charged and that this charge is shown on the ammonium ion using square brackets and a plus sign outside the square brackets

We can also show this as: Note that we do not use a line for the dative covalent bond

So in theory atoms that have at least one unpaired electron can form bonds with any other atom that also has an unpaired electron

If an atom has an electron pair it will normally not share that pair to form a bond

If an atom has more than one unpaired electron it can form multiple bonds to another atom

A dative covalent bond can be formed between an atom with no electrons and an atom with a lone pair

There are dots around element Y and from our knowledge of Lewis diagrams we know that these represent the valence electrons

X contributes one electron (represented by a cross) to the bond and X has no other electrons

From our knowledge of Lewis diagrams we look at how many cross and dot pairs there are in the molecule and that gives us the number of covalent bonds

These are single bonds since there is only one dot and cross pair between adjacent atoms

Note that Y could also be sulfur and X hydrogen and the molecule would then be hydrogen sulfide (sulfur dihydride)

The types of intermolecular forces that occur in a substance will affect its properties, such as its phase, melting point and boiling point

You should remember from the kinetic theory of matter (see grade ), that the phase of a substance is determined by how strong the forces are between its particles

The weaker the forces, the more likely the substance is to exist as a gas

This is because the particles are able to move far apart since they are not held together very strongly

If the forces are very strong, the particles are held closely together in a solid structure

Remember also that the temperature of a material affects the energy of its particles

The more energy the particles have, the more likely they are to be able to overcome the forces that are holding them together

Note that we are showing two dimensional figures when in reality these are three dimensional

The following five experiments investigate the effect of various physical properties (evaporation, surface tension, solubility, boiling point and capillarity) of substances and determine how these properties relate to intermolecular forces

A formal experiment on the effects of intermolecular forces is included in this chapter

In this experiment learners will investigate how intermolecular forces affect evaporation, surface tension, solubility, boiling points and capillarity

Some of the substances that are used (nail polish remover (mainly acetone if you use the non acetone free variety), methylated spirits (a mixture of methanol and ethanol), oil (a mostly non-polar hydrocarbon), glycerin (a fairly complex organic molecule)) are quite complex substances and learners may not have the skills needed to determine the types of intermolecular forces at work here

You should guide learners in this and tell them the intermolecular forces for these substances

You can help learners work out the strength of the intermolecular forces by telling them that larger molecules have stronger intermolecular forces than smaller molecules

This is often a big factor in determining which substance has the strongest intermolecular forces

Each experiment focuses on a different property and sees how that property relates to intermolecular forces

It will often not be easy for learners to see the small differences between some of the molecules chosen and so they need to use a combination of experimental results and knowledge about the strength of the intermolecular force to try and predict what may happen

Each experiment ends with a conclusion about what should be found to guide learners

It is very important to work in a well ventilated room (one with lots of air flow) particularly when working with methanol and ethanol

Many of the substances used (particularly nail polish remover, ethanol and methylated spirits) are highly flammable and so care must be taken when heating these substances

It is recommended that learners use a hot plate rather than a Bunsen burner to heat these substances as this reduces the risk of fire

When doing chemistry experiments it is also extra important to ensure that your learners do not run around, do not try to drink the chemicals, do not eat and drink in the lab, do not throw chemicals on the other learners and in general do act in a responsible and safe way

The guidelines for safe experimental work can be found in the science skills chapter from grade

From these experiments we can see how intermolecular forces (a microscopic property) affect the macroscopic behaviour of substances

If a substance has weak intermolecular forces then it will evaporate easily

Substances with weak intermolecular forces also have low surface tension and do not rise as far up in narrow tubes as substances with strong intermolecular forces

Substances are more likely to be soluble in liquids with similar intermolecular forces