Sodium hydroxide

Sodium hydroxide is probably the best known STRONG ALKALI. It is pH 11. If you see it as one of the products in a reaction, you will know that the solution has become alkali.
If you think about it, the pH has gone UP. pH11 is UP from pH7. The pH goes DOWN to make acids.

Intermolecular forces

Water has a low boiling point because the forces between the molecules are weak. On the diagram, the forces between H and O are strong covalent bonds. It takes a lot of energy to break up water into H and O. However, it is easy to separate the molecules. In an exam answer, you need to make it clear that it is the inter-molecular forces that you are talking about.

Metallic bonding

Metals have big atoms and so the force of attraction between the nucleus and the outer electrons is weaker. The electrons can wander off. Metals contain positive ions surrounded by delocalised free electrons.
Metallic bonding is shown in the diagram. Ion A pulls left on the free electrons (green arrow). Ion B pulls right on the free electrons (red arrow). Like two kids fighting over a toy, the ions are held together.

The difference between yield and rate of reaction

This is bugging me a bit, but I think it boils down to this:

Yield means what percentage of the reaction ends up doing what you want. Remember that if it is a reversible reaction, it can make the product and then undo it again.

Rate of reaction means how fast the reaction goes; how quickly you get what you want.

Questions dealing with pressure be about either:

• If you increase the pressure you push the particles closer together so that they collide more often and the rate of reaction increases.
• If you increase the pressure of a reversible reaction involving gases, you favour the direction that goes from more molecules to less molecules. If that is the forward direction, then increasing the pressure will increase the yield.

It is the same with temperature:

• If you increase the temperature, the particle move faster and so collide more frequently. More collisions reach the activation energy. Hence the rate of reaction increases.
• If a reversible reaction is endothermic in the forward direction and you increase the temperature, then you will increase the yield.

Kinetic energy

This is the energy that an object has when it is moving.

You can calculate knietic energy like this:

kinetic energy = 1/2 x mass x speed x speed

For example:

A 500kg car is travelling at 10 m/s.

kinetic energy = 1/2 x 500 x 10 x 10 = 25000 Joules

momentum

Momentum means how difficult it is to stop an object that is moving.

The bigger the momentum the harder it is to stop.

Momentum = mass x speed

Momentum is measured in kgm/s

Momentum can be passed on from one object to another in a collision.

Remember the clicking balls!

Precipitation reactions

This when you mix two liquids together and end up with a solid floating in the liquid. The solid is called a precipitate.

We did it when we made the chemical that they put on film. We got a white precipitate which we filtered out. We spread the white precipiate out on the filter paper and put it on the side in the sunshine with a coin over it.

Where the sun reached it, a reaction happened and the chemical went dark grey. Under the coin there was no light and no reaction so it stayed white.

Covalent bonds

This is where atoms share electrons to make them think that they have full outside layers. They are weak bonds, so the chemicals have low melting and boiling points.

This is chlorine.

An odd one is diamond. Each carbon atom is joined to 4 other atoms in a huge structure that is very strong.

Ionic bonding

Atoms are only "happy" when their outside layers are either completely full or completely empty. The atom on the left had 2 atoms on its outside layer.
To get a completely empty layer, it loses two electrons. Taking away 2 negative electrons makes it doubly positive (2+).

The atom on the right had only 6 electrons on its outside layer. It needs 8 to have a full layer so it gains two electrons. Gaining 2 negative electrons makes it doubly negative (2-).

Opposites attract so the (2+) and the (2-) stay together. This is an ionic bond. The bonds are very strong so ionic compounds have very high melting and boiling points.

How to work out where to put electrons

To work out where to put the electrons, start by looking at the Periodic Table. You need the bottom number to give you the number of electrons.

Let's start with ALUMINIUM. It has 13 electrons because the bottom number is 13.

We get the 13 like this:

• The first 2 go on the inside layer.
• The next 8 go on the second layer out.
• 2 and 8 make 10. That means there are 3 left for the outside layer.

Oxygen is the next example. It's bottom number on the Periodic Table is 8.

• 2 on the inside layer.
• That means that there are 6 left to go on the second layer. (It can have up to 8)
  
  
  

Final example is lithium. It's bottom number is 3.

• 2 on the inside layer.
• That leaves 1 on the second layer this time.
  
  

Electrolysis

Electolysis is where we use an electric current to split up an ionic compound that has been dissolved in water.

Our example was sodium chloride solution (salt water)

It contains H+ and OH- ions from the water.
It contains Na+ and Cl- ions from the chlorine.

Two Cl- ions lose 2 electrons at the positive electrode and turn into chlorine gas. It bleached blue litmus paper, turning it white.

The two electrons join up with two H+ ions at the negative electrode to make hydrogen gas.

Na+ and OH- are left behind. This makes sodium hydroxide which is an alkali and turns the universal indicator purple.

Re-sit papers

If you are re-sitting the year 10 modules on Monday 23 June, here's a link to get you the past papers that you need. There are mark schemes too.

http://www.aqa.org.uk/qual/gcse/newscience/extassessover.php

Star formation

Satellites

A geostationary orbit is one above the equator. It takes the satellite 24 hours to go round once. This means that satellite Y is always above town A. (Note that the satellite doesn't stay still. It does move but so does town A. They stay fixed relative to each other.)

• Used for communications eg phone and TV
• Because the satellite is in a fixed position relative to the Earth so dishes can always point in the same direction.
• GPS satellites are geostationary too.
• You SatNav constantly checks where it is in relation to these satellites.

Polar orbit goes over the poles and covers a lot of territory.

• Many countries visited so good for spying.
• Close to the ground so better photos (GoogleEarth)
• Weather photos

Ultrasound

Think about what happens when light hits a window. Some of it goes through and some is reflected. You can choose to focus on what is reflected or on what is on the other side of the window. We call this partial reflection.

In the same way, ultrasound is partially reflected when it goes from one medium into another medium. The ultrasound changes speed when it goes from one medium to another. The bigger the change in speed, the more is reflected. It is the reflection that allows us to make up a picture using ultrasound scanning for eg foetuses.

There is a big change in speed between air and muscle, so if you fire ultrasound towards a woman's stomach, it would mostly reflect and not get into her. For this reason, they smear saline gel on first because there is less of a reflection between saline gel and muscle.

Ray diagrams for convex lenses

Remember that they keep giving you mini-lenses on the diagram. Don't get fooled into drawing the rays through the lens. On their way of doing it, the vertical line is also part of the lens.

Reflection in a plane mirror

You've probably forgotten that we ever did this diagram for a plane mirror (flat mirror) but it would be a good idea to learn it.

Concave mirrors

Let's make a concave mirror. We start off with a complete circle. It seems quite obvious at the moment that the middle of the circle is the centre of curvature.



















Now let's mark out a section of the circle and then cut it out. You can see that it still has a centre of curvature even when it's just a cut section.



















Now let's fire rays of light at it. Notice that they don't focus at the centre of curvature. That means that we also need to know about a second point - the focal point (or principal focus).

















Now take a look at a proper ray diagram:

• It has C for centre of curvature and F for principal focus
• The curved lens has been drawn flat for the purposes of the diagram

There are 3 possible rays you could draw, although only 2 appear on any diagram:

Ray 1: Horizontally from the tip of the object arrow to hit the mirror and bounce back through F.
Ray 2: Slants down straight through F to hit the mirror then bounces back horizontally.

Ray 3: Is a perfectly straight line going through the top of the image arrow and through point C.

Real and virtual images

For the purposes of exam answers, this is what you have to say:

A real image can be projected onto a piece of paper.

• We did this is class to see an upside down picture of the room and to get pictures of the upside down triangle.
• You can spot it on a diagram because it is where solid rays cross.

A virtual image cannot be projected onto a piece of paper.

• You can spot it on a diagram because it is where dotted rays cross.

The National Grid

Click on the picture to get it to a bigger size.

On this diagram, A is a step up transformer at the power station.

• A step up transformer increases the voltage (potential difference).
• At the same time the current is reduced.
• Current is responsible for heating up wires and wasting energy.
• So reducing the current reduces the energy wasted by heat.

Near your house there is step down transformer C that decreases the voltage ( and increases the current). The reason is that the massive voltage could give a fatal electric shock.

The National Grid is the system of power stations and power lines. It means that each town no longer has to have its own power station. Clearly there are economies of scale.

PS B is an intermediate step down transformer. There's one for every town that reduces the voltage from 400,000V to 33,000V before it is sent to the local substations to be reduced to 230V for your house.