Chapter 2 Atoms and molecules

Important definitions-

Atom- The smallest particle of an element that can take part in a chemical reaction. All atoms of the same element have the same number of protons in the nucleus.

Pure substances can be of two types-

1. Element- Substances that cannot be chemically broken down into simpler substances. An element cannot be decomposed by passing electric current through it.
2. Compound- Pure substances made from two or more elements chemically combined together in fixed proportions. 

Mixtures can be of two types-

1. Homogeneous 
2. Heterogeneous (explained in the previous blog)

Difference between Mixtures and compounds

It is really important to understand the difference since most students don't understand the difference.

No	Mixtures	Compounds
1 2  3 4	Substances simply mixed together, no reaction takes place Composition of mixture can be varied like in air the carbon dioxide level could rise and fall, no fixed value Properties of the substances present remain the same. Substances in the mixture can be separated by physical means like filtration and distillation	Substances chemically react together to form a new compound Composition of new compound remains the same. Properties of the new compound formed are different form the elements it is made of. Compound cannot be easily separated into its elements

Kinetic Theory (the Dalton theory not required by syllabus,

• All matter is made up of small particles
• The particles are moving all the time. The higher the temperature the higher the average energy and speed of particles.
• Heavier particles move more slowly than lighter particles at the same temperature. Then take a look at http://igcsechemistryrevision.blogspot.in/2013/03/chapter-2-states-of-matter-and.html for application of kinetic theory in changes in physical states of matter.

Diffusion

• This is the process by which different substances mix as a result of the random motion of their particles. This is the movement of particles from a region where they are in a higher concentration to region of lower concentration
• This eventually leads to the particles spreading out. to make the concentration same throughout.
• Diffusion only takes place in fluids (liquids and gases)
• The lower the molecular mass the faster the particle will diffuse. EG- Hydrogen gas has molecular mass of 2 (H2 =1*2=2) is more likely to diffuse faster than chlorine gas with molecular mass of 71  (Cl2 =35.5*2=71) since it is lighter.

Structure of Atom

• Atoms have three sub-atomic particles- proton, neutron and electron.

Sub atomic Particle	Relative Mass	Relative Charge	Location in atom
Proton	1	+1	In nucleus
Neutron	1	0	In nucleus
Electron	1/1840	-1	Outside nucleus in orbits

• Atomic number = Number of Proton
• Atomic Mass = Number of protons+ Number of neutrons=nucleons
• Number of Electrons = Number of protons = Atomic Number
• Number of neutrons = Mass number-Atomic number=nucleons-protons
• Isotopes- Atoms of the same element with different mass number. They have the same number of protons and electrons but different number of neutrons.They have the same chemical properties since they have the same number of electrons but only physical properties differ. Some isotopes maybe radioactive known as radio-isotopes.(Extremely important to remember this definition and properties of isotopes) 

Radioactivity

• This is the spontaneous decay of unstable radio-isotopes. It is unaffected by temperature or whether the isotope is part of a compound or present as the free element, it is a totally random process.
• There are three types radioactivity

1. Alpha Radiation
2. Beta Radiation
3. Gamma Radiation (no mass)

Radiation	Isotopes involved	Nature	Distance travelled in air	Penetration Paper        Thin     Thick                     Al         Lead
Alpha Radiation	Atomic Number >83	Alpha Particles Helium He42 (double positive charge, no electrons)	A few Centimeters	No	No	No
Beta Radiation	Atomic Number ≤ 83	Beta Particles (electrons single     –ve charge)	A few meters	Yes	No	No
Gamma Radiation	Only occurs with one of the other forms of radiation	Electromagnetic Radiation	Many Kilometers	Yes	Yes	No

Alpha and beta radiation knocks out electrons out of atoms (ionization) to produce positive ions.

Uses of Radioactivity (Important to learn these uses, can be asked )

Industrial

• Radioactive Dating which measures age of an object. 
• Monitors the level of filling in containers
• Checks thickness of sheets of metal or paper
• Detects leaks in gas or oil pipes

Medical

• To kill cancer cells using gamma radiation from cobalt-60
• To sterilize medical instruments, dressings and syringes by gamma radiation to kill bacteria.
• Food treatment to kill bacteria, mould and yeast by gamma radiation 

Electron arrangements in atoms

• Electrons in atoms are arranged in an organised way in different shells
• These electron shells have different electron shells are at different distances from the nucleus of the atom
• Electrons are placed in the shells closest to the nucleus first,and each shell has a maximum number of electrons it can contain. This could be calculated using the formula 2n^{2                     where n is the shell number. For example shell 1 can hold 2 electrons at maximum and shell 2 can hold maximum 8 electrons.}

• Noble gas arrangement is when the outershell is filled with maximum electrons therefore is unreactive since its already stable. These elements are known as noble gases like helium has 2 electrons in outershell and the first shell can only hold 2 electrons not more than that therefore is stable.

• The number of electrons in the outershell play a very important role in deciding the chemistry of the element.
• Atoms gain or lose electrons or share electrons to attain noble gas arrangement. Only the outershell electrons are involved in bonding.Will be discussed later. But remember this, commonly asked question why atoms do this? To attain noble gas arrangement to be stable.

Chapter 2: Separating and purifying substances

Separating heterogeneous mixtures

• Decantation- This is the process of removing a liquid from a solid (Like sand and water) which has settled (sedimented) or form an immiscible heavier liquid (like oil and water) by carefully pouring. It is important you learn this definition, asked in paper 6.
• Filtration- This is where an insoluble material is collected on filter paper- this is the residue. The liquid phase obtained is called the filtrate. It is useful since both phases can be obtained.
• Centrifugation- The separation of an insoluble solid from a liquid in a test tube by rapid spinning during which the solid collects at the bottom of the sample. The liquid then can be decanted carefully.   
• Separating Funnel- This is used to separate immiscible liquids like oil and water. The tap is opened to let the water out first. Then close tap when water is finished and change beaker and empty oil. Hence both liquids are separated.

• Magnetic Properties- Suppose we have a mixture of iron and plastic. We can hence separate iron from the plastic by using a magnet to attract all the iron pieces from the plastic.
• Solubility- If we have a mixture of salt and sand. We can take water which dissolves the salt and not the sand therefore separating the mixture.
• Sublimation-  If we have ammonium chloride and sodium chloride (Salt), ammonium chloride sublimes while salt doesn't therefore the mixture is separated.

Separation of Homogeneous Mixtures

• Evaporation and Crystallization- Used to separated solid in liquid mixtures (like salt and water) Evaporation is used if you want the powder of solid like salt. Evaporation is done by heated the solution on a evaporating dish, letting the liquid evaporate and solid is left. In crystallization the solution is concentrated by evaporating only some of the liquid in a water bath. When small crystals form on glass rod when dipped, it goes for cooling and crystals of solid is formed. The crystals are then filtered off and dried in oven or using filter paper (like tissue paper) 

     Evaporation

• Distillation- Used to obtain liquid from solution. Liquid is first evaporated and then condensed in the condenser. The liquid obtained is known as distillate.

• Fractional Distillation- This is often asked in the exam so pay attention. This is used to separate a mixture of liquids. The liquids have different boiling points hence can be separated. Suppose we have ethanol (alcohol) and water. Ethanol has a lower boiling point (78C) than water (100C) therefore gets boiled first. The water vapor keeps condenses in the fractional column since its below its boiling point.

• Chromatography- This is used to separate a mixture of solids dissolved in a liquid like coloured dyes  by differences in their solubility in a solvent. Please learn the procedure for chromatography since it is frequently asked in most of the papers it is extremely important (bold) 

1.  First draw a line on the chromatography paper (about 1cm form the bottom) with pencil. This is called the base line.
2. Place a drop of the solution on the base line.
3. Place the chromatography paper in a solvent and the solvent shouldn't cross the base line.
4. As solvent moves up the line the dyes with it start to separate into different colours.
5. If the solution is invisible like amino acids, use locating agent to locate the spots and warm the paper in the oven then spots will reveal.
6. Calculate Rf value using the formula = Distance moved by substance/ Distance moved by solvent front. The solvent front is where the solvent stops. The identity of a substance in the dye can be checked by comparing its Rf value to that of a sample we know is pure. Suppose Rf value of an amino acid is 0.31 (random) and the Rf value of a spot on the chromatography paper is 0.31 therefore the solution has an amino acid.
7. One spot means that the solution is pure.

Solubility

• Solution is made of solute (solid dissolved) and solvent ( the liquid in which the solid dissolves in).
• Solubility of solids in liquids increase with temperature. At a fixed temperature there will come a point where no more sold will dissolve in the solvent. This means that the solution is saturated. Temperature has to increase then to dissolve more of the solid.
• Solubility of gases in liquid decreases with an increase in temperature.
• Experiment to find solubility of solid in liquid (asked in p6 once so I recommend you to learn the procedure)

1. Take known mass of the solid using balance and take 100 cm3 of water using burette in beaker.
2. Heat the water ti the required temperature like 30C
3. Add the solid to the liquid and stir and keep adding till no more solid will dissolve in liquid and solution becomes saturated.
4. Filter out excess solid that didn't dissolve in liquid.
5. Then evaporate the liquid off and solid that dissolved remains.
6. Measure the mass of solid that dissolved in the liquid obtained in the evaporation.
7. Calculate solubility of solid at the fixed temperature using mass of solid dissolved (say in grams) divided by volume of liquid used (100cm3 in this example). Units will be g/100cm3 then.

Chapter 2 States of matter and substances

(Chapter 1 is not so important for IGCSE so we can skip that, it can be a little bit long since I added other necessary things to the notes which are required by the syllabus.)

States of Matter
Matter is defined as anything that occupies space and has mass.
There are three states of matter

• Solid
• Liquid
• Gas

Its really important to remember this table, the ones in bold are the key ones to remember and write in the exam to get marks.

Remember liquid and gas are known as fluids.

	Solid	Liquid	Gas
Description	Fixed volume, own shape	Fixed volume, takes shape of container	Any volume, takes shape of container
Arrangement of Particles	In a regular pattern called a lattice	Random	Random
Separation of Particles	Close together, touching	Still close together, just slightly further apart than in the solid pace	Separated, far apart.
Movement of Particles	Vibration about a fixed point	Slow movement in a random way from place to place, sliding other each other	Fast random movement
Attractive forces between particles	Stronger than in the liquid pace	Slightly weaker than the solid pace	No attractive forces between particles

All states of matter respond to a change in temperature.

• An increase in temperature will cause the particles in a solid to vibrate more in their fixed positions and take up more space causing expansion. 
• The reverse is contraction which happens when solid is cooled and therefore less energy in the particles, less vibration therefore less space taken up by the particles.
• When the temperature reaches to melting point then the particle have enough energy to move past each other and change positions and the solid changes into a liquid. This is known as melting.
• The reverse of melting is solidification where liquid is cooled and becomes solid. This happens at freezing point which is the same as melting point. Like in water  water melts or solidifies at 0.C

Melting point is the definite temperature where solid changes to a liquid.

• When a liquid is heated, the particle move faster and take up more space causing expansion. 
• When a liquid is cooled, the particles have less energy and move less faster therefore causing a contraction. 
• When the liquid is heated up to its boiling point, the particle in the liquid have enough energy to break intermolecular forces and so the particles separate and become gas or vapor. This is known as boiling. 
• Vapor is a gas which can be compressed into a liquid without cooling.
• The reverse of boiling is condensation where gas is cooled to become a liquid.

Boiling point is the definite temperature where liquid changes to a gas at a constant temperature. If the surrounding pressure decreases the boiling point also decreases.

 Evaporation though happens at all temperatures.

• For solids like dry ice (solid carbon dioxide) they don't turn into a liquid, they turn directly into a gas when heated, This is known as sublimation. 

Pressure

Only fluids (gases and liquids) are compressible, not solids. Liquids are only slightly compressible while gases can be easily compressed. 

In a sealed container the pressure of gases increases when temperature increases. This is because particles of gas will have more energy and will more faster and therefore will hit the walls of container more frequently and harder leading to an increase in pressure. 

This is a frequently asked question IGCSE, why does this happen, therefore it will be good if you understand the answer properly.

Pure Substances

A pure substance consists of only one substance and boils and melts at definite temperatures. Boiling and melting points can be checked to test the purity of a substance. Water that boils at 100C is pure while water that boils at 110C isn't pure.

Impurities in a substance cause-

• The melting point to decrease
• The boiling point to increase.

Mixtures

Mixtures are a system of two or more substances that can be separated by physical means.

It can be of two types-

• Homogeneous- This means the substances are totally mixed up and are indistinguishable like salt and water. Alcohol and water are miscible meaning they make a solution
• Heterogeneous- This means that the substances remain separate and one substance is spread throughout the other as small particle, droplets or bubbles like sand and water. Water and oil are immiscible and separate into different layers. 

 

About me and the website

Hi there students. I love to share my knowledge with you guys about chemistry and help you out in IGCSE examinations. I am also a student by the way. I love chemistry and is one of the subjects I am best at. I hope these notes I made would help you out for revision.

I am referring to IGCSE Chemistry Richard Hardwood by the way. Thanks for taking a glimpse at my work. :) Please help me to improve the site by posting comments and suggestions.
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