Science Assessment 2 2024 Revision - Chemistry

Revision from the "Don't Over React" topic.

compound

when two or more elements are chemically combined.

element

when two of the same atoms join together.

molecules

when two DIFFERENT atoms join together.

why do elements join?

to become stable + achieve full outer shell.

ionic compounds

the transferring of electrons between a chemical compound.

what type of substances are ionic compounds between?

metals and nonmetals

covalent compounds

the sharing of electrons between a chemical compound.

what type of substances are covalent compounds between?

non-metals

polyatomic elements

when a lot of atoms join (e.g. sulphate, hydroxide, carbonate, phosphate etc.)

pH scale

ranges from 1-14, with 7 being in the middle/neutral. as you go down, the acidity increases.

base

anything with OH (hydroxide) ions.

indicators used to test the acidity or alkalinity of a substance:

• litmus paper

• universal indicator

• using a pH probe

characteristics of acids:

• taste sour

• need to be dissolved in water in order to act as an acid

• have a pH value less than 7

• are very soluble in water

• changes the colour of indicators

• contain hydrogen atoms that form H+ ions when acid is dissolved in water.

characteristics of bases

• tastes bitter

• pH ranges from 8-14

• soapy to touch

• releases hydroxide ions

• conducts electricity

• changes the colour of indicators

physical changes

change in the physical structure only, not chemically. no new product/substance has been formed. most of the physical changes can be reversed (e.g. the states of matter with water).

chemical change

change in the chemical composition/structure. generally irreversible and forms new product(s) (e.g. burning paper which turns into ash).

law of conservation of mass

mass cannot be created or destroyed, only transferred. Mass of reactants also needs to equal to mass of products.

exothermic reaction

a reaction or process that gives off heat.

endothermic reaction

a reaction or process that takes in thermal energy from its environment to either make or break the chemical bonds between atoms.

activation energy

minimum energy required to start a chemical reaction.

decomposition reaction

anything breaking/decomposing (corrosion, rusting)
equation is below:

AB —> A + B

combination reaction

anything combining
A + B —> AB (e.g. H2 + O2 = 2H2O)

H2SO4

sulphuric acid

HNO3

nitric acid

HCl

hydrochloric acid

CH3COOH

acetic acid

NaOH

sodium hydroxide

NH4OH

ammonium hydroxide

acid + metal reaction (or redox reaction but you don’t need to know that)

a salt + hydrogen

acid + base reaction (neutralisation reaction)

a salt + water

carbonates are normally what?

normally metals

acid + metal carbonates reaction

carbon dioxide + a salt + water

combustion reaction

anything being burned basically (but it will have oxygen and will produce carbon dioxide and water) so it’s fuel + O2 —> CO2 + H2O

test for hydrogen and explain

pop test - where if fire is put over the top of a test tube and it produces a distinctive ‘pop’ sound, that means that hydrogen is present.

test for carbon dioxide and explain

lime water test which involves air being funnelled into lime water and if the lime water turns into a milky white colour, then the gas is carbon dioxide.

indicator that there is hydrogen present?

when there is a distinctive “pop” sound

indicator that there is carbon dioxide present?

when the lime water turns into a milky white colour

precipitation reaction

when two clear solutions are mixed together, the formation of a solid is called a precipitation reaction. the formula is aqueous + aqueous —> solid (or precipitate) + aqueous

rate of reaction

the time taken for a reaction to occur

factors affecting rate of reaction

• temperature/heat

• surface area

• catalysts

• concentration of substance