Chemistry checkpoint

A Solid has a definite mass, a definite shape

and its volume does not change. It does not
flow and it is hard to compress ( squash) it

A liquid has a definite mass and its volume

does not change. It is hard top compress but it
flows easily. The shape of the liquid varies and
depends on the shape of the container
holding it.

A gas has a definite mass but its volume can

vary and it takes the shap[e of the container
holding it. It flows easily and it is easy to
compress.
 -a vacuum is a scientific term for a space that
is devoid of matter. Devoid means completely
without.

-a vacuum is a space is which there is no

particles of matter. In reality, this can never
occur; some particles are always present in a
vacuum but far fewer than outside a vacuum.

Space(between the planets and the stars) is

often described as a vacuum , but it still has a
few particles in it that spread over great
distances over each other. This means it not a
perfect vacuum but a partial vacuum. ,
although it does not contain much, does
contains atoms (e.g. hydrogen, helium).

Because space is a vacuum, this means there

is no air resistance to slow objects down-
meaning if you throw a ball in space it would
theoretically travel in a straight line forever.

Air resistance eg in parachutes slows down

In Solids, strong forces hold the particles motion
together in a three-dimensional structure. In
. if you throw a stone and a feather
many solids, the particlesd form an orderly
downwards in air, the stone will reach the
arrangement called a lattice. The particles in a
ground first because the air resistance on the
solid vibrate backwards and forwards about
feather is more.
one position.
. if you throw a stone and a feather
In liquids, the forces that hold the particles
downwards in a vacuum, they reach the
are weaker than in solids. The particles have
ground at the same time since there is no air
some freedom, they move over each other.
resistance .
In gases, the forces of attraction between the
Testing common gases
particles are very small, and the particles can
move freely , hitting each other or the surface
of the container.

CHANGE OF STATE

Vacuum
Oxygen -use the burning splint

- remove the stopper from the test tube with

carbon dioxide and dip the burning end of the
splint into the test tube.

* the splint stops burning

Another test for carbon dioxide

-pick up a splint and light it

-blow off the flame on the splint but make

sure it is glowing Bubble the gas through limewater ,
limewater changes from clear to milky
-remove the stopper from the test tube with
(cloudy).
oxygen and dip the glowing splint into the test
tube

* the splint produces a flame. Oxygen relights

a glowing splint
Atoms and elements

Hydrogen

- Use a burning splint , remove stopper

from test tube with hydrogen and dip
the burning end of the splint into the
- test tube
- * hydrogen burns with a pop sound

Carbon dioxide
In an atom there is always the same number
of protons as electrons. The negative charges
on the electrons are balanced by the positive Hydrogen -1
charges on the protons, to make the atom
Helium-2
electrically neutral. If any electron is added or
removed the atom becomes an ion. Lithium-3

Atoms can be different elements depending Beryllium-4

on the number of protons.

The number of protons is the atomic number

which you can find from a Periodic Table. So Hydrogen always has 1 proton. Helium
if, say, you are interested in sodium, Na, you always has 2 protons. Lithium always has 3
will find that it has an atomic number of 11. protons. Beryllium always has 4
That means that it has 11 protons, and the protons.
neutral atom must also have 11 electrons.

Each energy level can only hold a certain

number of electrons, and the first level
(closest to the nucleus) only has room for 2
electrons. The other levels we shall be
concerned with hold increasing numbers of
electrons.

level number of electrons it can hold

1 2
2 8

3 temporarily full with 8, but can hold

18

At this 14-16 year old level, we usually only

look in detail at the arrangement of the
electrons in the first 20 elements (up to
calcium in the Periodic Table), and you
probably won't need to worry about the fact
that the third level can eventually take 18
electrons.

Instead, we often use a quick short-hand to -an element is made from one particular type
represent electronic structures. opf atom nwhich has itz own properties. Every
element is made from a different atom, hence
the difference in physical and chemical
properties properties between different
Sodium, for example, has 2 electrons in the
elements.
first level, 8 in the second, and 1 in the third.

Before 1669, the following elements had

We write this as Na 2,8,1
already been discovered: carbon, sulphur,
iron, copper, arsenic, silver ,tin,antimony,
gold, mercury and lead

Date element discription

Elements of the periodic table discovere
d
- Elements were arranged based on 1669 phosphorou -white, red or
mass s black solid
- The number at the top is the atomic 1766 hydrogen Colourless gas
number ( proton number which is also 1771- oxygen Colourless gas
equal to the number of electrons) 1774
1772 nitrogen Colourless gas one or more elements joined together eg
1774 chlorine -green-yellow gas carbon dioxide, which is made from one atom
1807 sodium Silver –white of carbon and two atoms of oxygen.
solid
1808 Magnesium Silver –white Wax in candle is made from carbon and
solid hydrogen
1808 calcium Silver –white
solid -
1811 Iodine Grey-black solid
1825- Aluminium Silver –white
1827 solid
1868 Helium Colourless gas
1886 flourine -green-yellow gas

Chemical properties of elements

- A chemical property is one that is

measurable once the substance
undergoes a chemical change..
- Some elements produce a color eg in
a fireworks display

Physical properties of elements

A Physical properties of elements is one which

can be observed without altering the element
through any form of chemical reaction such as
whether it is a solid , liquid or gas, whethrt it
is shiny or dull.

Mercury and bromine are liquids at room

temperature and standard pressure.

- Each elemnt has its own special

propertoies eg Sodium is soft, silvery
–white metal with melting point 98
degree celcius and bp 884.
- Chlorine is a yellow-green gas with
mp -101 and bp -34

Substances and elements

A substance is any form of matter such as a

gas, liquid or solid. It may be made of the
atoms of just one element, like pure gold, or
Mixing elements and making a Compound
from Iron and Sulfur.

A mixture occurs when you combine matter

so that the components can be separated
again. A compound results from a chemical
reaction between components, forming a new
substance. For example, you can combine iron
filings with sulfur to form a mixture. All it
takes is a magnet to separate the iron (silvery
grey and magnetic) from the sulfur ( yellow).

On the other hand, if you heat the iron and

sulfur, you form iron sulfide, which is a
compound; the iron and sulfur can no longer
be separate from one another.
What You Need

Iron filings

Sulfur (powder or flowers of sulfur)

Magnet

Test tube or beaker

Burner or hot plate or stove

Creating a Mixture and Then a Compound

First form a mixture. Stir some iron filings and

sulfur together to form a powder. You have
just taken two elements and combined them
to form a mixture. You can separate the
components of the mixture by stirring the
powder with a magnet; the iron filings will
stick to the magnet while the sulfur will not.
Another (less messy) option is to swirl the
powder with the magnet under the container;
the iron will fall toward the magnet at the
bottom.

If you heat the mixture over a bunsen burner,

hot plate, or stove, the mixture will start to
glow. The elements will react and will form
iron sulphide (grey black), which is a
compound. Unlike the mixture, the formation
of a compound can't be undone so easily. Use
glassware that you don't mind ruining.

When you form a mixture, you can add

components in any ratio that you want. It
doesn't matter if there is more iron than
sulfur, for example.

When you form a compound, the components

react according to a set formula. If there is an
excess of one or the other, it will remain after ALLOYs
the reaction that forms the compound.
A metal alloy is a mixture of more than one
metal or mixture of a metal with other non-
metallic elements.
 Solutions

- Is a mixture
- A solution is made when a
subsdtance, called a solute, mixes
with a liquid called a solvent, in such a
way that the solute can no longer be
seen.
- This type of mixing is called dissolving.
- Solute is a substance that is dissolved
in a solution
- A solvent is any substance, usually
liquid, which is capable of dissolving
one or several substances, thus
creating a solution. Eg water

The reason that alloys are useful is the

PHYSCAL PROPERTIES OF
arrangement of their particles., particles of MATTER
the second elements disruprt the organised -Scientists divided elements into two groups-
layers and preventing the particles from matals and non-metals
sliding over each other.
 rigid –cannot be bent

Rigid

Rock has been used as building material

because it is rigid.

magnetism
CHEMICAL REACTIONS
-a chemical reaction can be modelled in two
dimensions by using two coloured circles to
represent the atoms.

- hydrogen can be represented by a white

circle and an oxygen atom by a shaded circle.

-Both oxygen and hydrogen occur natuarally

in pairs.

Oxygen atom
Hydrogen atom

+
.

Achemical reaction can be modelled in 3

dimensions using balls.
When a candle burns - substances formed during a reaction the
products
Physical changes
Physical and Chemical Changes Class 7 Extra
Solid wax is converted into liquid wax, which Questions Science Chapter 6
changes into a gas called wax vapour.
June 10, 2019 by Sastry CBSE
The chemical changes

-hot wax vapour mixes with oxygen , a

chemical reaction takes place, water and Physical and Chemical Changes Class 7 Extra
carbon dioxide are formed. Questions Science Chapter 6

-carbon dioxide is gtested using calcium Physical and Chemical Changes Class 7 Science
hydroxide ( limewater) , limewater turns milky Extra Questions Very Short Answer Type
from clear, Questions

-water is tested using cobalt chloride paper , Question 1.

the paper changes from blue to pink in the
presence of water, What kind of change is shown by tearing of
paper? [NCERT Exemplar]

Answer:
Preciptates
Tearing of paper is a physical change
- A precipitate is a solid that forms in although, it cannot be reversed.
aliquid.
- A precipitate may form when a
chemical reaction takes place
between two soluble compounds in a
solution. Question 2.

Eg carbon dioxide reacts ( dissolves) in Melting of wax is a change where a solid

limewater (calcium Hydroxide) to form the changes to liquid state. Give one more such
compound calcium carbonate which is change which you observe in your
insoluble in water. The precipitate makes the surroundings. [NCERT Exemplar; HOTS]
limewater cloudy.
Answer:
Signs of a chemical reaction
Melting of ice is also a change where solid
-formation of a precipitate changes into liquid state.

-formation of a gas

-substance changing colour Question 3.

-reduction of the substances taking part in the Name the gas which turns lime water milky.
reaction ( reactants)
Answer:
Carbon dioxide gas (CO2) turns lime water Ripening of a fruit is a chemical change
milky. because after ripening, a new product with
different properties is formed.

Question 8.
Question 4.
Is souring of milk a physical change or a
Give example of a physical change which chemical change? Why?
occurs by the action of heat.
Answer:
Answer:
Souring of milk is a chemical change because
Melting of ice to form water is a physical original substances present in milk lose their
change which occurs by the action of heat. nature and identity and form new chemical
substances.

Question 5.

Write the colour of copper sulphate solution Question 9.

obtained when iron nails are dipped in it? Complete the following reaction
Answer: Ca (OH)2 + CO2 → [HOTS]
When iron nails are dipped in copper sulphate Answer:
solution, then the colour of the solution
changes to green. Physical and Chemical Changes Class 7 Extra
Questions Science Chapter 6 1

Question 6.
Question 10.
What colour of flame is observed when
magnesium ribbon burnt in air. What is the nature of magnesium oxide
solution?
Answer:
Answer:
When magnesium is burnt in air then a
brilliant white flame is obtained. Magnesium oxide is basic in nature because it
turns red litmus solution to blue.

Question 7.
Question 11.
How can you say that ripening of a fruit is a
chemical change? [HOTS] Name the process by which common salt is
obtained from sea water.
Answer:
Answer: We should eat freshly cut apple because if we
leave the apple after cutting, it starts turn to
The common salt can be obtained by the brownish due to the oxidation of the essential
evaporation of sea water. nutrients present in it and its food value
decreases.

Question 12.

Name the metal which is used for galvanising Question 16.

iron. Write word equations for two chemical
Answer: reactions with the help of materials given in
the box.
Zinc metal is used for galvanising iron.

Air, copper sulphate, iron, vinegar, iron oxide,

Question 13. carbon, dioxide, iron sulphate, copper, lime
water, water
Name the metals which are mixed (alloyed)
with iron to make stainless steel. Answer:

Answer: (i) Iron + air + water → iron oxide

Metals like chromium and nickel are mixed (ii) Copper sulphate + iron → iron sulphate +
(alloyed) with iron to make stainless steel. copper

Physical and Chemical Changes Class 7 Science

Extra Questions Short Answer Type Questions
Question 14.
Question 1.
Suggest two methods to prevent rusting.
Classify the following processes into physical
Answer:
or chemical changes. [NCERT Exemplar]
The two methods to prevent rusting are
(a) Beating of aluminium metal to make
aluminium foil

Painting the iron articles. (b) Digestion of food

Greasing or oiling the iron articles. (c) Cutting of a log of wood into pieces

Question 15. (d) Burning of crackers

We should eat freshly cut apple. Why? Answer:

Answer: Physical changes are beating of aluminium

metal to make aluminium foil and cutting of a
log of wood into pieces.
Chemical changes are digestion of food and between sodium hydrogen carbonate and
burning of crackers. acetic acid to form three new substances.

The change in the test tube is as follows:

Physical and Chemical Changes Class 7 Extra

Questions Science Chapter 6 4
Question 2.
Physical and Chemical Changes Class 7 Extra
Explain the following. Questions Science Chapter 6 5
(a) Lime water turns milky on passing carbon
dioxide gas through it.

(b) Bubbles are produced when acetic acid is

added to a solution of sodium hydrogen Question 3.
carbonate. [NCERT Exemplar]
Is cloud formation a physical change or
Answer: chemical change? Explain.

(a) Carbon dioxide gas produced in the Answer:

reaction passing through freshly prepared
lime water as shown in figure. Formation of clouds is a physical change.
Clouds are formed by the condensation of
Physical and Chemical Changes Class 7 Extra water vapours present in the atmosphere.
Questions Science Chapter 6 2 When rainwater goes back on the earth, no
new product is formed. Therefore, it is a
Lime water is calcium hydroxide solution. physical change.
When carbon dioxide gas is passed through
lime water, then calcium hydroxide combines
with carbon dioxide to form a white solid
substance, calcium carbonate which makes Question 4.
lime water milky. This chemical change can be Write the differences between physical and
written in the form of word equation as chemical changes.
follows:
Answer:
Physical and Chemical Changes Class 7 Extra
Questions Science Chapter 6 3 Differences between physical and chemical
changes are
The reaction between lime water and carbon
dioxide gas is a chemical change because a
new substance calcium carbonate is formed
Physical change Chemical
during this change. The turning of lime water
change
into milky is a standard test of carbon dioxide.
No new substance is formed. New
(b) When baking soda (sodium hydrogen
substance is formed.
carbonate) and vinegar (acetic acid) are mixed
together, then a chemical change takes place It is a temporary change. It is a
permanent change.
Physical change is easily reversible.
Chemical change is irreversible.

Very little energy (heat, etc) is absorbed or

given out in a physical change. A lot of
energy (in the form of heat, light, sound etc) is
absorbed or given out in a chemical change.

ACIDS AND ALKALIS

ATOMS
-Dalton discovered an atom , matter is made
of small tiny particles

-Thompson devised a model of the structure

of an atom, he described the atom as being
like a plum pudding , with the negatively
charged electrons being surrounded by a
positively charged “pudding”.

The4 electrostatic charge inside an atom

- The nucleus has a positive charge and

- THE MODEL SHOWS THAT the atom is an electron a negative charge, the
neutral as +ve protons must be ba;lanced with difference in charge creates an
the negative electrons electrostatic force ,
No mentioning of the netrons on the model. - an attraction force between the
proton s and the electrons , that
keeps each atom together.
 MIXTURES AND IMPURITIES.
PURE AND IMPURE SUBSATANCES

- if a substance is made up of one kind

of element,or from just one kind of
-
comppund , then it is a pure
substance
- some substances contain other
elements or compounds called
impurities. These substances are said
to be impure.
- A pure substance has a definite
melting point and boiling point.
- Use intenet to find mp and bp of
- a) vinegar
- b) iodine
- c) sodium chloride (salt)
- d) pure gold

Causes of impurities

- the temperature of the reactants may

be too low for all of them to
completely react , eg copper does not
react with sulphuric acid at room
temperature.
- A reactant may come to the reaction
with impurities already present
- sometimes reactants are present in
such large quantities that not all of
the reactant is used up in the
reactaion

expt: is all acid used up in a chemical

reaction?

Apparatus

Test tube rack, boiling bube with10 cm of

hydrochloric acid, universal
indicator,dropper, measuring cylinder.,
beaker with dil sodium hydroxide

Method

1. set up a boiling test tube in the testb

tube rack
 2.add three drops of indicator solution Solubility

3. measure 5cm of sodium hydroxide usin -solubility is the ability of a substance to

a measuring cylinder dissolve in a particular solvent.

4. pour the sodium hydroxide into the - in scientific terms, it is the maximum amount
boiling acid and make note of the colour of a substance that will dissolve in a certain
changes amount of solvent at a certain temperature.

5. reapeat steps 3 and 4 until green 23.1 means 23.1 g in 100ml

coluour is green (neiutralisation has
occurred) substance 00C 1000C
Copper 23.1 114
Conclusion sulphate
Magnesium 52.9 73.3
Draw a conclusion chloride
Potassium 105 156
What improvements could be made? carbonate
Silver nitrate 122 733
Zinc chloride 342 614

Calculating purity

%purity =
Why solubility increases with temperature

- when temp rises:

Dissolving and concentration .energy of the particles rises
.the solid particles vibrate more and
- the liquid in which the solid dissolves solvent particles move faster in all
is called the solvent directions
- the solid that dissolves is called the - the increased movement of particles
solute causes the solid particles to separate
- the particle theory says: ande spread out between the solvent
particles.
there are gaps between the particles in a
liquid , when a substance dissolves , its Chromatography
particles spread out and fill the gaps.
Is a way to check the number of solutes in a
solvent

Concentration is the amount of solid

substances that are dissolved in a volume of a
solvent

-high concentration is when a large number of

solid particles dissolve in a volume of solvent(
the solution is darker)

diag
Stage 1

- a pencil line is drawn across a strip of

chromatography paper near one end.
- Why pencil not ink- pencil does not
dissolve in the solvent
- A drop of ink is placed on the line

Stage 2

- A solvent is placed in the beaker.

- It should reach a depth which is below
the level of the pencil mark
- The solutes in the sample dissolve in
the solvent and move up thje paper
with it.
- Each solute moves at a different
speed to the others, this is caused by
the difference in solubility of the
solutes. The most soluble move
furthest upthe paper , so the solutes
spread out up the paper.

Stage 3

- When the solutes has risen almost to

the top of the paper, it can be
roemoved and dried, and the
separated solutes can be observed.
- Each solute will move up the paper to
the same position, no matter which
sample itb is in, if the same solvent is
used.
- Thius means two samples can be
tested and compared.
- Chromatography is used to test for
the presence of chemicals in the
environment , to check if a drug is
pure.

REACTIVITY SERIES Metal + oxygen =

Metal + water =

Metal + acid =
EXOTHERMIC
What is the Rate of The
Reaction?
 The speed of the reaction
 Reactions take place when
particles collide with a certain
amount of energy
 For a reaction to take place
between two reactant particles,
there are three conditions that
are necessary
o The two-particle must
Burning -SOME fuels are burned to provide collide with each other
haet and light, eg o They must collide in the
correct orientation
-th heaT is used to warmbuildings, cook  The reactive parts
meals, to turn water into steam to drive of each particle will
generators in power stations. come into contact
with each other
Rusting- it is also an exothermic reaction
o The reactants must
although it is very slow
collide with sufficient
Is respiration an exothermic reaction., kinetic energy to bring
reseasrch about the reaction
 The rate of reaction depends on
Expt two things:
o The frequency of collision
between particles
o The energy with which
particles collide
 If the particles collide with less
energy than the activation
energy, they will not react.
Factors Affect The Rate of
Reaction
Sherbet- when you put sherbet in your mouth
Temperature
, it feels cool.  The higher the temperature, the
faster the reaction
This is b/c heat is taken from the body whe  An increase in temperature
sherbt reacts with chemicals in saliva. causes a rise in the energy level
of the molecules involved in the
cooking
 reaction, so the rate of the
reaction will increase.
Concentration
 The higher the concentration at a
dissolved reactant, the faster the
reaction
 Increasing the concentration of
one or more reactants will
increase the reaction rate. This
occurs because a higher
concentration of a reactant will
lead to more collisions of that
reactant in a specific time period.
Particle Size
 The surface area increases when
the solid is split into several
pieces.
 The smaller the pieces, the
larger the surface area, the
greater the chance of reaction
Catalyst
 A catalyst speeds up a chemical
reaction, without being
consumed by the reaction.
 It increases the reaction rate by
lowering the activation energy
(minimum amount of energy
needed for the particles to react)
for a reaction.