science std 7

Chapter 14Elements,compounds & Mixtures

matter Elements compounds

picking

sieving

sifting

churning

filteration

All the objects or materials in the world are made from elements,compounds or mixtures

Metals gold -Au silver Ag , iron Fe, copper Cu, sodium Na, platinium Pt

chemical Physical chemical

physical chemical chermical chemical

harmful

chemical,useful

Heat the glass to 600 + Celsius and mold it

Answer. Just heat the wax of original candle and pour it into any container and yes don't forget to put a thread into it so you can burn it later also and in this way you can use it

Mixing substances can cause an irreversible change. For example, when vinegar and bicarbonate of soda are mixed, the mixture changes and lots of bubbles of carbon dioxide are made. Burning is an example of an irreversible change.

slow changes -curdling of milk,rusting of iron,growth of nails and hair fast changes- bursting of cracker ,burning of paper, melting of ice

Ans 1.It is temporary change. Because summer,winter and rainy season come again and again Ans 2.As the clock's hands move throughout the day from 6.00 am to 6.00 pm, the minute and hour hands are pointing in the same direction 11 times

Ans1 ,Changes of state are physical changes. They occur when matter absorbs or loses energy. Processes in which matter changes between liquid and solid states are freezing and melting. Processes in which matter changes between liquid and gaseous states are vaporization, evaporation, and condensation.

Answer: Physical Change making table from wood breaking glass object Chemical Change burning wood ripening tomato rusting

slow changes -curdling of milk,rusting of iron,growth of nails and hair fast changes- bursting of cracker ,burning of paper, melting of ice

Question 1: Distinguish between the following. (a) Physical change and chemical change (b) Periodic change and non-periodic change (c) Natural change and man-made change ANSWER: a. Physical Change Chemical Change The chemical composition of a substance does not change The chemical composition of a substance does not remain same Easily reversible Not easily reversible No new substance is formed New substances are always formed Melting of ice is an example of physical change Burning of paper is an example of chemical change b. Periodic changes Non periodic changes 1. Events that repeat themselves after a fixed period of time and whose ocurence can be predicted are called periodic changes. The events that do not repeat themselves after regular interval of time are called non periodic changes. 2. Example- rotation of earth, revolution of earth, day and night, seasons etc. Example- natural disasters like earthquake, cyclones etc. c. Natural changes Man made changes 1. The changes which are brought about by nature itselfand are not under the control of humans are called natural changes. The changes which are brought about by humans are called man made changes. 2. Examples- change of season, tides in sea, ageing of plants etc. Examples- burning of fuels, cutting hair, switching on lights etc.

Question 2: Under which different types can the following changes be classified? (a) Conversion of milk into yoghurt. (b) Bursting of a cracker. (c) Occurence of an earthquake. (d) Revolution of the earth around the sun. (e) Stretching of a spring. ANSWER: (a) Conversion of milk into yoghurt- Chemical change (b) Bursting of a cracker- Chemical change (c) Occurence of an earthquake- Natural change (d) Revolution of the earth around the sun- Periodic change (e) Stretching of a spring- Physical change

Question 3: Give reasons. (a) While purchasing canned food, its expiry date should be checked. (b) An iron article should be given a coat of paint (c) A wooden article should be polished. (d) Copper and brass utensils should be tinned. (e) A dry handkerchief gets wet at once on dipping in water, but it takes long for a wet handkerchief to dry. ANSWER: a. While purchasing canned food their expiry date should be checked because an expiry date indicates that for how long a canned food is safe for consumption. Canned foods with expired dates are not considered worth consumption as they could affect your health. They might get contaminated with microorganisms like bacteria or may have no nutritional value left after its expiration . b. Iron articles often undergo rusting resulting in the formation of a reddish brown layer on its surface. Rusting is caused by the exposure of iron to the air and water in the surroundings. An iron article which is coated with a paint prevents the interaction of iron with the surrounding air and water and thus protects it from rusting. c. A wooden article should be polished because it protects the wooden article from sunlight and all other external conditions and also it reduces the probability of wooden mites damaging the furniture. d. Copper and brass materials also undergo corrosion resulting in the formation of a greenish layer on this surface. When the surface of copper/ brass comes in contact with air or water, it results in corrosion and to prevent this, the process of tining is used. In this process copper and brass articles are coated with tin . e. A dry handkercheif gets wet at once because it quickly absorbs the water in which it is dipped. However, it takes longer for a wet handkerchief to dry because the water needs to evapourate from the handkerchief. Evaporation is a slow process which is dependant on other factors as well like relative humidity.

Question 5: Read the paragraph and note down the various types of changes it mentions. It was nearing six o'clock in the evening. The sun was setting. A breeze was blowing. Leaves on the tree were shaking. Sahil was sitting in the courtyard, rolling balls of wet soil and shaping them into various toys. Then he felt hungry and went into the house. Mother made a dough from wheat flour and fried purees. While eating hot purees, his attention was drawn outside the window. It had started raining. There was lightning, too. Sahil was enjoying his dinner in the dim light. ANSWER: It was nearing six o'clock in the evening (Periodic change). The sun was setting (Periodic change). A breeze was blowing (Natural change). Leaves on the tree were shaking (Physical change). Sahil was sitting in the courtyard, rolling balls of wet soil and shaping them into various toys (Physical change). Then he felt hungry (Chemical change) and went into the house. Mother made a dough from wheat flour and fried purees (Irreversible change). While eating hot purees, his attention was drawn outside the window. It had started raining (Natural change). There was lightning (Natural change), too. Sahil was enjoying his dinner in the dim light.

Question 4: What will you take into account while identifying the following? (a) A physical change in a substance. (b) A chemical change in a substance. ANSWER: a. Physical changes are those changes in which only the forms of substances get modified; the chemical natures and compositions of the substances involved are not altered. Characteristics: No new substance is formed during a physical change. Most physical changes can be reversed easily. The chemical composition of a substance undergoing physical change remains the same. b. Chemical changes are changes that involve reaction of substances with one another. Such reactions result in alterations in the chemical compositions of the substances involved. These changes lead to the formation of new substances. Characteristics: One or more new substances are formed during a chemical change. Most chemical changes cannot be reversed easily. The chemical composition of a substance undergoing chemical change does not remain the same. A chemical change is always accompanied by a change in energy.

Non metals -carbon C, sulphur S,Phosphorus P Physical properties 1.physical state-At ordinary temperature nonmetals occur as solid, liquid and gases . solids-C,S,P liquids- Br Gases- hydrogen , nitrogen, oxygen 2.Lustre - do not lustre exception Diamond and iodine crystals some non metals are colourless some have different colours sulphur is yellow bromine is brown 3.Brittleness- solid non metals are brittle , some non metals are soft except diamond 4. Ductility and malleability- non metals are neither malleable or ductile . 5. Conduction of heat and electricity - bad conductors of heat and electicity exception Graphite 6. Density- low density 7.Melting and boiling point - non metals have low melting and boiling point exception carbon and boron melt at high temperature.

Chemical properties of metals

1.Electronic configuration- metals have upto 3 electrons in outermost orbit 2. Formation of ions- metals have tendency to lose their valence electrons to form positively charged ions- cations 3. reaction with oxygen - metals combine with oxygen to form their oxides

Physical properties of metals

1. physical state -under ordinary temperature metals are in solid state with the exception of mercury and gallium in liquid state. 2.Lustre - it reflects light 3. Hardness metals are hard exception sodium and potasssium which are soft and can be cut. 4. Ductility- metals drawn into wires 5. Malleability -- metals if hammered can be beaten into thin sheets. 6. conduction of heat- good conductors of heat . Ag, Cu, Al are best conductor of heat 7.Conduction of Electricity- good conductors of electricity. lead is exception. 8. Density -Metals have high density ,sodium , potassium and lithium are exception 9. Melting point and boiling point - metals have high MP and BP exceptions Hg,Ga,Na,K . 10.Sonority- metals produce sound on striking

Metalloids - some elements like arsenic -As, silicon -Si, Germanium- Ge, Antimony -Sb have properties which are intermediate between metals and non metals

copper electric wires oxygen respiration gold ornaments sulphur fireworks

Rhodium plating

Merits of Mendeleevs Periodic Table

• Atomic masses of some elements were revised and given proper place in the periodic table according to their properties.example Be 14.09 was changed to correct value 9.4 and beryllium was placed before boron

Merits of Mendeleevs Periodic table

Merits of Mendeleevs Periodic table

Merits of Mendeleevs Periodic table

Merits of Mendeleevs Periodic Table

When noble gases such as helium, neon, and argon were discoverd at the end of 19 century ,mendeleev had created a zero group without disturbing original periodic table

Demerits of Mendeleevs Periodic Table

1. Atomic mass of the elements Cobalt and Nickel is the same .
2. Isotopes were discovered long time after mendeleevs periodic table and it was a challenge to place them in his table.
3. When elements are arranged in an increasing order of atomic masses,the rise in atomic mass does not appear to be uniform so it was not possible to predict how many elements could be discovered between two heavy elements.

Demerits of Mendeleevs Periodic Table

Position of hydrogen