Mixtures, elements, compounds

1. Mixtures, elements, compounds • Scientists like to classify things. • One way that scientists classify matter is by its composition. • Ultimately, all matter can be classified as mixtures, elements and compounds.

2. Why isn’t it a good idea to classify matter by its phases? • Because one kind of substance can exist in more than one phase – such as H20. And matter changes phases rather easily.

3. Why isn’t matter classified according to its physical characteristics, such as color? • Scientists wouldn’t find it very useful to group gold, sunflowers, and the sun together.

4. Scientists ask themselves these questions? • Is the matter uniform throughout? • Can it be separated by physical means? • Can it be separated by chemical means?

5. By asking these questions scientists can classify matter into: • Mixtures – two or more substances that are not chemically combined with each other and can be separated by physical means. The substances in a mixture retain their individual properties. • Solutions – a special kind of mixture where one substance dissolves in another. • Elements – simplest form of pure substance. They cannot be broken into anything else by physical or chemical means. • Compounds – pure substances that are the unions of two or more elements. They can be broken into simpler substances by chemical means.

7. Is it uniform throughout? • If the answer is no, the matter is a heterogeneous mixture. • Considered the “least mixed.” • Does not appear to be the same throughout. • Particles are large enough to be seen and to be separated from the mixture.

8. Examples of heterogeneous mixtures • Sand and pebbles • Oil and water • Powdered iron and powdered sulfur Granite is a heterogeneous mixture

9. Homogeneous Mixtures • A mixture that appears to be the same throughout. • It is “well mixed.” • The particles that make up the mixture are very small and not easily recognizable.

10. Examples of homogeneous mixtures Milk, toothpaste, and mayonnaise are homogeneous mixtures. They are also colloids.

11. Colloids • In a colloid the particles are mixed together but not dissolved. • The particles are relatively large and are kept permanently suspended.

12. Colloids • A colloid will not separate upon standing. • The particles are constantly colliding, and this allows a colloid to scatter light – thus colloids often seem cloudy.

14. Solutions • A solution is a type of homogeneous mixture formed when one substance dissolves in another. • It is the best mixed of all mixtures. • A solution always has a substance that is dissolved and a substance that does the dissolving. • The substance that is dissolved is the solute and the substance that does the dissolving is the solvent.

15. Ocean water is a solution The universal solvent: Water

16. Water as a solvent • Many liquid solutions contain water as the solvent. • Ocean water is basically a water solution that contains many salts. • Body fluids are also water solutions.

17. Types of solutions Solute Solvent Example Metals dissolved in metals are called alloys.

18. Air is a solution of oxygen and other gases dissolved in nitrogen

19. Alloys Stainless steel is a mixture of iron and chromium. Brass is an alloy of copper and zinc.

20. Can it be separated by physical means? • If the answer is no, the matter is a pure substance. • An element • Or a compound

21. Elements An element is a pure substance which is made up of atoms of same kind • Elements are the simplest pure substance. • An element can not be changed into a simpler substance by heating or any chemical process. • The smallest particle of an element that has the properties of that element is called an atom. • An atom is the basic building block of matter. • There are more than one hundred known elements in the universe listed on the periodic table of elements. • These elements combine in such a way to create millions of compounds.

22. Elements • All elements are made of atoms. • Atoms of the same element are alike. • Atoms of different elements are different.

23. Elements • In 1813, a system of representing elements with symbols was introduced. • Each symbol consists of one or two letters. • Two letters are needed for a chemical symbol when the first letter of that element’s name has already been used.

24. Common Elements

25. Compounds • Compounds are also pure substances which are made from molecules. • Water is a compound. • Water can be broken down into simpler substances – hydrogen and oxygen. A compound is a substance formed when two or more chemical atoms are chemically bonded together.

27. Folding of Paper Tearing of Paper Melting of Ice , Wax Freezing of Water Boiling of Water Change of water into vapour (Vaporisation) Change of vapour into water (Condensation) Stretching of a rubber band

28. Reaction between vinegar and Baking Soda Burning of magnesium ribbon Rusting of Iron

29. Characteristics of Physical change: Only physical properties of substance change. No new substance is formed in this change. Most of the Physical changes are reversible. Characteristics of Chemical change: Chemical properties of substance change. New substance is formed. Some of the Chemical changes are irreversible. Chemical change cannot be reversed by simple physical means. Signs of Chemical Change: Evolution of heat. Production of sound. Change in colour. Change in smell. Evolution of gas.

30. In the setting of curd, milk changes into new substance, curd. This is an irreversible process that means milk cannot be made from curd. Formation of new substance and irreversible process shows that setting of curd is a chemical change. Iron get rusted because of reaction with oxygen present in moist air. By painting iron gate, iron is prevented to come into contact with oxygen present in moist air, which prevent it be getting rusted.  Anaerobic bacteria digest animal waste and produce biogas (Change – A). The biogas is then burnt as fuel (Change – B). The following statements pertain to these changes. Choose the correct one. (i) Process – A is a chemical change.(ii) Process – B is a chemical change.(iii) Both processes A and B are chemical changes.(iv) None of these processes is a chemical change.

31. Classify the changes involved in the following processes as physical or chemical changes: (a) Photosynthesis(b) Dissolving sugar in water(c) Burning of coal(d) Melting of wax(e) Beating aluminium to make aluminium foil(f ) Digestion of food True Or False

32. ATOM All matter is made up of smallest particles called atom. Ex- Hydrogen atom, Oxygen atom, Nitrogen atom When two or more atoms combine together chemically is called molecules Diatomic molecules are made of two atoms of the same element. Ex- Hydrogen – H2, Oxygen – O2 PURE SUBSTANCES Sample of the Element Lead Sample of the Element Chlorine Materials made up of same kind of atoms or molecules are called Pure Substances. Pure Substances is classified into ELEMENT or COMPOUND ELEMENTS Pure Substances which is made up of same types of atoms are called Element. It Cannot be broken down by physical and chemical methods Examples: Oxygen, Nitrogen, Carbon

33. COMPOUNDS -Compound are substances made of two or more different kinds of elements chemically combined in a specific ratio. H2O – ratio of Hydrogen is 2:1 Oxygen Compound is made up of same types of molecules Compound Can be separated only by chemical methods Properties of a compound are totally different than the properties of the elements that form them Examples: Water, Carbon dioxide, Sugar . A formula is just chemical shorthand for the compound. The subscript lets us know how many atoms are present. The coefficient lets us know how many molecules or moles are present.

35. CHEMICAL FORMULA indicates the types & number of atoms (composition) of each kind present in that substance. IN CHEMICAL FORMULA , the symbols of all the elements and the number of atoms of each element is written as a subscript to the right side of its symbol. The chemical formula of water i.(2 atom of hydrogen + 1 atom of oxygen) Similarly, the formula of carbon dioxide. (1 atom of carbon + 2 atom of oxygen) CATIONS :- Positively Charged Atoms i.e. Na+ ANIONS :- Negatively Charged Atoms i.e. Cl-

36. Writing chemical Formulas of Steps 1- • 1st word = CATION Symbol (positive ion) • 2nd word = ANION Symbol (negative ion) Steps 2- Their charge is written on the top right corner of symbol Steps 3- Remove common factor in charge (if any present) Steps 4- Charges of two ions are criss-crossed Steps 5- Criss-crossed numbers are now written at bottom right (as subscript) of symbols of ions.

37. A chemical equation is a short, easy way to show a chemical reaction, using symbols instead of words. • Although chemical equations are shorter than sentences, they contain more information. • Chemical equations use chemical formulas and other symbols instead of words to summarize a reaction. CHEMICAL EQUATION are the representation of a chemical reaction using symbols and formulae of substances involved in the reaction .

38. Chemical Reactions • Chemical Reactions form new substances by breaking and making chemical bonds • Chemical reactions change the way the atoms are arranged • Reactants: present at the beginning of the reaction • Products: are the substances formed by the chemical reaction • Example burning natural gas • CH4 + 2O2 CO2 +2H2O • Reactants  Products

39. Physical A change in the state Solid to liquid Liquid to gas Gas to liquid Atoms/molecules of the substances do not change Chemical Atoms/molecules broken down or combined to form new substances Properties of new substances are different than properties of the substances that make them up Difference between Physical and Chemical Changes

40. Evidence of Chemical Reactions • Color Change • Formation of Precipitate (solid formed from two liquids) • Formation of gas • Temperature change: endothermic (temp down) exothermic (temp up) • Change in smell

41. Use Coefficients to Balance Atoms • To balance the equation, use coefficients. • A coefficientis a number placed in front of a chemical formula in an equation. • It tells you how many atoms or molecules of a reactant or a product take part in the reaction. • If the coefficient is 1, you don’t need to write it.

42. Balancing a Chemical Equation

43. Balancing • Balancing the oxygen atoms throws off the hydrogen atoms. • There are now two hydrogen atoms in the reactants and four in the product. • How can you balance the hydrogen? • Try doubling the number of hydrogen atoms on the left side of the equation by writing the coefficient 2 for hydrogen.

45. Look Back and Check • The equation is balanced. • It tells you that two molecules of hydrogen react with one molecule of oxygen to yield two molecules of water. • Count the atoms in the balanced equation again to see that the equation is correct.

46. Classifying Chemical Reactions • Substances may combine to make a more complex substance. • They may break apart to make simpler substances. • Or, they may even exchange parts. • In each case, new substances form. • Many chemical reactions can be classified in one of three categories: synthesis, decomposition, or replacement.

47. Reactions Three categories of chemical reactions are synthesis, decomposition, and replacement. Making Models How do these different geometric shapes act as models for elements and compounds in reactions?

48. Synthesis • Have you ever listened to music from a synthesizer? • You can hear many different notes and types of sounds combined to make music. • To synthesize is to put things together. • In chemistry, when two or more elements or compounds combine to make a more complex substance, the process is called synthesis (sin thuh sis). • The reaction of hydrogen and oxygen to make water is a synthesis reaction.

49. Decomposition • In contrast to a synthesis reaction, a process called decomposition breaks down compounds into simpler products. • You may have a bottle of hydrogen peroxide (H2O2) in your house to clean cuts. • If you keep such a bottle for a very long time, you’ll have water instead. • The hydrogen peroxide decomposes into water and oxygen gas.

50. Replacement • When one element replaces another in a compound, or when two elements in different compounds trade places, the process is called replacement. • Look at this example: • Copper metal can be obtained by heating copper oxide with carbon. • The carbon takes the place of copper.