Chemical Changes

CHEMICAL CHANGES

Conservation of Mass The mass of the product is always equal

to the mass of the reactants. This is known as the Law of Conservation of Mass : that mass is neither created nor destroyed in a chemical reaction.

Chemical Changes 1.Odor or Change in taste: (sour), Curdling of milk ,

browning fruit 2. Change in Color : Rusting/tarnishing, hair dye or

bleach, toast, burning toast or food 3. Production of gas:(expanding /rising & bubbles), Baking

a cake or bread, Making cheese 4. Hissing or Loud Noise (fire works) 5. Formation of precipitate 6. Protein rearrangement: frying and egg, eggs in baking

a cake

Corrosion & Rusting Corrosion: is the loss of metallic

properties of a metal due to oxidation and is accompanied by the formation of unwanted products. Copper, iron and aluminum metals all corrode over time loosing strength, luster and electrical conductivity.

Rusting is the corrosion of iron and readily occurs in the alloy steel. The formation of a reddish brown flakes which loosely adheres to the iron is called rust.

RUST The overall chemical equation for the

formation of rust is Iron + water → oxygen rust

4 Fe(s) + 6 H2O(l) + 3 O2(g) → 4 Fe(OH)3(s)

Recognizing chemical changes Changing color is a clue that a new

substance is formed. A bracelet turning darker such as tarnishing.

2 Ag + H2S ----> Ag2S + H2.

Baking soda + Vinegar NaHCO3 + CH3COOH => CH3COO-Na+

+ H2O + CO2 Sodium bicarbonate and vinegar =>

Sodium acetate and water and carbon dioxide

Bicarb of soda can also neutralize both acids and bases

Formation of precipitate:

A precipitate is a solid that forms out of solution. A common example is that of the mixing of two clear solutions: (1) silver nitrate (AgNO3) and (2) sodium chloride (NaCl): The reaction is

Because lead iodide is not soluble it comes out of solution, or precipitates. It is yellow because that is just the color of the substance. Although the color can be changed by heating (to red), it will precipitate as a yellow solid every time

Curdling of milk When raw milk is left standing for a while,

it turns sour. This is the result of fermentation: lactic acid bacteria turning the milk sugar into lactic acid. This fermentation process is exploited in the production of various dairy products such as cheese and yogurt

Chemistry of cheese Separation of Curds and Whey Cheese making is essentially the process

by which water, lactose and other minerals are removed from milk to produce a concentrate of protein and fat. It involves the separation of milk into a solid mixture of coagulated protein and fat (called curds) and a liquid mixture of water and lactose (called whey).

Soda bottle & Balloon experimentMeasure 1/2 tablespoon of yeast and pour it into the soda bottle. Measure 1/2 tablespoon of sugar and pour it into the bottle. Measure 1/2 cup warm water and pour it into the bottle. Swirl the bottle so that all of the contents are well mixed. Ask an adult or a friend to hold the bottle while you put your balloon over the mouth of the bottle. Observe your bottle every five minutes.

Yeast Yeast is one kind of fungus. Yeast can

use sugar as food. Several chemical changes are occurring inside the bottle. The yeast causes the sugar to turn into alcohol, carbon dioxide, and energy. Carbon dioxide fills the balloon.

Baking Bread Baking bread

The formation of carbon dioxide — a byproduct of ethanol fermentation — causes bread to rise.

Ethanol fermentation causes bread dough to rise. Yeast organisms consume sugars in the dough and produce ethanol and carbon dioxide as waste products. The carbon dioxide forms bubbles in the dough, expanding it into something of a foam. Nearly all the ethanol evaporates from the dough when the bread is baked.

Baking a Cake

Baking Soda Baking soda is a leavening agent that causes a cake to rise by producing bubbles of carbon dioxide, or CO2, as well as the byproducts of water (H2O) and sodium carbonate (Na2CO3).

This occurs according to the following formula: 2NaHCO3 = Na2CO3 + H20 + CO2.

Baking a Cake Baking Powder

Baking powder is baking soda (NaHCO3) that has already been neutralized with the addition of an acid . Baking powder produces more CO2 carbon dioxide than baking soda, which means more bubbles in the cake mix and a softer finished product.

Baking a Cake Eggs

Egg yolks are an emulsifier that helps the oil-based and water-based components of the cake mix together. When eggs are cooked, the protein molecules become uncurled when exposed to heat and create new molecular bonds with other nearby protein molecules. When the egg is completely cooked, it helps form a protein network that gives the cake structure.

Speeding up Chemical Reactions 1. Breaking a solid into smaller sizes (to

increase its surface area) 2. Increase the concentration (↑

substance) 3. Increase the temperature ( more kinetic

energy) 4. Decreasing volume 5. Adding a catalyst

Chemical Equations Reactants Products Carbon + Oxygen Carbon Dioxide C + O2 CO2

Chemical Weathering The breakdown of rocks resulting in a

change of chemical composition. 1. Oxidation : Oxygen combines with the

elements in the rock and it reacts. (Rust) 2. Hydration: water can dissolve away

and change the chemical composition of rocks.

3. Carbonation: Carbon dioxide dissolves in water to form carbonic acid. This makes acid rain which chemically weathers (dissolves rocks.