Ch 2.3 Chemical Properties When you describe the candles at the left you may talk about the color, the size, the smell, or the fact that they float on water. Those are all physical properties…. You may also notice that the candles are burning. This is a Chemical property because you cannot test it without changing the composition of the candle The burning of the candle creates new compounds like ash and smoke. • Chemical Properties – any ability to produce change in the make up of matter. This means that chemical properties can only be observed as the chemical itself changes into different materials. Flammability and reactivity are both examples of chemical properties. Flammability Flammability is a material’s ability to burn when oxygen is present. Gasoline is burned to power cars and wood is burned for heat and light. Flammability is not always considered a good thing. Cloth is often flammable but if your clothes are burning they are not doing their intended job. Reactivity Reactivity is the ability to combine chemically with other substances. Oxygen is a very reactive element, while Nitrogen is has a very low reactivity. Oxygen in the presence of water and iron creates rust. Oxygen with apples causes them to brown. What is the ability to produce a change in the make up of matter called? a. Chemical change b. Chemical property c. Physical change d. Physical property Answer : B Which of the following is an example of a chemical property? a. Color b. Density c. Reactivity d. Hardness Answer: C Which element is more reactive, Nitrogen or Oxygen? Answer: Oxygen Other reactive elements are iron, sodium, and chlorine Chemical Changes A chemical change is any change that cause the substance to turn into a new and different substance. • Cutting an iron beam in half is not a chemical change, because you still have iron, just in smaller pieces. • Water and oxygen acting on iron to cause it to change to iron oxide, or rust is a chemical change because rust is a new compound separate from the iron we started with. Which of the following is a chemical change? A. Peeling a banana B. Turning copper into a wire C. Burning a log on a fire D. Driving over a watermelon Answer: C Detecting Chemical Change There are 3 main ways to detect chemical change: 1. Change in color 2. Production of gas 3. Formation of a precipitate Changes in color Shiny silver left out and exposed to the air will dull and darken A lit match will turn dark black. Copper turns green with time and air. Banana peels turn brown with spots as they ripen. These are all changes of color from chemical change. Production of a Gas Think about mixing vinegar with baking soda. The foaming bubbles produced are the result of a chemical change. This is similar to what happens when baking soda or yeast is used in baking. The gas produced creates bubbles of carbon dioxide that causes the bread or cake to rise. Formation of a Precipitate A precipitate is any solid that forms and then separates when liquids mix. When an acid is added to milk, the proteins and fat in the milk undergo a chemical change to form cottage cheese as a precipitate. When a marshmallow is held over a camp fire but doesn’t catch fire, what tells you a chemical change has occurred on the outside? Answer: The marshmallow changes color as it cooks on the outside. What tells you a candle wick is undergoing a chemical change? a. It changes color as ash forms on it b. A small trail of smoke rises from the wick c. The wick begins to grow shorter d. Both a and b Physical and Chemical Change Gas, color change, and even precipitate does not always mean a chemical change. A hot piece of iron is still iron though it turns orange or white. Water releases steam as it boils, but it is still water. A chemical change always produces new substances. The composition of matter changes whenever a chemical change occurs. A pad of butter is put in a hot pan and begins to melt. Is this a chemical or physical change? Answer: Physical, it is still butter, just melted. The butter then begins to burn and smokes and turns to a brown ash on the pan. Is this a chemical or physical change? Answer: Chemical the butter is changed into a gas and a carbon residue left behind.