Acids, Bases, pH, Chemical Change, Physical Change

a measure of acidity or alkalinity
Key Context
The pH factor is a quick method for identifying whether a solution is acid or alkaline. A pH value of 7 or lower means the liquid is acid. Higher than 7, the solution is alkaline. Fruit juices like apple or orange have pH values in the range of 3-4, noticeably acidic. Eggs and human blood are in the 7-8 range, definitely alkaline. Distilled water is a 7, perfectly neutral.
A substance that will change colors in the presence of an acid or a base.
Examples: Litmus paper turns red in an acid.
Litmus paper turns blue in a base.
Phenolphthalein remains colorless in an acid.
Phenolphthalein turns bright pink or magenta in a base.
pH Scale
Based on the hydronium ion concentration.
Ranges from 0 – 14.
A pH of 7 is considered neutral meaning it is neither an acid nor a base.
Measures how acidic or basic a solution is.
pH of an acidic solution is <7 and the lower the pH number the stronger the acid. pH of a basic solution is >7 and the higher the pH number the stronger the base.
When a substance changes directly from a gas to a solid – such as the formation of frost from water vapor
When a substance changes directly from a solid to a gas – such as dry ice or solid air fresheners
any substance with a pH less than 7; acids turn blue litmus paper red (related word: acidic)
Key Context
Have you ever been sick with a stomachache and vomited? Did it burn your throat? There are acids in your stomach that help you digest your food. Sometimes when we are sick, the liquids come up and out of the stomach. The stomach acids will give you a burning sensation in your throat. But not all acids are bad for you. Orange and lemon juices are acidic and will not harm our throat. These juices are not as acidic as the acids in your stomach.
any substance with a pH higher than 7; bases turn red litmus paper blue (related term: basic, alkali)
Key Context
Did you ever accidentally taste soap? Soap tastes bitter and it is very slippery. Soap is an example of a chemical called a base. Bases are the opposite of acids. Both can be very strong. Other bases like baking soda are weak bases and are safe to touch. Some bases, like ammonia and bleach are very strong bases and are very dangerous.
describes something that is not charged or is neither an acid nor a base: In chemistry, neutral substances are neither acids nor bases. In physics, neutral objects have no electrical charge.
Key Context
Two friends are having an argument but you stay out of it. You are neutral. You do not side with either friend. In science when something is not positively or negatively charged we say it is neutral. Or if it is not an acid or a base, we call that neutral as well.
litmus paper
a tool used to find out whether a substance is an acid or a base: Acids turn blue litmus paper red. Bases turn red litmus paper blue.
Key Context
Did you know that some things you drink are acids? It’s true. You can use litmus paper to find out which drinks are acids. Dip a blue litmus paper into the liquid. If the paper turns pink, then it is an acid. If it stays blue, it’s not an acid. Lemonade is one example of a weak acid that we drink. Litmus also comes in pink, which is used to test for bases such as bleach.
chemical change
a chemical reaction; a process that changes substances into new substances (related word: chemical reaction)
Key Context
You light a fire while you camp to keep warm and to cook. You also see a chemical change. Once the wood turns to ashes, it cannot turn back into wood. The wood has become something different. That’s what happens in a chemical change.
physical change
a change in matter that does not affect its chemical composition
Key Context
You can melt ice and then refreeze the water to make ice again. The ice changes states but it is always water. You can tear up a piece of paper and it’s still paper. You can mix sugar in water and still taste that the sugar is there. These are all physical changes. Physical changes mean that matter has changed states but remains the same.

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