A substance has immutable properties
- Pure salt is always white and has all the same salty taste: Salt is a substance. - Pure iron everywhere has the same density, the same hardness etc. .. Iron is a substance. - Pure water has the same density everywhere, the same transparency etc. .. Water is a substance
.A mixture consists of several substances mixed, the components of the mixture
A mixture has the properties of each component, which are all the more pronounced as the predominant component is: - Saltwater has the colorless appearance of water and the taste of salt is all the more pronounced it contains more salt. Salt water is a mixture of water and salt. - A gold and copper alloy becomes the more red it contains morecopper and the more yellow it contains more gold. The 18-karat gold (75% gold and 25% copper) is a mixture of gold and copper.
A mixture is called homogeneous if the substances which it is made of cannot be discerned separately by sight.
- Vodka, made out of water and alcohol is a mixture. - Steel, made out of iron and carbon is a mixture
.A mixture is called heterogeneous if the constituants of which it is composed can be distinguished by sight, possibly under a microscope.
- Pastis, made out of water and anisole is a heterogeneous mixture. You can see the droplets of anisole under a microscope. - Granite, made out of quartz, feldspar and mica is a heterogeneous mixture. Its constituents can be distinguished by the naked eye.
The methods for separating substances out of mixtures can be very diverse: - Filtration is used to separate liquid/solid heterogeneous mixtures. - Decantation is used to separate heterogeneous mixtures of two immiscible liquids. - Distillation separates mixtures of two liquids. - Chromatography is used with various mixtures, often colored substances. etc ...
These separation methods are all physical changes, because by remixing the constituants obtained, one obtains mixtures with the same properties as before.